Separation of Cobalt and Nickel Using CYANEX 272 for Solvent Extraction
2/2/2021 Separation of Cobalt
and Nickel using CYANEX 272 for Solvent Extraction In the presence of other contaminates in sulphate based leaching solution
Caroline Kihlblom BOLIDEN MINERAL AB Supervisor: Mohammad Khoshkhoo Examiner: Kerstin Forsberg
2021-04-07
Abstract This project aimed to examine the separation of cobalt and nickel using solvent extraction (SX) with the extractant CYANEX 272 (C272). It was intended to investigate the Co-Ni separation in a sulphate-based leach solution in presence of other contaminants. This is an area of interest because of the difficulty of separating metals of similar properties within the field of hydrometallurgy.
Batch tests, with varying modifiers and diluents, were carried out to examine the effect of organic phase composition on phase separation. The effect of pH on equilibrium was investigated by constructing equilibrium curves. Through various shaking tests, different separation parameters were studied. McCabe-Thiele diagrams were constructed to predict design parameters. In order to simulate a continuous 3-stage countercurrent solvent extraction, batch tests were performed. Scrubbing, as means of impurity removal was also investigated. Finally, the product’s purity was examined by the help of crystallization.
The organic feed mixture that resulted in a sufficient phase separation consisted of C272, tributyl phosphate and naphtha. At pH 4, equilibrium curves showed that equilibrium was either not reached or affected by competing metal ions. A standard equilibrium curve appearance was seen at pH 4.5, resulting in that the theoretical required stages for extraction was calculated to 3 stages (A/O=1). However, a McCabe-Thiele diagram did not give an accurate representation of the more complex case (presence of contaminants). Batch simulation results gave a cobalt recovery of 69% and 100% at pH 4.5 and 4.8, and a nickel recovery of 0% and 3%, respectively. A recommended pH-value for solvent extraction could not be stated, because the choice must be based on operation specifications. Therefore, several different aspects (Co recovery, purity, and economical etc.), must be accounted for. A similar pH-trend was shown in scrubbing, where an increase of pH resulted in an increase of metal ions’ organic concentration. For stripping, acid test results proved 24 g/L sulphuric acid to give the highest cobalt concentration, with a marginal difference in concentration of impurities.
An overview of the entire SX process, indicated that extraction, scrubbing, and stripping were all successful operations. The extraction stage showed a Co and Ni recovery of 99% and 0.02%, respectively, and a separation factor of 14250. Distribution results indicated that Al was difficult to remove and was transferred with Co into the product. Therefore, this element must be removed before SX. From noticing an increase of Co:Ni ratio throughout the process, solvent extraction was considered an effective separation method for cobalt and nickel separation. A considerably high purity of cobalt sulphate was produced. However, impurities Al and Ca were also detected in the product. Increasing the acetone volume in crystallization resulted in an increase of Co purity. An increase of the cobalt sulphate crystals formed was observed when increasing the acetone volume, where no impurities were detected.
Key words: Solvent extraction, Cobalt-Nickel separation, CYANEX 272, pregnant leach solution
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Table of Contents 1 INTRODUCTION ...... 4 1.1 Aim and objectives ...... 5 1.2 Scope ...... 5 1.3 Boliden mineral ab ...... 6 2 BACKGROUND ...... 7 2.1 Solvent extraction ...... 7 2.1.1 Extraction ...... 7 2.1.2 Scrubbing ...... 7 2.1.3 Stripping ...... 7 2.1.4 Solvent Extraction Chemistry ...... 8 2.2 Seperation of cobalt and nickel ...... 8 2.2.1 Solvent extraction in sulphate solutions ...... 8 2.2.2 Selectivity ...... 9 2.2.3 Extractant and diluent ...... 10 2.2.4 Impurities ...... 12 2.3 Definitions ...... 13 2.3.1 Distribution coefficient ...... 13 2.3.2 Separation factor ...... 13 2.3.3 McCabe-Thiele diagram ...... 14 3 METHODS AND MATERIALS ...... 16 3.1 Materials ...... 16 3.2 Experimental set-up ...... 17 3.2.1 Process overview ...... 17 3.2.2 Effect of saponification and organic phase composition ...... 18 3.2.3 Effect of pH on extraction equilibrium ...... 19 3.2.4 Batch Simulation of Continuous Multistage Countercurrent Solvent Extraction ...... 19 3.2.5 Scrubbing tests ...... 21 3.2.6 Stripping tests ...... 21 3.2.7 Crystallization of Cobalt ...... 21 3.2.8 Analytical methods ...... 22 4 RESULTS AND DISCUSSIONS ...... 24 4.1.1 Effect of saponification and organic phase composition ...... 24 4.1.2 Effect of pH on extraction equilibrium ...... 26 4.1.3 Batch Simulation of Continuous Multistage Countercurrent Solvent Extraction ...... 29 4.1.4 Scrubbing ...... 33 4.1.5 Stripping ...... 34 4.1.6 Solvent extraction overview ...... 35 4.1.7 Overall distribution and process summary ...... 38 4.1.8 Crystallization of Cobalt ...... 39 4.1.9 Source of error ...... 40
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5 CONCLUSION ...... 41 5.1 Future research ...... 43 6 ACKNOWLEDGMENTS ...... 44 7 BIBLIOGRAPHY ...... 45 8 APPENDIX ...... 47 8.1.1 Effect of pH on extraction equilibrium ...... 47 8.1.2 Batch simulation of continuous multistage countercurrent solvent extraction ...... 47 8.1.3 Stripping ...... 47 8.1.4 Crystallization of cobalt ...... 48
Table of Figures Figure 1: pH-dependency of Co, Ni and Ca extracted in a sulfate solution using CYANEX 272 ...... 10 Figure 2: Molecular structure of Di (2,4,4-trimethylpentyl) phosphinic acid (CYANEX 272)11 ...... 11 Figure 3: Different metal ion extractions (in %) with Cyanex 272 vs. pH 9 ...... 12 Figure 4: Schematic of a McCabe-Thiele diagram5 ...... 15 Figure 5: A flowsheet of the overall experimental solvent extraction design...... 17 Figure 6: Batch simulation of 3-stage countercurrent solvent extraction. The red and black lines represent the aqueous and organic phase, respectively. Note that SX A3 was not conducted, but is presented for the sake of visualization...... 20 Figure 7: Continuous countercurrent solvent extraction, where the aqueous raffinate and organic extract are denoted with an R and E, respectively. The symbol F denotes the fresh feed in and S denotes fresh extractant...... 20 Figure 8: A photograph representing test 2, where a blue color is seen in both phases ...... 24 Figure 9: The organic feed tests, test 8-10, from left to right ...... 25 Figure 10: Plotted equilibrium curve for cobalt at pH 4.0...... 26 Figure 11: Equilibrium curve for cobalt at pH 4.0 with a lower aqueous to organic ratio. Notice that the red points represent Co ions being pushed out ...... 27 Figure 12: McCabe Thiele diagram for cobalt at pH 4.5 with a slope of 1 ...... 27 Figure 13: The metal ions’ concentration in the aqueous phase vs. batch simulation cycles, for cobalt and nickel ...... 29 Figure 14: Metal ion recoveries into organic phase vs. cycles at pH 4.5 ...... 30 Figure 15: Metal ion recovery into organic phase vs. cycle, at pH 4.8 ...... 32 Figure 16: Schematic of experimental design with Co and Ni in-and-out streams presented with unit g/L, and with A:O=1 ...... 35 Figure 17: A photography of all the formed crystals from crystallization ...... 48
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1 INTRODUCTION At Boliden’s mine in Kylylahti, Finland, ore is being processed and mined to produce different kinds of concentrate, namely copper, zinc, and Knelson gravity concentrate. The flotation tailings, also called pyrite- or sulphur concentrate, which contains significant amount of pyrite, is deposited in lined ponds. In the pyrite concentrate, cobalt (Co) and nickel (Ni) can be found, which is of high interest for the metal company Boliden. The reason for interest of recovering of these two base metals from the sulphur concentrate is their corrosive, resistance, and magnetic properties, alongside their high economic value due to their applications in manufacturing of electrical vehicles batteries. To obtain as much of the desired product as possible, Boliden must oxidize an enormous amount of pyrite first, which is done by bioleaching, followed by a series of purification and separation processes. The leach solution consists of Co and Ni, alongside the main impurities Ca, Al, Mg and Mn.
However, it is difficult to separate cobalt from nickel in a sulphate-based solution, because of their similarity in properties. Therefore, it is of great importance to use an effective separation method for this purpose. Applying solvent extraction by using CYANEX 272 has shown good separation of Co and Ni in similar systems and will therefore be used in this project.
In this master thesis project, different conditions will be investigated in batch tests, in order to observe the effect on separation. To find out how many separation stages are required for the specific separation process, design parameters will be investigated by using theoretical calculations. An evaluation of where metal ions end up after solvent extraction will also be studied.
The main research questions for this project are: • What effect does organic phase composition have on phase separation? • What effect does pH have on equilibrium? • How many minimum stages are required for the separation process? • How are other elements distributed throughout solvent extraction? • What is the purity of a possible cobalt sulphate product?
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1.1 AIM AND OBJECTIVES The aim of the project is to investigate the separation of cobalt and nickel by employment of solvent extraction using the extractant CYANEX 272. The possibility of producing a pure cobalt sulphate solution from a sulphate-based leach solution using this separation method will be studied in the paper.
In order to achieve the aim, some objectives are formulated, which are as follows: • Planning and design of the experimental work by the means of a literature study. • To test different diluents and modifiers to find a suitable organic feed composition for phase separation • To examine different separation parameters (pH and acidity) to find recommended values for obtaining maximum cobalt recovery without co- extraction of impurities. • To investigate how many extraction stages are required for the operation, by using McCabe-Thiele diagram. • Conducting an analysis of metal ions and contaminants using ICP-OES and XRF, respectively.
1.2 SCOPE The project scope was divided into three different parts. The first part consisted of performing batch tests on the leach solution under different conditions, in order to verify the effect on phase separation. This was done in lab scale in Boliden’s pilot facility.
After the initial tests were conducted design parameters were investigated by the help of a McCabe-Thiele diagram. This method was used to calculate the required stages for the extraction process. Batch tests were conducted in this part as well, but to simulate a continuous multistage countercurrent solvent extraction.
The third part consisted of analyses of metal ions in different streams. Through calculating various mass balances, distribution of elements throughout the process were studied. Impurities were studied closely, to justify if pure cobalt sulphate could be obtained.
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1.3 BOLIDEN MINERAL AB Boliden Mineral AB has requested that this master thesis project is conducted at one of their many facilities, in the Boliden area. Boliden Mineral AB, also known as Boliden, is a well-known Swedish metal company which mines, produces and sells valuable metals to the world’s metal market. For more than 90 years, the company has been exploring, extracting, processing, and selling base- and precious metals from their many mines and smelters. They have over five mines distributed across Sweden, Finland and one mine in Tara, Ireland. Whereas, their smelters are located in Sweden, Finland and Norway, which accounts for a total of five smelter facilities. Their head office is situated in Stockholm. Boliden is today the European leader of producing the base metals copper and nickel and one of the world’s leading zinc producers. 1
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2 BACKGROUND 2.1 SOLVENT EXTRACTION Solvent extraction (SX) is a separation process often used in hydrometallurgy to obtain a highly purified and concentrated metal. This specific separation technique was introduced in connection with the Manhattan project in the early 1950s to produce pure uranium. Later, the first commercial copper solvent extraction plant was introduced in 1968. After that, the interest for solvent extraction grew as a unit operation used for the recovery of a wide range of metals. The separation process is constantly being improved by development of new extractants and equipment.2
Solvent extraction consists of contacting an aqueous phase, containing the metal of interest, with an organic phase, containing an extractant. The metal of interest is transferred from the aqueous phase to the organic phase after the phases are put in contact via agitation. The phases can separate due to their immiscibility and their difference in density. The metal chemically reacts with the extractant to form a metal complex in the organic phase, while the aqueous phase often contains remaining impurities. By reversing the chemical reaction, it is possible to recover pure metal in a highly concentrated aqueous solution. Solvent extraction consists of two operations, extraction and stripping, which are described more thoroughly below.3 4
2.1.1 Extraction In the extraction circuit, metal ions from the aqueous phase (SX feed) bind to the extractant in the organic phase when they are put in contact. By the means of stirring, the two phases are mixed, allowing them to separate. From the extraction step a loaded organic phase is produced together with an aqueous phase called a barren or raffinate. The raffinate is then either recycled back to the SX feed or discharged for further treatment, while the loaded organic continues to the stripping circuit.3–5
2.1.2 Scrubbing If impurities are transferred to the loaded organic, it may be required to add a scrubbing stage before the stripping step. This is in order to wash and remove all metal impurities by treatment with a fresh scrub solution or a bleed of the recycled strip liquor.3–5
2.1.3 Stripping In the stripping stage, the valuable metal is recovered by reversing the chemical reaction in the extraction circuit. In other words, the metal complex in the organic phase is back extracted (stripped) into the aqueous phase by using a suitable solution. The product after stripping is a strip liquor containing a high concentration of pure metal. Here, the raffinate is the organic strip solvent, which can also be recycled back to the extraction step.3–5
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2.1.4 Solvent Extraction Chemistry On a chemical level, the solvent extraction process is an equilibrium process, distinguished by three different type of extraction mechanisms i.e. cation-, anion- and solvation extraction. In this report, only cation extraction mechanism will be focused on and described in more details. The extractant, the extracted metal ions and the chemical environment determine which type of extraction process is obtained.
In cation extraction mechanism, the metal ions (e.g. Cu2+, Fe3+, Co2+) are mainly exchanged with an acidic hydrogen from the extractant. The extractant used for this purpose is for example alkyl phosphoric acids, sulphonic acids and carboxylic acids.6 The cation extraction, for both extraction and stripping, is a pH-dependent equilibrium process and is described by the following formula:
2+ + 푀푒 (푎푞) + 2푅퐻 (표) ↔ 푀푒푅2(표) + 2퐻 (푎푞)
Where, Me and R represents the metal and extractant functional group, respectively. The formula indicates that a high pH value is desired for extraction, whilst the opposite is true for stripping i.e. a low pH value favors the reaction.7 3
2.2 SEPERATION OF COBALT AND NICKEL In hydrometallurgy, the separation of cobalt and nickel in aqueous solutions has always been problematic. The reason is their aqueous chemical similarities, due to that they are positioned adjacent to each other in the periodic table (in the transition metal series). Therefore, an effective separation technique is required (a high separation factor) to achieve this separation. To separate the two base metals, one takes advantage of their difference in water exchange rate of the metal ions, where cobalt ions have a much higher rate of water exchange compared to nickel ions. This implies that complex ions form more easily for cobalt.2
Today, a commonly used separation process for cobalt-nickel separation from sulphate solutions is solvent extraction (SX). This separation alternative is preferred over the traditional separation processes, such as selective oxidation and precipitation. The explanation for the choice is the high degree of separation (high separation factor) that SX offers, together with its ability to fulfill the industry’s high separation demand2. Another advantage of using solvent extraction for the separation of cobalt from nickel is its ability to separate metals that have very similar properties. The separation process has a much higher degree of separation for cobalt and nickel compared to for instance crystallization or precipitation8.
2.2.1 Solvent extraction in sulphate solutions Solvent extraction for sulphate-based leach solutions involves mixing of the leach solution (aqueous phase) with a liquid organic (organic phase), where the leach solution contains Co and Ni and the organic phase contains an extractant. The cobalt
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ions in the liquid phase are transferred to the organic liquid, forming an organic complex. In other words, cobalt is extracted from the leach solution leaving nickel in the aqueous phase. Later, the loaded organic, which is loaded with cobalt, can be separated from the aqueous solution by means of gravity. The specific gravity of the aqueous- and organic phase is 1.1 and 0.85, respectively, resulting in the organic phase floating on top of the aqueous phase. The raffinate, containing mostly nickel, is further processed, while the loaded organic is sent for stripping in the stripping circuit. In the stripping stage, the loaded organic is mixed with a suitable solution, for instance sulfuric acid, and the revers reaction to the extraction step takes place. The product from stripping is the aqueous phase, enriched in cobalt, while the organic phase is recycled back to the extraction circuit.6
Solvent extraction of cobalt from sulphate-based solutions is commercially available today. At, for instance, Norilsk Nickel in Harjavalta, Finland, Minara Resources in Murrin Murrin, Australia; and Sherritt in Toamasina, Madagascar. Through extraction, a large amount of pure cobalt and nickel is produced each year. The extraction reaction is described by:
2퐻2퐴2(푙) + 퐶표푆푂4(푎푞) → 퐶표퐴2 ∙ 퐻2퐴2(푙) + 퐻2푆푂4(푎푞)
, where A represents 2,4,4-trimethyl phosphonic conjugate base in the organic phase. The stripping reaction is described by reversing the extraction reaction. A low concentration of sulfuric acid is preferred in extraction, whilst the opposite is true for stripping i.e. a high concentration.6
2.2.2 Selectivity The selectivity of a specific separation determines the outcome from the separation process i.e. extraction and stripping. Consequently, one must be aware of the system’s selectivity, in order to control it more easily. For separation of cobalt from nickel in a sulphate solution the selectivity, when using an alkyl phosphorous acid, is dependent on:
➢ Temperature ➢ pH ➢ Acid type ➢ Co concentration ➢ Diluent ➢ Modifier
Also, the separation factor for alkyl phosphoric acids changes with an increase of 10-1000 in the given order:
푝ℎ표푠푝ℎ표푟𝑖푐 < 푝ℎ표푠푝ℎ표푛𝑖푐 < 푝ℎ표푠푝ℎ𝑖푛𝑖푐 푎푐𝑖푑푠
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The increase of separation factor can be explained from the change of the cobalt complex’s nature. In chemical terms, the hydrated octahedral complex changes into the anhydrous tetrahedral complex.2
When a cation exchange chemistry exists, extraction and stripping is pH- dependent7. Therefore, it is important to control the pH in order to obtain specified process requirement. Hence, selective extraction is possible if the equilibrium concentration of the two metals ions are substantially different at a specific pH- value8.
Earlier studies have tested the effect of pH on the Co-Ni selectivity when using CYANEX 272 as extractant, which is demonstrated in Figure 1.
Figure 1: pH-dependency of Co, Ni and Ca extracted in a sulfate solution using CYANEX 272 Figure 1 shows that depending on the solution pH, different amount of metals can be extracted. 9
2.2.3 Extractant and diluent 2.2.3.1 Extractant Due to the high degree of separation required for cobalt and nickel, choice of a suitable extractant is vital. The properties of a suitable extractant are listed below10:
➢ Selective ➢ Easy to strip ➢ Easily handled ➢ Non-toxic and non-hazardous ➢ Easy to separate from water i.e. different density from water ➢ Low aqueous solubility ➢ High surface tension ➢ Has a high flash-point
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➢ Has a low vapor pressure ➢ Does not bind too strongly to metal ion ➢ Cheap
It may be difficult for the extractant to achieve all the given properties, but it should be carefully chosen on these basis.3, 8Therefore, one must take into consideration what type of chemical environment is present.
If a cation exchange chemistry exists, alkyl phosphoric, phosphonic or phosphinic acids are the preferred extractant, because of the selectivity for Co over Ni. If a weakly acidic sulphate solution is present, an alkyl phosphorous acid has shown to be the preferential extractant option for the separation of cobalt and nickel. The commercially available dialkyl phosphinic acid CYANEX 272 (C272) has been proven to give a high separation factor (>1000), which, as mentioned earlier, is required for cobalt and nickel separation.2
Figure 2: Molecular structure of Di (2,4,4-trimethylpentyl) phosphinic acid (CYANEX 272)11 Di (2,4,4-trimethylpentyl) phosphinic acid (Figure 2), more commonly known as CYANEX 272, is the extractant of choice when extraction of cobalt from nickel in a sulfate or chloride solution is wanted. The extractant is non-toxic and easy to handle, which is an advantage. However, one disadvantage is its high cost.9 The selectivity series of C272 is:
퐹푒3+ > 푍푛 > 퐶푢 > 퐶표 > 푀𝑔 > 퐶푎 > 푁𝑖
, which indicates the selectivity of Co over Ni2. According to Sole, the selectivity of cobalt over nickel depends on the organic complex of cobalt being very hydrophobic and having a simple structure12. The extractant’s concentration is usually in the range of 10-25% by volume, when diluted with an aliphatic diluent 6.
Soldenhoff et.al showed that cobalt recovery of 97.5% could be achieved when using CYANEX 272 for solvent extraction in continuous mini plant trials. The results from the trials also showed a high separation factor of >1000 (Co:Ni ratio 1000:1) and a >99% removal of impurities.13 This indicates that SX using CYANEX 272 can be a suitable separation option for Co and Ni separation.2 Other literature indicates a maximum separation factor of C272 at 6700 for Co and Ni, which means that 6700 times more cobalt will be extracted from an aqueous sulfate
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solution into the extractant C272. Note that the prior sulfate solution contains the same amount of both metals.6
2.2.3.2 Diluent An organic extractant is seldom used in its pure form, but instead dissolved in an organic diluent. The diluent used possess a low density and viscosity, in order to be immiscible with aqueous solutions and easily separated by gravity from the aqueous phase. Typically, aliphatic diluents are mixed with CYANEX 272 due to their low vapor pressure, a high oxidation resistance, a high flash point temperature, and, foremost, being totally miscible with C272.6
2.2.4 Impurities Since solvent extraction does not remove all of the impurities, this must be done before or after the process, which is mainly done by precipitation. After the extraction, some unwanted nickel from the aqueous phase can be co-extracted. To purify the loaded organic, both washing and scrubbing can be conducted. The difference between the two is that washing is done by physical removal, whereas scrubbing is the chemical removal of the co-extracted nickel. An example of the scrubbing of nickel from the organic phase is to add an aqueous solution containing high concentration of cobalt, where the intention is to “push out” the nickel. Meanwhile, Ni can be physically removed (washed) by water.6
M.B.Kime et.al concluded in their paper, that cobalt was sensitive to impurities such as Ni, Zn, Al, Mn, which were often co-extracted in their experiments (when using IONQUEST 290 as extractant)14. To avoid the problem a pH selective to extracting cobalt over other metal ions (impurities) should be chosen. Figure 3 demonstrates the single metal extraction versus pH using C272.
Figure 3: Different metal ion extractions (in %) with CYANEX 272 vs. pH 9
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Note that this figure is valid for single metal extraction under the specific experimental conditions performed in the mentioned reference and may vary when other conditions apply9.
2.3 DEFINITIONS In this section some of the essential definitions will be described more thoroughly.
2.3.1 Distribution coefficient The distribution coefficient is defined by the following equation:
[푀푒](표) 퐷 = [푀푒](푎푞)
The distribution coefficient describes how much of the metal component is distributed between the two phases after the aqueous solution (containing the metal of interest) is brought into equilibrium with the organic phase. In other words, D indicates the extractant’s effectivity. Obtaining a high distribution value means that a high amount of metal is extracted from the aqueous phase to the organic, which is favored in the extraction stage. The opposite applies for the distribution coefficient for stripping, D1, which is described by reversing the equation above:
[푀푒](푎푞) 퐷1 = [푀푒](표)
A high D1 value is the result of a successful stripping, because then a high amount of the metal has been stripped accordingly. However, if a low value is obtained after the stripping stage, this is an indication that the metal has bonded too strongly to the extractant, disabling the metal ion to be stripped to the aqueous phase. To be clear, [Me](o) and [Me](aq) is the concentration of metal in the organic phase and aqueous phase, respectively. D is dependent on different factors such as: temperature, composition of the respective phases, and nature of solvent etc.8
2.3.2 Separation factor The separation factor α is the ratio of distribution coefficients for the two metal components that are to be extracted, described by: 퐷 훼 = 푖 퐷푗 Where, i represents one metal component and j the other metal component. The separation factor is an indication of the ease or difficulty of a separation. The greater the difference of the components’ distribution coefficients, the better the separation. Thus, a separation is possible if α is not equal to one. The reason that solvent extraction is a suitable separation process for separating and purifying certain
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metals, is because of their great difference in distribution coefficients, which in turn means a great separation factor.8
2.3.3 McCabe-Thiele diagram A design of a solvent extraction operation can be made by the help of a McCabe- Thiele diagram. This diagram consists of an equilibrium curve and an operating line (working line), as shown in Figure 4. By equilibrating a leach solution (aqueous phase) with an organic phase in different aqueous to organic phase ratios, an equilibrium curve for the specific operation is determined. Thereafter, the equilibrium curve can be drawn into the diagram from analyzing the metal content in the two phases after each new aqueous to organic phase ratio. The equilibrium curve is constructed by plotting metal concentrations in the organic phase against metal concentrations in the aqueous phase. The metal concentration in the aqueous feed is drawn as a vertical line, known as the feed line. An operating line is inserted into the diagram by drawing a straight line of slope Aq/O, which passes through the coordinates of the fresh aqueous stream to the coordinates of the raffinate stream leaving. Note that a phase ratio (Aq/O) is assumed to fit the operation requirements.3, 5, 7
In order to investigate the amount of extraction stages required for the given process, the constructed McCabe Thiele diagram is used to calculate number of required stages. The first extraction stage is obtained by drawing a horizontal line from the intersection point between the working- and feed line to the equilibrium curve and then vertically back to the working line. After one stage the metal concentration in each phase can be obtained, where metal concentration in the loaded organic phase is found on the y-axis, while metal concentration in the aqueous phase (barren) is found on the x-axis. This can be given after each stage. Similarly, the second extraction stage is acquired by continuing horizontally to the equilibrium curve and vertically back again to the working line. This is repeated until the desired minimum metal concentration level in the raffinate is achieved. Worth mentioning is that the slope of the working line can be varied by varying the Aq/O ratio, which affects the amount of extraction stages required for the separation specifications. Note that the same procedure is true for calculating the theoretical stages for stripping, but by reversing the axes.3,5,7
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Figure 4: Schematic of a McCabe-Thiele diagram5
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3 METHODS AND MATERIALS 3.1 MATERIALS The aqueous feed used in all extraction experiments was a pregnant leach solution (PLS) containing cobalt and nickel, where Fe, Cu, Zn were removed beforehand by precipitation. Aluminum was partially removed from the leach solution. The leach solution was obtained after bioleaching and precipitation of the metals mentioned had been performed at Boliden’s pilot facility. An analysis of the different feed is presented in Table 1 below. The strip feed for the acid tests are given in Appendix (section 8.1.3).
Table 1: The feed composition in each experiment, determined by ICP-OES or XRF (*XRF-analyses of Mg, Mn, Al, Ca inaccurate)
Test Co Ni Mg Mn Al Ca (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) Effect of pH 1792 1002 5094 101 910 143 (pH 4) Effect of pH 1821 1023 6462 44.8 986 103 (pH 4.5) Effect of pH 1385 800 14933 38.0 794 96.8 (pH 4 low A:O) Batch 1803 990 4176 178 772 156 simulation (pH 4.5) Batch 1716 989 2197 193 369 130 simulation (pH 4.8) Scrub (pH- 35251 23.0 14.1 5.55 7.05 68.0 tests) Scrub batch 30065 91.0 340 103 1738 154 (pH 3.7)* Stripping batch 22161 13.68 4.40 3.62 0.96 11.81
Crystallization 23309 14.83 0 0 537 111
For the organic feed, CYANEX 272, provided from Solvay, was used as extractant. Tributyl phosphate (Merck) and 1-Decanol (Aldrich) were the modifiers used. Naphtha and Kerosene (low odor), provided from Starta and Alfa Aeser, respectively, were used as diluents for the organic feed.
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3.2 EXPERIMENTAL SET-UP 3.2.1 Process overview An illustration of the solvent extraction experimental design for this project is presented in the flowsheet below, Figure 5. The design consists of an extraction part, where cobalt is extracted by the help of an extractant, followed by scrubbing, and lastly stripping is carried out to strip cobalt from the organic phase back to the aqueous phase. Scrubbing is added to the experimental set-up to “push out” impurities, especially nickel, to obtain pure cobalt sulphate as final product. The aqueous phases were analyzed using ICP-OES or XRF and mass balance equation were used to calculate the concentrations in the respective organic phases. Process efficiency was calculated through percentage of Co and Ni extracted and the separation factor between these metal ions (α=DCo/DNi). In order to indicate if stripping was successful, respective distribution coefficients D1 were calculated, see 2.3.1 for more detail. Cobalt to nickel ratios were also measured to determined process effectivity.
Figure 5: A flowsheet of the overall experimental solvent extraction design A more thorough description of the different streams will be mentioned in conjunction to the results, section 4.1.6. Every choice was clarified after each test was made i.e. organic composition test, pH-tests, equilibrium tests, and acid-testing. Temperature and contact time were held constant throughout the experiments, at 22 °C (room temperature) and 5 min, respectively. A contact time of 5 min was chosen based on experimental grounds, because it was regarded as a sufficient time to reach equilibrium. The pH-value was adjusted continually using a dilute ammonia solution after each shake.
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3.2.2 Effect of saponification and organic phase composition These tests consisted of varying different modifiers and diluents, in order to observe the effect on phase separation. This was done through visual observations of phase separation and separation time. The effect of saponification was also investigated and compared with the classical solvent extraction method.
In preliminary tests, saponification was tested, which is defined by Anne Marie Helmenstine as “a process by which triglycerides are reacted with sodium or potassium hydroxide (lye) to produce glycerol and a fatty acid salt called "soap"15. In other words, a proton in the organic extractant is exchanged (saponified) against a sodium ion, by mixing with sodium hydroxide. The reason for the saponification test was to investigate if this process could be implemented before solvent extraction with the intention of avoiding a control of pH during the extraction stage. These four tests (Test 1-4) are summarized in Table 2.
Table 2: Conducted batch tests, with varying compositions, in order to examine the effect on phase separation
Tests Extractant Modifier Diluent Dilution Saponified (15%) (5%) factor 1 C272 TBP Kerosene 2 x 2 C272 Isodecanol Kerosene 2 x 3 C272 TBP Naphtha 2 x 4 C272 Isodecanol Naphtha 2 x 5 C272 TBP Naphtha 2 - 6 C272 Isodecanol Naphtha 2 - 7 C272 - Naphtha 2 - 8 C272 TBP Naphtha 4 - 9 C272 Isodecanol Naphtha 4 - 10 C272 - Naphtha 4 -
50 ml of organic phase was prepared based on the composition given in Table 2. The organic feed was then mixed with 50 ml of 32 %w/v NaOH solution (VWR Chemicals) in a 250 ml separation funnel at room temperature for 2 min. After phase separation, the aqueous phase was tapped off. A volume of 50 ml pregnant leach solution was added to the funnel and mixed for an additional 2 min at room temperature. After equilibrium was reached, pH of the aqueous phase was measured with a pH-meter (pH 3210 WTW) and adjusted to pH 4.5±0.1 by adding diluted NH3 (14%), which was provided from VWR Chemicals.
The experimental tests, Test 5-7, were conducted without saponification, using naphtha as diluent (Table 2). Only the modifier was varied in these tests to investigate its effect on separation of phases. The same method was repeated for these tests, but by removing the addition of NaOH, to avoid saponification. Three additional tests (Test 8-10) were carried out, where the aqueous feed’s dilution factor was doubled, to examine its effect on phase separation.
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3.2.3 Effect of pH on extraction equilibrium The equilibrium curves were constructed for Co extraction at pH 4.0 and 4.5. A McCabe-Thiele diagram was constructed for cobalt extraction at pH 4.5.
Table 3: The different aqueous to organic ratios tested at pH 4.0 and pH 4.5
Test A:O Aqueous feed (ml) Organic feed (ml) 11 10 50 5 12 5 50 10 13 2 50 25 14 1 50 50 15 0.5 50 100 16 0.2 50 250
Table 4: Tests at pH 4.0 with lower aqueous to organic ratios
Test A:O Aqueous feed(ml) Organic feed (ml) 17 1 25 25 18 0.5 25 50 19 0.2 10 50 20 0.1 10 100 21 0.05 10 200
The organic feed used for these tests was 15% CYANEX 272 + 5% TBP + naphtha, which was mixed and shook with the aqueous feed in a separation funnel for 5 min at room temperature. Tests 11-16 (Table 3) and Tests 17-21 (Table 4) were conducted at pH 4.0±0.1, with varying A:O ratios. The pH-value was adjusted by adding ammonia. The same procedure and aqueous to organic ratios as Tests 11-16 were conducted at pH 4.5±0.1, under the same conditions. Cobalt concentrations in the aqueous solutions were determined by XRF analysis, and cobalt concentrations in the loaded organic phase were calculated by mass balance, see Appendix (section 8.1.1.)
3.2.4 Batch Simulation of Continuous Multistage Countercurrent Solvent Extraction Batch tests were carried out in laboratory scale to simulate the continuous multistage countercurrent process. Figure 6 illustrates how the batch simulation tests were performed to simulate the 3-stage countercurrent solvent extraction process. The method consisted of continuously introducing the aqueous feed solution with the organic feed solution into a series of batch extractions, with an aqueous to organic feed ratio of 1:1. After the two feeds were transferred into a 500 ml separation funnel and the pH was set to 4.5±0.1, a contact time of 5 min was used by shaking the funnel. The aqueous phase (raffinate) and organic phase (loaded organic) were removed from the funnel by tapping them off separately. This procedure was done for 8 cycles (A-H). The circles in Figure 6 represent a batch
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extraction step in a separation funnel. The aqueous and organic phases are denoted by red and black lines, respectively, in both figures below. The aqueous raffinate and organic extractant out of the system resemble the actual streams out of a continuous countercurrent extraction process (Figure 7). Two batch simulation of countercurrent solvent extraction experiments were conducted, one at pH 4.5±0.1 and the other at pH 4.8±0.05. The pH-value was increased, in order to examine if a higher cobalt recovery and a less co-extraction of impurities was possible. Note that in the second batch simulation experiment an aqueous feed with dilution factor of 5 was used instead of 4.
Figure 6: Batch simulation of 3-stage countercurrent solvent extraction. The red and black lines represent the aqueous and organic phase, respectively. Note that SX A3 was not conducted but is presented for the sake of visualization.
Figure 7: Continuous countercurrent solvent extraction, where the aqueous raffinate and organic extract are denoted with an R and E, respectively. The symbol F denotes the fresh feed in and S denotes fresh extractant.
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3.2.5 Scrubbing tests The loaded organic phases from the batch simulation at pH 4.8 were blended and homogenized in a glass flask. A 2 L scrub feed was prepared by adding cobalt sulphate heptahydrate (CoSO4·7H2O), provided from VWR Chemicals, with a concentration of 30 g/L, into water, which were mixed by being placed on a magnetic stir plate. Three different scrubbing tests were carried out for 5 min at room temperature in a separation funnel containing 50 ml of respective phase. The pH-value was varied from 3.5, 3.7 and 4.0 between each test. Thereafter, the remaining loaded organic solution was scrubbed by the help of the scrub feed containing CoSO4·7H2O, where a pH-value of 3.7 and a contact time of 5 min were held constant. Even here, the scrubbing process was conducted by shaking using a separation funnel.
3.2.6 Stripping tests The stripping experiment was divided into two parts, an acid testing part, and a batch simulation counter-current testing. The method for the first part was to prepare a 2 L strip solution as base, by blending CoSO4·7H2O (concentration of 20 g/L) diluted with water in a beaker placed on a magnetic stir plate. The pH was brought to 2.5 at room temperature. Three different 30 ml acid tests were prepared by adding different H2SO4 concentrations to the base solution, which had concentrations of: 12, 24, and 48 g/l. A volume of 5 ml was saved for analysis from each sample. A volume of 25 ml of the blended and homogenized loaded organic from scrubbing was mixed with 25 ml of the respective acid solution for 5 minutes in a separation funnel. After equilibration, the aqueous phase was tapped off and taken for further analysis.
For the second part of the experiment, a batch simulation multistage countercurrent test was conducted, in order to simulate the continuous stripping process. A three- stage system was assumed to be required. The same method for the batch simulation test was used in stripping as earlier for the extraction process, see Figure 6. The only difference was that the experiment was run for 7 cycles (A-G) without controlling the pH-value. The fresh strip feed contained the prepared base solution mixed with a 24 g/L H2SO4 concentration. The organic phase in the experiment was the scrubbed loaded organic taken directly out from the previous scrubbing process. For a more thorough description of the method see section 3.2.4.
3.2.7 Crystallization of Cobalt To investigate the purity of a possible cobalt sulphate product from the stripped solution, batch simulation tests of crystallization were made. Crystallization was carried out by adding different amount of acetone to reduce the solubility of cobalt, and thus precipitating it as sulphate. The purity is defined by the given equation:
푚 퐶표 푃푢푟𝑖푡푦(%) = 퐶표 × 100% 푚푐푟푦푠푡푎푙푠(푡표푡) Where the mass is determined from ICP-analyses of the aqueous phase.
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The feed used in the crystallization test was prepared by mixing the remaining of the aqueous phase from batch simulation of the stripping stage. Five different tests were performed (Table 5). The solutions were stirred using a magnet stirrer for 80 min, after addition of the antisolvent, to facilitate the formation of crystals. Solid crystals formed were separated from the liquid decantation, before drying in an oven at 50 ºC until completely dried. The decanted solution samples were analyzed by ICP-OES.
Table 5: Experimental solute and solvent volumes used for crystallization tests, where the solvent’s volume is the dependent variable
Aq. solution (ml) Anti-solvent (ml) 20 5 20 10 20 15 20 20 20 25
After drying, the crystals formed for each case (10-25 ml) were weighed, see Table 19 (section 8.1.4) for more detail. Lastly, crystal samples were solubilized in 1% nitric acid solution for analyses. As a reference, a cobalt sulphate heptahydrate (CoSO4·7H2O) sample purchased form WVR was also prepared using similar method for analyses. Mass balance calculations were utilized for obtaining the crystallized mass, which was determined from ICP-OES analyses of the aqueous phase.
3.2.8 Analytical methods Throughout the experimental work, both XRF and ICP instrument were used as analytical means for the sake of comparison. However, the analytical result used depended on the given case. For instance, XRF was the analytical method of choice when a fast analysis was desired (mainly for Co and Ni) and for an experiment where focus was to examine a specific trend. This applied for the experiment were the effect of pH was studied. Whereas, when an analysis of the impurities (with smaller concentration) was required, ICP-OES was the optimal decision. These analysis results were used for all experiments, except for section 4.1.1.
XRF Before analysis of each sample, 5 ml of the feed or raffinate was filtered using a syringe filter and acidified with 100 µl of nitric acid, in order to decrease the pH and stabilize the dissolved species. The samples from stripping were already acidic and therefore they were acidified. Thereafter, the sample was inserted into the XRF instrument SPECTRO XEPOS 5 (60 kV, 2mA) for analysis.
ICP-OES
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Before running the samples in the ICP instrument Agilent Technologies 5110 ICP- OES, they were prepared by diluting procedure. The samples were prepared beforehand by filtering each aqueous raffinate and then diluting each aliquot (a 10 potency less than aimed for) with a diluent of 2% HNO3 (nitric acid) together with tap water, after which they are diluted an additional 10 times. The last dilution of the samples was done by using a 2% HNO3 + 10 mg/L yttrium + H2O as diluent. The four standards used had four different dilution factors, namely 5, 10, 20 and 30.
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4 RESULTS AND DISCUSSIONS 4.1.1 Effect of saponification and organic phase composition The four different organic feed mixtures that were tested with saponification all showed poor and slow phase separations, except for Test 3. In Test 1 no phase separation was achieved and a pH-value of 4.5 was not possible to reach, without addition of significant amount of sulphuric acid. In Test 2 a phase separation was observed, but not a clear one. The aqueous phase was hazy with significant amount of organic phase entrapment (Figure 8). Likewise, Test 4 resulted in a slow and insufficient separation of the phases.
Figure 8: A photograph representing test 2, where a blue color is seen in both phases However, results from Test 3 gave a distinct phase separation. These preliminary tests, generally, verified that saponification was not a suitable process for this project, because the intention of avoiding pH-control during extraction could not fulfill. From these tests, it was also observed that when naphtha was used as diluent, the separation was better, compared to using kerosene. This could be due to aliphatic nature of naphtha which, according to the producer, is a more suitable diluent for C272. As a result, it was used in all tests during this project.
In experiment tested without saponification all tests (Test 5-7) showed poor and slow phase separations. Aqueous phases in the three tests were cloudy, which could possibly due to gypsum formation. To avoid this problem, the aqueous feed was diluted four times, instead of twice. As a result of increasing the dilution factor the
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gypsum formation was avoided, because of a lower calcium concentration. As expected, the more diluted aqueous feed, gave better phase separation, therefore this was kept constant in the following experiments.
Tests 8-10 all resulted in acceptable clear phase separations, indicated by clear aqueous phase, together with a reasonable separation time. The color of the organic phase determined the suitable organic feed mixture, where theory states that cobalt has a dark blue color in the organic phase. From visual observation, the dark blue color in Test 8 illustrated that there was a high concentration of cobalt present in the organic phase (top phase in Figure 9). Test 9 had a light blue organic phase, which confirms the absence of cobalt. A “cobalt blue” color was observed in Test 10 as well, but not as dark blue as Test 8. Therefore, Test 8 was chosen over Test 10.
Figure 9: The organic feed tests, test 8-10, from left to right From these results, C272 + TBP + Naphtha (Test 8) proved to be the organic phase composition that resulted in a sufficient and immediate phase separation. Therefore, this composition was used for further experimental work.
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4.1.2 Effect of pH on extraction equilibrium The equilibrium curve for cobalt at pH 4.0±0.1 is presented in Figure 10. The curve’s appearance differs from that of a standard polynomial curve (Figure 4). A possible explanation for the deviation is the effect of competing metal ions in the leach solution. Therefore, tests at lower A:O ratios were conducted to examine the equilibrium phenomena further.
1200,0
1000,0
800,0
600,0
[Co (mg/L) [Co org.] 400,0
200,0
0,0 600 800 1000 1200 1400 1600 1800 [Co aq.] (mg/L) Figure 10: Plotted equilibrium curve for cobalt at pH 4.0. The equilibrium curve presented in Figure 11 gives a better representation of a standard curve and shows that equilibrium was reached. However, the two red points in the figure, with A/O of 1 and 0.5, demonstrate that Co has been pushed out by other metals. The competing metal ions in this case are Al and Mg because there is a high concentration of them in the aqueous feed, which is verified in Table 1. This result was suspected due to that Al2+ is favored by C272 at pH 4.0, see Figure 3. Cobalt recoveries into the organic phase at lower A/O ratios were greater compared to earlier samples with a higher A/O ratio. This is an expected result. The explanation is that with a lower A/O ratio there is more organic present, leading to that there is more extractant for the metal ions to extract to.
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160 140 120 100 80 60 40 [Co [Co (ppm) org.] 20 0 0 200 400 600 800 1000 1200 1400 -20 -40 [Co aq.] (ppm)
Figure 11: Equilibrium curve for cobalt at pH 4.0 with a lower aqueous to organic ratio. Notice that the red points represent Co ions being pushed out Both equilibrium curves presented above make it difficult or even impossible to construct a McCabe-Thiele diagram. Whereas, experimental data from pH 4.5 gave a polynomial curve, similar to a standard equilibrium curve. With this equilibrium curve it was possible to insert into a McCabe-Thiele diagram and calculate the required extraction stages, which was one of the purposes with these tests. Figure 12 presented below, shows the constructed McCabe-Thiele diagram with an operating line slope of 1 at pH 4.5. Note that point A/O=2 was an outlier to the curve and was therefore removed from the plot.
3000 y = -0,001081x2 + 3,196298x R² = 0,998558 2500
2000
1500
[Co [Co (ppm) org.] 1000
500
0 0 500 1000 1500 2000 [Co aq.] (ppm) Figure 12: McCabe Thiele diagram for cobalt at pH 4.5 with a slope of 1
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By reading off Figure 12, it was evident that 3 stages were required for solvent extraction operation to reach to 0.085 g/L cobalt in the final raffinate. Based on the grounds discussed above i.e. similarity to standard appearance and possibility for constructing a McCabe-Thiele diagram, a pH-value of 4.5 was chosen for the SX operation, i.e. batch simulation experiment.
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4.1.3 Batch Simulation of Continuous Multistage Countercurrent Solvent Extraction Figure 13, demonstrates the concentration of nickel and cobalt in the aqueous phase, after each cycle. Shown in the figure, is that the nickel concentration has reached equilibrium at around 1.20 g/L, which is represented by the blue points. Furthermore, nickel’s concentration in the aqueous phase was higher compared to cobalt’s, resulting in a successful separation between the two metals. The concentration of cobalt in the aqueous phase reached equilibrium after three cycles, see the black dots in the figure. This means that simulation tests can be run for four cycles.
1400
1200
1000
800
600
[Me](aq.) [Me](aq.) (mg/L) 400 Co 200 Ni 0 0 1 2 3 4 5 6 7 8 Cycle Figure 13: The metal ions’ concentration in the aqueous phase vs. batch simulation cycles, for cobalt and nickel From examining Figure 14 and Table 6, it is possible to notice that Co was recovered at an average of 69 % into the organic phase. Whereas, Ni remained in the aqueous phase and was not co-extracted into the organic phase. The blue points in Figure 14 , verify that aluminum has a 100 % recovery and is competing against cobalt for a spot in the organic phase. Both Al and Mn are the main impurities, which were co-extracted into the organic phase (Table 7).
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120
100
80
60
Me Me org. (%) in 40 Co recovery 20 Al recovery
0 0 1 2 3 4 5 6 7 8 Cycle
Figure 14: Metal ion recoveries into organic phase vs. cycles at pH 4.5
Table 6: Different metal ion recoveries into the organic phase after each cycle, at pH 4.5
Cycles Co Al Mg Ca Mn (%) (%) (%) (%) (%) B3 88 100 39 32 99 C3 76 100 36 15 99 D3 54 100 34 6 97 E3 64 100 35 10 98 F3 63 100 34 9 98 G3 73 100 37 18 99 H3 66 100 37 15 98 Average (%) 69 100 36 15 98
When comparing organic recoveries for each metal ion from the table above with literature (Figure 3) it is worth noticing similarities. For instance, recovery of Co into the organic phase at pH 4.5 is around 70 % in Figure 3, which complies with batch simulation tests at an average of 69%. The same is true for the other metal ion recoveries when compared to literature. Hence, the experimental data at pH 4.5 agrees to a great extent with theory. It is worth noting that Al, which was partially removed from the leach solution beforehand, shows a 100% recovery into the organic phase.
One source of error in the batch simulation tests was the pH interval of ±0.1, where a majority of the samples had a pH of 4.4. In other words, the results may differ significantly between having a pH-value of 4.4 compared to 4.5. The difference in pH between samples makes the results less reliable, due to inconsistency.
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Therefore, a smaller interval (0.05) was used in future tests to minimize this source of error.
Batch simulation results, at pH 4.5, showed a higher cobalt concentration in the raffinate, at 0.56 g/L, compared to 0.085 g/L, which was read from the McCabe- Thiele diagram (Figure 12). As a result of the difference in Co purity, it is worth discussing if a McCabe-Thiele diagram is necessary to use for this case. In other words, a McCabe-Thiele diagram may not be an optimal representation of the required stages for a more complex system, where many metal ions are present. In an ideal case it is assumed that only cobalt and nickel are present in the solution, whereas in this specific case there were several different metal ions present in the leach solution. This may lead to other metal ions competing against the desired metal ions, which affects separation factor, equilibrium curve etc.
A possible factor to the difference between the real and theoretical case, can be referred to the source of error discussed earlier, concerning the interval gap of ±0.1. Namely, that if the experimental points to construct the McCabe-Thiele diagram where mostly conducted at pH 4.5, whereas the batch simulation tests had a pH of 4.4, it may not give an accurate comparison between the two cases. This results in a less reliable comparison. However, this difference should not be so significantly large.
Due to that both dilution factor and pH-value was changed in the next set of batch simulation tests a full comparison between the two batch tests was not possible. With a higher dilution factor, it is evident that lower concentrations are expected. Therefore, concentration difference between the two different tests is ignored. However, a comparison between the recoveries is possible because a percentage calculation is made . Table 7: The metal ions’ recoveries into the organic phase after each cycle, at pH 4.8.
Cycles Co Ni Al Mg Ca Mn (%) (%) (%) (%) (%) (%) B3 100 6 100 35 62 100 C3 99 8 100 23 43 100 D3 99 1 100 14 30 100 E3 100 2 100 14 33 100 F3 100 1 100 16 35 100 G3 100 4 100 17 35 100 H3 100 1 100 9 31 100 Average (%) 100 3 100 18 38 100
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100 90 80 70 60 Co 50 Ni
40 Mg Me Me org. (%) in 30 Ca 20 10 0 0 1 2 3 4 5 6 7 8 Cycle
Figure 15: Metal ion recovery into organic phase vs. cycle, at pH 4.8
As predicted from literature, a higher cobalt recovery in the organic phase was seen at pH 4.8, with an average of 100 %. However, a small percentage of Ni (3 %) was co-extracted into the organic phase, which was not seen at pH 4.5. Operating SX at pH 4.8 resulted in a higher recovery of impurities, which is not desired when pure cobalt is wanted. One the other hand, less Mg was co-extracted at pH 4.8. Both Al and Mn had a 100 % recovery into the organic phase. All these statements can be verified in Table 7 and Figure 15 above. Experimental data at pH 4.8 compiles with literature, see Figure 3. Figure 15 demonstrated that equilibrium was reached after 2 cycles, leading to that a maximum of 3 cycles is required.
When deciding on a pH-value for the operation, many parameters must be weighed against each other e.g. Co recovery, co-extraction of impurities (Co purity), economical aspects etc. For instance, when operating at pH 4.8 a high cobalt recovery is obtained, but some unwanted nickel is also extracted into the organic phase, followed by other impurities. In order to avoid this problem, the solvent extraction process should be operated at pH 4.5 instead. Nevertheless, more stages are required in this case compared to operating at pH 4.8, which is economically unfavorable. One aspect that is worth mentioning, is that a scrubbing process is necessary for the system operation at pH 4.8, in order to remove co-extracted Ni, which also requires stages. Therefore, the scrubbing process was also examined in the project. All these aspects must be taken into consideration when choosing a pH- value for solvent extraction.
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4.1.4 Scrubbing The intention of scrubbing was to purify the loaded organic (after batch simulation countercurrent tests) from impurities, especially nickel, by addition of cobalt as sulfate. This was done in order to push out Ni for instance. Table 8 shows ICP- OES-results from the pH-tests, which indicates that at pH 4.0 the highest cobalt concentration in the loaded organic out was acquired, with a concentration of 7.06 g/L. However, a higher concentration of impurities was also detected here. As expected, the trend of an increasing pH-value is that the loaded organic concentration increases, which is verified in the table. This was suspected, due to earlier observation from extraction, were the metal ions examined were extracted to a higher extent at higher pH-values, and the same phenomena is true for scrubbing.
Table 8: Concentrations of loaded organic out after scrubbing at different pH-values pH-value Co Ni Al Mg Ca Mn (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (m/L)
3.5 3668 21.7 373 184 79.7 80.8 3.7 4340 22.8 373 187 80.1 92.8 4.0 7058 23.2 374 190 81.5 112
Results from scrubbing will affect stripping results because the scrubbed loaded organic continues to the stripping stage. In other words, the more Co is present after scrubbing the more there is room to strip. However, to obtain a high purity of cobalt, other impurities must be minimized in the scrubbing process. Both these parameters must be taken into consideration when deciding on a pH-value.
A pH-value of 3.7 was chosen for further scrubbing, because of marginal difference in cobalt concentration between the other pH-values. Furthermore, less impurities were seen at pH 3.7 compared to at pH 4.0, as mentioned earlier. If the intention is to obtain as high Co concentration as possible pH 4.0 would have been the optimal decision. Additionally, concentrations of impurities at pH 4.0 were marginally higher than for pH 3.5 and 3.7. However, in this report the aim is to develop a deeper understanding of separation parameters for Co-Ni separation, opposed to obtaining an optimal value.
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4.1.5 Stripping The strip results from acid testing, demonstrated that with a sulphuric acid concentration of 24 g/L the highest cobalt concentration was obtained, which is verified in Table 9, with a Co concentration of 24.3 g/L. Regarding the other metal ions, there was a marginal difference in acid concentrations between the tests. Therefore, 24 g/L H2SO4 was the acid concentration chosen to continue with for the second part of the stripping experiment.
Table 9: Concentration of aqueous product out for each metal ion with varying acid concentrations
H2SO4 Co Ni Al Mg Ca Mn conc.(g/l) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)
12 23740 13.93 349 4.72 12.54 24.53 24 24333 13.95 394 4.79 12.92 24.69 48 23236 13.53 394 4.72 12.84 24.86
A 3-stage stripping process was assumed, when conducting a McCabe-Thiele diagram was neglected (due to inaccuracy representation of the real case). Conducting tests for the batch simulation countercurrent process for stripping resulted in that equilibrium was reached after one cycle. This leads to the knowledge that a minimum of 2 cycles was required to reach equilibrium, and that the average concentration of cobalt stripped was 23.2 g/l. Table 10 indicates that a high amount of Co was stripped compared to the other metal ions. This means that the stripped aqueous product has a high cobalt purity, with small amounts of impurities. However, Al concentration was 10 times higher than the other metal ions.
Table 10: Average aqueous concentrations out from batch simulation process for each respective metal ion
Co (ppm) Ni (mg/L) Al (mg/L) Mg (mg/L) Ca (mg/L) Mn (mg/L) 23180 13.45 313 4.59 12.51 20.09
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4.1.6 Solvent extraction overview
Figure 16: Schematic of experimental design with Co and Ni in-and-out streams presented with unit g/L, and with A:O=1 Figure 16 shows a flowsheet of the entire solvent extraction process, to demonstrate in-and-out streams. Only Co and Ni concentrations are presented, because these were the metals of interest to separate.
By reading off the left-hand-side of the figure, it is noticed that most of the cobalt has been extracted into the organic phase, leaving nickel in the aqueous phase. The calculated distribution coefficients for SX for cobalt and nickel are 294 and 0.02, respectively, resulting in a separation factor of 14250, proving CYANEX 272 to be an appropriate extractant for Co-Ni separation. When comparing the obtained separation factor with earlier studies (see section 2.2.3.1) it confirms a significantly higher value i.e. 14250 vs. 6700. Whereas, the continuous mini plant trials conducted by Soldenhoff et.al achieved α>1000. It is worth discussing if these results are comparable to the continuous systems. From one perspective the batch tests throughout the experiment were made to simulate a continuous countercurrent process and should therefore be comparable to other existing continuous systems. However, when scaling up a chemical process many aspects are changed, and this must also be taken into consideration. The extraction process offered a 99% recovery of cobalt and 2.0% recovery of nickel. This result displays a similar recovery to the one mentioned in section 2.2.3.1 , obtained from Soldenhoff et.al’s mini plant trials, where a 97.5% Co recovery was achieved. The Co:Ni ratio after extraction was calculated to 85.5.
Co-extracted nickel is successfully removed through scrubbing, where the cobalt ions manages to “push out” impurities (namely Ni), as intended. Nickel from the loaded organic in is scrubbed to half its original value, which is seen in the figure, by observing that 0.01 g/L remains in the scrubbed organic out. This implies, that the scrubbed organic stream that continues to stripping is nearly free from nickel and contains practically only cobalt, leading to more room for stripping of pure Co. Another indication of a successful scrubbing process, is a higher cobalt to nickel ratio (Co:Ni=321) obtained after scrubbing compared to earlier observed ratio in extraction.
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Out of the stripping process, Co was stripped to the final product with a value of 1 g/L, while the Ni stream was unchanged. The distribution coefficients are 10.5 and 1.00 for Co and Ni, respectively. A low distribution coefficient is aimed for in this stripping process because it is not desired to strip all cobalt due to the risk of stripping other elements (impurities). Consequently, obtaining a low D value minimizes this risk. A Co:Ni ratio of 2320 indicates that stripping was successful.
4.1.6.1 Distribution of elements In order to get an overview of other metal ions (contaminants) in the streams, distribution results were analyzed. Presented in Table 11, are metal ion distributions between the four streams in extraction. It is of interest to give focus on the impurities (other elements), while earlier observation can be justified for. Cobalt, Aluminum and Manganese were evenly distributed between the streams. They were also extracted to the organic phase to the highest extent of the elements. This implies, that employment of scrubbing was necessary. While, Mg and Ca were not co-extracted to the same extent and remained mainly in the aqueous phase. The distribution of Ni confirmed earlier observations, i.e. that Ni practically remained in the aqueous phase enabling Co-Ni separation.
Table 11: Distribution of elements in the different extraction streams
Extraction Co Ni Mg Al Mn Ca Stream (%) (%) (%) (%) (%) (%) Aq. feed 33.4 49.5 61.0 33.3 33.3 42.8 Org. feed 0.00 0.00 0.00 0.00 0.00 0.00 Aq. out 33.4 49.5 30.5 33.3 33.3 42.8 Org. out 33.3 0.9 8.6 33.3 33.3 14.4 Total 100 100 100 100 100 100
Distribution of different metal ion scrub streams are summarized in Table 12. As mentioned earlier, scrubbing of Ni was successful, and this statement is strengthen through noticing that the distribution of Ni was lower in the outgoing organic stream compared to the ingoing. The same applies for the other impurities (Mn, Ca), besides from Al and Mg. Distribution of Mg was even. Aluminum distribution results show that approximately 40% is distributed to the organic stream out, which implies that the metal ion was difficult to scrub.
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Table 12: Distribution of elements in the different scrub streams
Scrub Co Ni Mg Al Mn Ca Stream (%) (%) (%) (%) (%) (%) Aq. feed 47.3 41.4 26.2 41.2 17.4 38.9 Org. feed 2.69 8.59 23.8 8.75 32.6 11.1 Aq. out 44.9 45.5 25.4 10.4 19.6 43.6 Org. out 5.07 4.49 24.6 39.6 30.4 6.41 Total 100 100 100 100 100 100
Through reading off Table 13, a remark was the high distribution of Co in the aqueous product stream out, opposed to other elements. This proves that cobalt has been stripped accordingly. Whereas, the other metal ions are mostly distributed between the organic streams. Note that nickel is distributed evenly, while a considerably high distribution of aluminum was seen in the aqueous stream out. Table 13: Distribution of elements in different strip streams
Strip Co Ni Mg Al Mn Ca Stream (%) (%) (%) (%) (%) (%) Aq. feed 43.6 29.0 0.68 0.03 0.99 15.9 Org. feed 6.35 21.0 49.3 50.0 49.0 34.1 Aq. out 45.7 28.5 0.71 9.37 5.48 16.8 Org. out 4.34 21.5 49.3 40.6 44.5 33.2 Total 100 100 100 100 100 100
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4.1.7 Overall distribution and process summary The metal ions’ distribution, seen from an overall process perspective, is demonstrated in Table 14. For Ni, Mg, Mn, and Ca a high percentage of the total distribution was seen in the raffinate stream leaving extraction, resulting in less room for co-extraction. Scrubbing proved to be effective, because a high distribution of elements was detected in the scrub raffinate. Confirmed was that a small percentage of impurities were distributed to the product compared to cobalt. However, distribution results show that the product stream was not free from contaminants. Al proved to be difficult to remove, because it was recognized to some extent in every out stream.
Table 14: An overall distribution of elements in streams throughout solvent extraction
Stream Co Ni Mg Al Mn Ca (%) (%) (%) (%) (%) (%) Aq. feed 1.57 44.8 51.2 8.05 24.4 20.5 Org. feed 0.00 0.00 0.00 0.00 0.00 0.00 Aq. 1.57 44.8 25.6 8.05 24.4 20.5 raffinate Scrub feed 27.4 4.13 7.92 37.9 13.0 24.2 Scrub 26.1 4.53 7.69 9.55 14.7 27.1 raffinate Strip feed 20.2 0.62 0.10 0.02 0.46 1.86 Stripped 2.01 0.46 7.43 29.60 20.62 3.87 org. Stripped 21.2 0.61 0.11 6.83 2.54 1.97 aq. product
The increase of Co:Ni ratio between each stage in solvent extraction (an increase of 10 potency each time), verifies that solvent extraction was successful in separating cobalt from nickel.
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4.1.8 Crystallization of Cobalt The purity of produced cobalt sulphate crystals is summarized in Table 15, where no Ni, Mn and Mg was detected. The trend in Table 15 shows that with an increase of solvent the Co purity increases, reaching a Co purity of 34.1% with a 25 ml solvent addition. Aluminum and Calcium were detected to some extent.
From comparison with the reference, the percentage of metals in cobalt sulphate show similar results, except for aluminum. The result gave considerably higher Al percentage than the reference, where no Al was present. This indicates, that in order to obtain a high purity cobalt sulphate product it is important to remove Al beforehand. Worth discussing, is the acceptable concentration limit of impurities. For instance, amount of Al in the product can be regarded as too high for utilizing in laboratory work, but acceptable for production of vehicle batteries. In other words, impurity limitations depend on the buyer’s specifications.
Worth noting is that a full comparison cannot be made between the produced cobalt sulphate and the reference. This is because in our case cobalt sulphate monohydrate was produced, while the reference utilized was heptahydrat. In other words, there are less water molecules in the product compared to in the reference, resulting in higher purities.
Table 15: Purity of the cobalt sulphate produced after stripping
Solvent added Co (%) Al (%) Ca (%) (ml) 10 32.5 0.10 0.20 15 33.2 0.10 0.30 20 33.5 0.10 0.10 25 34.1 0.10 0.10 Ref 22.1 0.00 0.10
A similar pattern to the one above is observed in Table 16, where an increase in solvent leads to an increase in mass crystallized. With a 25 ml acetone volume added together with 20 ml stripped product (solute) a value of 0.42 g Co is precipitated through crystallization. No impurities were present; therefore, they are not presented in the table. Worth discussing, is the quantity of cobalt sulphate crystallized as product of the entire SX process. It may not be considered that high, but by taking the volume quantities used into account it is more reasonable to expect these values. Additionally, it could be favorable with a lower Co amount crystallized if it results in a higher purity product, which is the case here when no impurities are produced.
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Table 16: Amount of cobalt sulphate salt produced that was crystallized with different volumes of solvent added
Solvent added Mass of Co (ml) crystallized (g) 10 0.17 15 0.30 20 0.30 25 0.42
4.1.9 Source of error Many potential sources of errors are associated with human error. Some examples of these are material loss (spill), washing, bad dilution or bad tapping off technique. However, the errors due to the human factor can be considered constant because the experiments were conducted by the same laborant throughout this project. Contamination and instrumental error are other potential source of errors.
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5 CONCLUSION The questions which were intended to be answered after all experimental tests are presented below: • Using naphtha opposed to kerosene as diluent resulted in distinct and immediate separation of phases. Both modifiers (and without) gave distinct phase separation, but a higher amount of Co was observed in the TBP composition. • Equilibrium curves for cobalt extraction at pH 4, indicated that equilibrium was either not reached or affected by the presence of competing metal ions (Mg and Al). A standard equilibrium curve was seen from Co extraction equilibrium curve at pH 4.5. • A minimum of 3 stages was required for extraction at pH 4.5, with an aqueous to organic ratio of 1. • Distribution of Al and Mn showed that they were co-extracted into the organic phase to a high extent compared to other elements. Out of the scrub streams most of impurities (expect Al) were distributed in the aqueous raffinate, resulting in a successful scrubbing process. Compared to Co, a small distribution percentage of other metal ions were detected in the product stream, with the exception of Al. • The Cobalt sulphate product’s purity is regarded as high, in respect to a reference of pure CoSO4·7H2O. However, substantial amount of Al was detected. The acceptable amount of impurities in the solution, i.e. product purity, should be determined through the buyer’s specification.
Worth noting is that no conclusion can be drawn regarding the recommended pH- value for solvent extraction. The reason is that pH-value is related to process specifications. Consequently, many different aspects must be considered and weighed against each other before deciding on a preferable pH-value. For instance, cobalt recovery, co-extraction of impurities, and economical factors are regarded. The same applies for separation parameters e.g. sulphuric acid concentration in stripping and pH-value in scrubbing.
One general conclusion was that a McCabe-Thiele diagram did not give an accurate representation of the batch simulation case, where a more complex system was in place. Therefore, it should not be a method as means for predicting required stages for SX process in this project.
Solvent extraction proved to be an effective separation process for the separation of cobalt and nickel. Furthermore, scrubbing showed to be an adequate method for removal of impurities and is a choice to consider when a high amount of impurities is co-extracted in the extraction circuit. Due to that Al proved to be difficult to
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remove, and is selective over Co, it must be totally precipitated before solvent extraction.
In conclusion, the aqueous feed’s (PLS) composition determines the operation parameters and phase separation. In other words, every case is unique and must be analyzed beforehand. The process specifications should also be accounted for in that respect. Worth noting, is that these given recommendations are based on these specific grounds and may vary on slightest change.
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5.1 FUTURE RESEARCH In this report a focus was given on examining if a high purity cobalt sulphate product could be obtained. For future research it would be interesting to investigate nickel production methods more, for instance electrowinning. Another possible future study could be to examine this case but applied in bigger scale and using actual continuous reactors. The scaling-up project could be conducted at Boliden’s pilot facility by constructing a pilot scale multistage counter-current solvent extraction. Work in this area has already started, by development engineer Mohammad Khoshkhoo. An on-going project is to produce Ni/Co sulfide from the same leach solution used in this project, and then conduct bioleaching again. Therefore, it would be of interest to examine if the newly produced leach solution could be an alternative (to the one used in this project) for SX. Investigating the effect that kinetics and thermodynamics has on the Co-Ni separation was outside of the project scope. Even though the chemical process is very pH-dependent it would be of interest to examine these parameters in the future. Finding a better theoretical method for predicting design parameters for solvent extraction could also be studied in the future.
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6 ACKNOWLEDGMENTS A great gratitude is due to the company Boliden for providing me with this exciting master’s thesis project and for the opportunity to work alongside some great colleges within the team. I want to give an extra thanks to my supervisor Mohammad Khoshkhoo for all the help and support and for the interesting discussions. Lastly a thanks is given to Kerstin Forsberg, my examiner at KTH, for recommending this company to me and for her encouragement throughout the project. Lastly, thank you to all the ones concerned.
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7 BIBLIOGRAPHY
1. Boliden – Metals for modern life. Accessed January 15, 2021. https://www.boliden.com/
2. Lett DS. Cobalt-Nickel Separation in Hydrometallurgy: a Review. :11.
3. Sandström Å. HYDROMETALLURGY. Published online January 2016.
4. SX Kinetics, Inc. - Specialists in Solvent Extraction and Electrowinning Pilot Plants. Accessed February 1, 2021. http://www.sxkinetics.com/sxprocess.htm
5. Habashi F. A textbook of hydrometallurgy. Métallurgie Extractive; 1999.
6. Crundwell FK, ed. Extractive Metallurgy of Nickel, Cobalt and Platinum- Group Materials. Elsevier; 2011.
7. Coulson JM, Richardson JF. Chemical Engineering. Vol. 2: Particle Technology and Separation Processes. 4. ed., reprinted (eith revisions). Pergamon Press; 1993.
8. Venkatachalam S. Hydrometallurgy. Narosa; 1998.
9. CYANEX® 272. Solvay. Accessed September 29, 2020. https://www.solvay.com/en/product/cyanex-272
10. Kordosky G, Virnig M, Mackenzie M. SOLVENT EXTRACTION - REAGENTS AND SELECTIVITY CONTROL. Henkel Corporation MID; :20.
11. Souza M, Mansur M. COMPETING SOLVENT EXTRACTION OF CALCIUM AND/OR NICKEL WITH CYANEX 272 AND/OR D2EHPA. Braz J Chem Eng. 2019;36:541-547. doi:10.1590/0104- 6632.20190361s20170527
12. Sole KC, Cole P. Ion Exchange and Solvent Extraction. 15th ed.; 2001.
13. Soldenhoff K, Hayward N, Wilkins D. The Minerals, Metals & Materials Society. Mishra. EPD Congress; 1998.
14. Kime MB, Kanowa EK. Valorization of low-grade copper-cobalt ore from the Mukondo mine by heap leaching and solvent extraction. CIM J. 2017;8(4). doi:10.15834/cimj.2017.25
15. Ph. D. BS, B. A. P and M, Facebook F, et al. Know the Definition of Saponification. ThoughtCo. Accessed November 12, 2020. https://www.thoughtco.com/definition-of-saponification-605959
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8 APPENDIX 8.1.1 Effect of pH on extraction equilibrium When the equilibrium curves were constructed, loaded organic concentrations were calculated by the help of the given mass balance:
푉푎푞. 푉표푢푡 퐶푙표푎푑푒푑 표푟푔. = ∗ 퐶푓푒푒푑 − ∗ 퐶푎푞.푟푎푓푓푖푛푎푡푒 푉표푟푔. 푉표푟푔. , where Vout=Vaq+Vadded. See Table 17, which demonstrated that there is more Al and Mg present in the organic phase compared to Co. In that respect, Al and Mg are seen as competing metal ions to Co. Table 17: Metal recoveries into the organic phase at pH 4.0 and low A:O ratios
A:O Co (%) Al (%) Mg (%) 1 3 82 73 0.5 20 88 74 0.2 59 84 83 0.1 73 81 69 0.05 85 83 76
8.1.2 Batch simulation of continuous multistage countercurrent solvent extraction The calculated recoveries into the organic phase are calculated through the equation presented below. 퐶푓푒푒푑 − 퐶푎푞.푟푎푓푓푖푛푎푡푒 %푅푒푐표푣푒푟푦 = ∗ 100 퐶푓푒푒푑 This equation is mostly used in the batch simulation experiment but may also be used for other experiments throughout the report.
8.1.3 Stripping An analysis of the strip feed for the acid tests are presented in Table 18.
Table 18: The feed analysis of stripping tests (acid testing)
Acid Co Ni Mg Mn Al Ca conc.(g/L) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) 12 22189 14.1 4.58 3.64 0.02 12.38 24 22160 14.4 4.75 3.78 0.06 12.67 48 22890 14.5 4.66 3.72 0.00 14.31
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8.1.4 Crystallization of cobalt The experimental data on the mass of crystals obtained after drying can be read from Table 19.
Table 19: The weighed crystals after drying for different solvent volumes added
Solvent volume (ml) Mass of crystals (g) 10 0.60 15 0.96 20 0.94 25 1.11 Ref 1.01
The formed crystals are illustrated in Figure 17 below.
Figure 17: A photography of all the formed crystals from crystallization
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