Article Recovery of Cobalt from the Residues of an Industrial Zinc Refinery
Laurence Boisvert 1, Keven Turgeon 1, Jean-François Boulanger 2 , Claude Bazin 1,* and Georges Houlachi 3
1 Department of Mining, Metallurgical and Materials Engineering, Université Laval, Québec, QC G1V 0A6, Canada; [email protected] (L.B.); [email protected] (K.T.) 2 Institut de Recherche sur les Mines et l’Environnement, Université du Québec en Abitibi Témiscamingue (UQAT), Rouyn-Noranda, QC J9X 5E4, Canada; [email protected] 3 Institut de Recherche, Hydro-Québec, Varennes, QC J3X 1S1, Canada; [email protected] * Correspondence: [email protected]; Tel.: +1-418-656-5914
Received: 4 November 2020; Accepted: 19 November 2020; Published: 22 November 2020
Abstract: The electrolytic production of metallic zinc from processing zinc sulfide concentrates generates a residue containing cadmium, copper, and cobalt that need to be removed from the electrolytic zinc solution because they are harmful to the zinc electro-winning process. This residue is commonly sent to other parties that partly recover the contained elements. These elements can generate revenues if recovered at the zinc plant site. A series of laboratory tests were conducted to evaluate a method to process a zinc plant residue with the objective of recovering cobalt into a salable product. The proposed process comprises washing, selective leaching, purifying and precipitation of cobalt following its oxidation. The process allows the production of a cobalt rich hydroxide precipitate assaying 45 4% Co, 0.8 0.2% Zn, 4.4 0.7% Cu, and 0.120 0.004% Cd at a 61 14% Co recovery. ± ± ± ± ± Replicating the whole process with different feed samples allowed the identification of the critical steps in the production of the cobalt product; one of these critical steps being the control of the oxidation conditions for the selective precipitation step.
Keywords: zinc residue; cobalt hydroxide; cementation; leaching; oxidative precipitation
1. Introduction The conventional roast-leach zinc extraction process yields a solid residue consisting of a mixture of zinc, copper, cadmium, and cobalt. Some zinc smelting plants process that residue [1–3] to recover the contained valuable metals. The high value of cobalt makes it an excellent candidate for a first step in the development of a process to recover the metals contained in that residue [1]. Indeed, cobalt is currently considered as a critical material [4] as it is used in the making of Li-ion batteries [1,5], increasingly strategic for the shift toward green energy or more precisely toward a 100% electric vehicle market [4]. The «critical» status of cobalt is related to uncertainties about the supply of the metal. In fact, 60% of the world’s cobalt is mined in the Congo and 80% of its production is processed in China [4]. In the case of a cobalt supply disruption due to a natural disaster, a change of government or a boycott [6], a zinc residue that contains more than 2% Co [1] becomes an interesting alternative to primary cobalt. The extraction of cobalt from zinc plant residues is not discussed in many papers except in a recent review [1]. Few papers [2,3,7,8] were found to describe processes to recover cobalt from zinc residues. Wang and Zhou described a process [2] to treat a zinc residue containing active carbon and organic compounds used to capture the cobalt and the manganese from the zinc solution prior to the Zn electro-winning step. The process developed for that residue consists of a washing stage followed by
Metals 2020, 10, 1553; doi:10.3390/met10111553 www.mdpi.com/journal/metals Metals 2020, 10, 1553 2 of 15 two roasting steps at different temperatures, leaching, and precipitation of iron and manganese followed by anion exchange and solvent extraction of cobalt using extractant P507. Fattahi et al. described a process [3] to extract cobalt from the zinc residues of Iranian Zn smelters that add permanganate to the zinc solution in order to oxidize Co(II) to Co(III) which is precipitated by increasing the pH. The precipitated zinc residue contains about 2% Co and more than 10% Mn. The authors [3] propose a reductive leaching followed by a precipitation of cobalt sulfide (CoS) using Na2S to selectively recover the cobalt from the manganese. Although the conventional approach for the purification of zinc solution is the cementation of Cu, Cd, and Co onto zinc dust [1,9], it was not possible to find in the Western literature a process dedicated to the treatment of this type of residue, although a qualitative description of a possible process can be found in [1]. Li et al. [10] studied the rate of leaching of a zinc residue but did not indicate if the residue is obtained by cementation on zinc dust and the authors did not attempt to process the leach solution to obtain a salable cobalt product. This paper describes a processing scheme to produce a cobalt rich compound from a zinc plant residue produced by the cementation process. The objective of the test work presented in this paper is not to optimize an existing flowsheet but to propose a process to obtain a salable cobalt compound from a zinc residue produced by cementation on zinc dust. As requested by the industrial partner, the proposed process should use only open tank reactors for leaching and precipitation without resorting to solvent extraction nor ion exchange to obtain the cobalt product. This last constraint complicates the process to be developed as selective extractants are available for cobalt [1,7]. However, if the economics of the process are not favorable, the use of SX will be investigated but it is unlikely that the addition of a SX plant could move the process economics toward more profitable conditions.
2. Materials and Methods
2.1. Instrumentation and Reagents The solid and liquid samples are assayed using a MP-AES 4200 (microwave-plasma atomic emission spectrometer) from Agilent Technology (Santa Clara, CA, USA). For solid samples, 0.5 g sub samples are taken and digested in aqua regia for analysis using the MP-AES. The pH and Eh of the solution are, respectively, measured using a Fisher Scientific accumet XL600 pH-meter (Waltham, MA, USA), an Orion pH probe, and an Orion Oxidation Reduction Potential (ORP) probe from Thermo scientific (Waltham, MA, USA). Table1 presents the reagents used for the experimentation.
Table 1. Reagents used for the experimentation.
Reagent Composition Brand and Purity
Sulfuric acid H2SO4 Fisher, 98% purity Sodium hydroxide NaOH Fisher, 98.8% purity Ammonium persulfate (NH4)2S2O8 Alfa Aesar, 98% purity
2.2. Provenance of the Zinc Residue The zinc residue used for the test work is provided by the CEZinc refinery [11] in Valleyfield, QC, Canada. Figure1 shows the zinc extraction process and identifies the origin of the Co-bearing zinc residue. The plant processes zinc sulfide concentrates assaying more than 50% Zn, 0–5% Pb, less than 2% Cu, 0.5% Cd and from 50–200 g/t Co. The main impurities are iron (>8%) and sulfur (>30%). The extraction process of zinc begins by a roasting of the zinc sulfide concentrate to oxidize sulfur into SO2 that is subsequently converted into sulfuric acid (H2SO4). The roasting also transforms the zinc sulfide into zinc oxide which is soluble in weakly acidic solutions. The roasted product is leached in sequence with weak and strong sulfuric acid to solubilize the zinc oxide. The metallic impurities (Fe, Cu, Cd, Co) follow the zinc into the solution. The solution then undergoes a neutralization, during which the solubilized iron is precipitated as jarosite [8]. The purification of the iron-free solution from Metals 2020, 10, 1553 3 of 15
Metals 2020, 10, x FOR PEER REVIEW 3 of 15 the remaining metallic impurities is done using cementation on zinc dust [9] where copper, cadmium, solution from the remaining metallic impurities is done using cementation on zinc dust [9] where and cobaltcopper, displace cadmium, the and zinc cobalt of the displace zinc powder the zinc throughof the zinc the powder reaction: through the reaction: 2+𝑀 +Zn →Zn2 + +𝑀 M + Zns Zn + M s (1) (1) aq → aq where M stands for copper, cadmium, or cobalt. The reduced copper, cadmium, and cobalt are plated where M stands for copper, cadmium, or cobalt. The reduced copper, cadmium, and cobalt are plated onto the surface of the zinc powder. The cemented powder is separated from the solution by a leaf ontopress the surface filter. The of recovered the zinc powder. solid is the The «Zinc cemented Plant Residue» powder (ZPR) is separated considered from in the the following solution study. by a leaf pressThis filter. residue The recoveredis currently solid transferred is the «Zincto another Plant pl Residue»ant for further (ZPR) processing. considered Zinc in is the finally following electro- study. This residuewon from is the currently purified transferred solution (See to anotherFigure 1). plant The sp forent further electrolyte processing. is recycled Zinc back is finallyto the leaching electro-won fromstep. the purified solution (See Figure1). The spent electrolyte is recycled back to the leaching step.
Spent electrolyte Zinc sulfide concentate Zinc dust Air Zinc Solution (Fe, Zinc Cd,Co, Cu) oxide+impurities Jarosite Soluton Zinc electro Roasting Leaching precipitation purification winning
SO2 Zinc Solution Purified (Cd,Co, Cu) L solution L S Cathode Acid Plant S Sulfuric acid Metallic zinc
Jarosite Zinc residue Studied residue containing Zn- L Solids/liquid separation Co-Cu-Cd S FigureFigure 1. 1.Simplified Simplified flowsheet flowsheet of the zinc zinc extraction extraction process. process.
2.3. Characterization2.3. Characterization of theof th Zince Zinc Plant Plant Residue Residue (ZPR) (ZPR) A 20A kg 20 sample kg sample of ZPRof ZPR was was collected collected byby thethe CEZinc personnel personnel at atthe the discharge discharge of the of press the press filter. filter. The sampleThe sample was was shipped shipped wet wet to the to the laboratory laboratory for for the the test test work. work. The The received received sample sample waswas splitsplit wetwet into ten partsinto afterten parts spreading after spreading the material the material onto a plastic onto a sheet. plastic One sheet. 2 kg One sample 2 kg sample was put was in anput oven in an for oven drying overnightfor drying and theovernight remaining and the material remaining was material kept wet was for kept the subsequentwet for the subsequent test work. test A portionwork. A ofportion the dried sampleof the is showndried sample in Figure is shown2a. The in materialFigure 2a.is The strongly material agglomerated is strongly agglomerated with practically with practically unbreakable unbreakable lumps. Sulfuric acid and sulfates are likely responsible for the observed particles lumps. Sulfuric acid and sulfates are likely responsible for the observed particles binding. It rapidly binding. It rapidly appeared that this material could not be characterized (chemical composition, size appeared that this material could not be characterized (chemical composition, size distribution, etc.) as distribution, etc.) as is and it was decided to wash the residue in water to remove any excess of is andsulfuric it was acid decided before to drying. wash theWashing residue was in carried water toout remove by mixing any 500 excess g of ofwet sulfuric residue acid in 2700 before mL drying.of Washingwater was (15% carried solids outin mass) by mixing in a beaker 500 g for of 90 wet min residue at room in temperature 2700 mL of water(25 °C). (15% The solidswashed in residue mass) in a beakerwas for separated 90 min atfrom room the temperature solution by vacuum (25 ◦C). filtration The washed and residuedried overnight was separated in an oven. from The the dried solution by vacuumwashedMetals filtration residue2020, 10, x isFOR and shown PEER dried REVIEW in Figure overnight 2b and in is an found oven. to The be more dried amenable washed to residue characterization is shown 4 inthan of Figure16 the 2b raw residue. and is found to be more amenable to characterization than the raw residue.
(a) (b)
Figure 2.FigureZPR 2. as ZPR is andas is afterand after washing washing with with water. water. ( a)) Raw Raw residue residue after after drying; drying; (b) Residue (b) Residue after after washingwashing and drying. and drying.
2.3.1. Specific Gravity The measured specific gravity (Gas pycnometer, HumiPyc model 2 from Instruquest, Coconut Creek, FL, USA) of the washed ZPR is 3.95 g/cm3. Since the densities of zinc and of the cemented metals are all above 5 g/cm3 this result indicates that the residue is not made exclusively of pure metals.
2.3.2. Size Distribution Figure 3 shows the particle size distribution obtained by sieving the washed ZPR on a Tyler screen series from 1.2 mm down to 0.038 mm. The ZPR is coarse with a D80 of about 900 µm. The material coarseness will pose a problem for the sampling for assaying of the residue. Indeed, since assaying of the sample implies collecting a 0.5–1.0 g sample for the digestion prior to analysis using the emission spectrometer (Section 2.1), one can expect a significant variability in the assays of the residue due to the fundamental error of sampling [12]. For ore type material, this error is reduced by pulverizing the sample prior to sampling. However, the metallic and ductile nature of the ZPR makes impossible the pulverization of the sample, and thus one should expect a variability in the assays that data reconciliation [13–16] as applied here should be able to attenuate.
100
80
60
40 % passing
20
0 1 10 100 1 000 10 000 Particle size (um)
Metals 2020, 10, x FOR PEER REVIEW 4 of 15
(a) (b)
Figure 2. ZPR as is and after washing with water. (a) Raw residue after drying; (b) Residue after washing and drying.
2.3.1. Specific Gravity Metals 2020, 10, 1553 4 of 15 The measured specific gravity (Gas pycnometer, HumiPyc model 2 from Instruquest, Coconut 3 Creek,2.3.1. FL, Specific USA) of Gravity the washed ZPR is 3.95 g/cm . Since the densities of zinc and of the cemented metals are all above 5 g/cm3 this result indicates that the residue is not made exclusively of pure metals. The measured specific gravity (Gas pycnometer, HumiPyc model 2 from Instruquest, Coconut Creek, FL, USA) of the washed ZPR is 3.95 g/cm3. Since the densities of zinc and of the cemented metals are all above 5 g/cm3 this result indicates that the residue is not made exclusively of pure metals. 2.3.2. Size Distribution 2.3.2.Figure Size 3 shows Distribution the particle size distribution obtained by sieving the washed ZPR on a Tyler screen seriesFigure from3 shows 1.2 themm particle down size to distribution0.038 mm. obtainedThe ZPR by is sieving coarse the with washed a D80 ZPR of onabout a Tyler 900 screen µm. The materialseries coarseness from 1.2 mm will down pose to a 0.038 problem mm. Thefor the ZPR sampling is coarse withfor assaying a D80 of about of the 900 residue.µm. The Indeed, material since assayingcoarseness of the willsample pose implies a problem collecting for the sampling a 0.5–1.0 for g assaying sample offor the the residue. digestion Indeed, prior since to assayinganalysis ofusing the emissionthe sample spectrometer implies collecting (Section a 0.5–1.0 2.1), g one sample can for expe thect digestion a significant prior tovariability analysis using in the the assays emission of the residuespectrometer due to the (Section fundamental 2.1), one error can expectof sampling a significant [12]. For variability ore type in thematerial, assays this of the error residue is reduced due to by the fundamental error of sampling [12]. For ore type material, this error is reduced by pulverizing pulverizing the sample prior to sampling. However, the metallic and ductile nature of the ZPR makes the sample prior to sampling. However, the metallic and ductile nature of the ZPR makes impossible impossible the pulverization of the sample, and thus one should expect a variability in the assays that the pulverization of the sample, and thus one should expect a variability in the assays that data data reconciliation [13 [13–16]–16] as as applied applied here here should should be able be toable attenuate. to attenuate.
100
80
60
40 % passing
20
0 1 10 100 1 000 10 000 Particle size (um) Figure 3. Size distribution of the residue. Figure 3. Size distribution of the residue. 2.3.3. Chemical Composition
2.3.3. ChemicalTable2 givesComposition the chemical composition of the ZPR sample. The measured assays of the sample are compared (see Table2) to the typical composition of the ZPR provided by the CEZinc plant. TheTable general 2 gives proximity the chemical of the sample composition composition of the with ZP theR sample. typical composition The measured of the assays residue of provided the sample are comparedby CEZinc (see confirms Table that 2) to the the received typical sample compositio is representativen of the ZPR of the provided residue usuallyby the CEZinc released plant. by the The generalplant. proximity The difference of the in sample the composition composition is attributed with the to the typical washing composition as discussed of later. the Theresidue Co content provided by CEZincis consistent confirms with that typical the ZPR received from othersample zinc is smelters representative [1–3]. of the residue usually released by the plant. The difference in the composition is attributed to the washing as discussed later. The Co content is consistent with typical ZPR from other zinc smelters [1–3].
Metals 2020,, 10,, 1553x FOR PEER REVIEW 55 of of 15
Table 2. Composition of the ZPR. Table 2. Composition of the ZPR. Elements CEZ Typical Composition Sample Composition * Elements CEZ Typical% Composition Sample Composition% * Zn 20–25%% 16.0 ± 0.5 Zn 20–25 16.0 0.5 Cd 2–6 3.5 ±± 0.2 Cd 2–6 3.5 0.2 S - 5.7 ±± 0.8 S - 5.7 0.8 ± CuCu 15–25 15–25 11.911.9 ± 0.40.4 ± CaCa - - 0.760.76 ± 0.040.04 ± CoCo 2–4 2–4 1.71.7 ± 0.10.1 ± Ni 0–2 0.44 0.04 Ni 0–2 0.44 ±± 0.04 Fe 0–1 0.34 0.02 Fe 0–1 0.34 ±± 0.02 Mn - 0.22 0.01 ± MnPb 9–12- 4.00.22 ± 0.010.1 Pb 9–12 4.0 ±± 0.1 *: Average standard deviation of three samples. *: Average ±± standard deviation of three samples.
2.3.4. X-Ray X-ray DiDiffractionffraction (XRD)(XRD) Figure4 4 shows shows thethe XRDXRD patternpattern (Instrument:(Instrument: Aeris,Aeris, MalvernMalvern PanalyticalPanalytical (Malvern,(Malvern, UK)UK) obtainedobtained for the residue. residue. The The XRD XRD shows shows the the presence presence of meta of metallicllic zinc, zinc, copper, copper, and and possibly possibly metallic metallic lead. lead. The Thepeaks peaks for Cd for and Cd andCo are Co not are visible not visible due to due the to low the co lowntents contents of these of these elements. elements. The presence The presence of Pb ofis Pb is confirmed by the assays (see Table2) and is reported under the form of PbO for the residue confirmed by the assays (see Table 2) and is reported under the form of PbO2 for the2 residue of an of an Iranian zinc smelter [3] and under the form of PbSO [10] for a Chinese zinc residue. The XRD Iranian zinc smelter [3] and under the form of PbSO4 [10] for4 a Chinese zinc residue. The XRD results resultsof Figure of Figure4 also 4show also showthe presence the presence of copper, of copper, zinc, zinc,and andcalcium calcium sulfates sulfates likely likely responsible responsible for forthe theobserved observed low low specific specific gravity gravity of ofthe the residue residue (Secti (Sectionon 2.3.1). 2.3.1). These These sulfates, sulfates, except except gypsum, gypsum, could be completely removed by a longer or slightly acidic or hot water wash of the residue as reported in [2]. [2].
Figure 4. XRD powder pattern of the CEZinc ZPR. 3. Results 3. Results 3.1. Washing of the ZPR 3.1. Washing of the ZPR The first step of the proposed process is the washing of the ZPR. Initially done to eliminate The first step of the proposed process is the washing of the ZPR. Initially done to eliminate the the sulfuric acid from the ZPR, the washing step provides an economic way to pre-concentrate the sulfuric acid from the ZPR, the washing step provides an economic way to pre-concentrate the cobalt by allowing a partial elimination of the soluble zinc and cadmium sulfates contained in the ZPR. The
Metals 2020, 10, 1553 6 of 15 Metals 2020, 10, x FOR PEER REVIEW 6 of 15 cobaltresults by of allowing a washing a partial test are elimination shown in of Figure the soluble 5 and zinc Table and 3. cadmium Washing sulfates is carried contained out at inroom the ZPR.temperature The results using of awater washing at 15% test solids are shown in mass. in Figure Figure5 and5 shows Table the3. Washingmeasured is concentrations carried out at room of Zn, temperatureCd, Ca, Co, usingand Cu water during at 15% washing. solids inThe mass. solution Figure pH5 shows falls from the measured 7.2 to 6.3 concentrationsduring the washing of Zn, Cd,step Ca,due Co, to the and release Cu during of the washing. residual Thesulfuric solution acid. pH Table falls 3 fromgives 7.2the to proportions 6.3 during theof metals washing removed step due from to thethe releaseZPR calculated of the residual using: sulfuric acid. Table3 gives the proportions of metals removed from the ZPR calculated using: 𝑉𝑥 𝐷 = 100 Vxm (2) Dm = 100 𝑉𝑥 +𝑊 𝑦 (2) Vxm + WZPR yZPR where Dm is the dissolved proportion of metal m, V is the volume (L) of the wash solution, xm the where Dm is the dissolved proportion of metal m, V is the volume (L) of the wash solution, xm the metal content (g/L) in the solution. WZPR and 𝑦 are, respectively, the weight (g) and the fraction metal content (g/L) in the solution. WZPR and yZPR are, respectively, the weight (g) and the fraction of metalof metalm in m the in driedthe dried washed washed ZPR. ZPR. Results Results presented presented in Table in 3Table show 3 that, show respectively, that, respectively, 20% and 20% 43% and of the43% zinc of the and zinc cadmium and cadmium contained contained in the rawin the ZPR raw are ZPR removed are removed by a 30 by min a 30 wash. min wash. About About 4% of 4% the of cobaltthe cobalt contained contained in the in ZPRthe ZPR is lost is lost during during the operation.the operation. Higher Higher impurity impurity removal removal can can be achievedbe achieved by washingby washing the ZPRthe ZPR for 90 for min 90 butmin at but increased at increased cobalt cobalt losses losses as shown as shown in Figure in Figure5. The zinc5. The is likelyzinc is under likely anunder insoluble an insoluble metallic metallic form as form metallic as metallic zinc dust zinc is du usedst is for used the for cementation, the cementation, while cadmiumwhile cadmium is likely is presentlikely present as a sulfate. as a sulfate. The 4% The Co dissolution 4% Co dissolution is an indication is an indication that some that cobalt some is under cobalt the is under form of the sulfate, form withof sulfate, the remaining with the beingremaining metallic. being The metallic. washing The step washing can be step viewed can be as aviewed selective as a leaching selective operation leaching foroperation which thefor leachingwhich the conditions leaching are conditions adjusted toare target adjust specificed to target metals specific [1,2]. The me usetals of [1,2]. diluted The acid use inof replacementdiluted acid ofin waterreplacement could yield of water a better could elimination yield a better of zinc elimination and cadmium of zinc at theand expenses cadmium of at more the importantexpenses of cobalt more losses important into the cobalt wash losses solution into [the10]. wash solution [10].
100 Zn Cd 80 Ca Cu 60 Co
40
Proportion dissovled, % dissovled, Proportion 20
0 0306090 Water leach duration, min.
Figure 5. Dissolved proportion of metals in time (25 ◦C, 15% solids in mass). Figure 5. Dissolved proportion of metals in time (25 °C, 15% solids in mass).
Table 3. Dissolved metals after 30 min of washing with water (25 ◦C, 15% solids in mass). Table 3. Dissolved metals after 30 min of washing with water (25 °C, 15% solids in mass). Metal % Dissolved * Metal % Dissolved * Zn 20 3 ± CdZn 43 20 ± 3 2 ± CuCd 0.0343 ± 2 0.02 ± Ca 16 1 Cu 0.03 ±± 0.02 Co 4 1 Ca 16 ±± 1 Ni 2 1 ± FeCo 0.4 4 ± 1 0.2 ± MnNi 84 2 ± 1 3 ± PbFe 0.1 0.4 ± 0.2 0.2 ± *: average Mnstandard 84 deviation ± of three3 tests. ± Pb 0.1 ± 0.2 *: average ± standard deviation of three tests.
Metals 2020, 10, 1553 7 of 15
3.2. Preparation of ZPR Samples for the Co Extraction Tests The leaching tests are carried out using samples prepared by washing 75 g of raw ZPR for 30 min using the above described approach. The washed material is dried at low temperature in an oven. The use of 75 g batches was found to be a good compromise between the variability of the 75 g batch composition [17] and the mass of material to be manipulated during the experimentation. Table4 gives the assays obtained by assaying three (3) randomly selected 75 g batches.
Table 4. Chemical assays (%) of three randomly selected feed samples (Smp = Sample; Avg = Average; Std-dev =Standard deviation RSD: = Std-Dev/Avg).
Elements Smp #1 Smp #2 Smp #3 Avg Std-Dev RSD (%) Zn 16.4 16.2 15.5 16.0 0.5 3 Cd 3.7 3.5 3.3 3.5 0.2 6 Cu 11.5 12.1 12.2 11.9 0.4 3 Ca 0.80 0.77 0.72 0.76 0.04 5 Co 1.68 1.80 1.63 1.71 0.09 5 Ni 0.47 0.47 0.40 0.44 0.04 9 Fe 0.36 0.34 0.31 0.34 0.02 6 Mn 0.23 0.21 0.23 0.22 0.01 5 Pb 3.8 4.1 4.0 4.0 0.1 3
3.3. Leaching of the Washed ZPR The next step of the process is to leach the metals out of the ZPR into an aqueous solution from whichi it will be possible to separate the cobalt from the other metals. The leaching tests are conducted in a mechanically agitated beaker for 30 min using a weight L/S ratio of 15/1 at 80 ◦C. Leaching tests are carried out with sulfuric acid as it is the acid used in the zinc plant that provided the ZPR. Table5 gives the average proportion of metals dissolved ( standard deviation) of three (3) leaching tests and ± the average metal contents in the liquor. About 65% of the solids in the ZPR are dissolved during the leaching process with 98% of the cobalt effectively leached off the ZPR, a performance similar to that reported in [10].
Table 5. Proportion of the metals dissolved during the leaching of the ZPR (Average ( standard ± deviation) of the results of three (3) leaching tests and the average composition of the liquor (Ratio L/S:
15/1; 100 g/LH2SO4, Eh = 90 mV, pH = 0.2, 30 min, 80 ◦C).
Species % Dissolved Liquor Concentration (mg/L) Solids 65 1 ± Zn 97.1 0.4 11,800 ± Cd 91 2 370 ± Cu 37 3 3600 ± Ca 88 3 252 ± Co 97 1 1170 ± Ni 97 1 345 ± Fe 98 2 270 ± Mn 91 16 26 ± Pb 0.2 0.3 6 ±
3.4. Selective Precipitation of the Cobalt from the Pregnant Liquor Solution The main impurities in the pregnant liquor solution are Zn, Cd, Ni, Fe, and Cu (Table5). The approach used to selectively separate cobalt from these elements begins with an oxidation of iron and manganese to precipitate of these oxidized elements by increasing the pH near 3.0 while Cu, Zn, Cd, and Co(II) remain in the solution. The next step is to oxidize Co(II) to Co(III) and to precipitate Co(III) by increasing the pH above 3.0 leaving Zn, Cd, Ni and Cu in the solution. Metals 2020, 10, x FOR PEER REVIEW 8 of 15 and MetalsCo(II)2020 remain, 10, 1553 in the solution. The next step is to oxidize Co(II) to Co(III) and to precipitate8 of Co(III) 15 by increasing the pH above 3.0 leaving Zn, Cd, Ni and Cu in the solution. Ammonium PerSulfate or APS ((NH4)2S2O8), as discussed in [1,8], provides the required Ammonium PerSulfate or APS ((NH4)2S2O8), as discussed in [1,8], provides the required oxidation oxidationpotential potential for iron, for manganese iron, manganese and subsequently and subseque cobalt. Thently sequence cobalt. ofThe the sequence steps followed of the to steps precipitate followed to precipitatethe cobalt isthe shown cobalt in is Figure shown6. in Figure 6.
ZPR 75 g Water W A Loaded batch washing solution Caustic to A V S L pH 3.0 Sulfuric acid (100g/ A L) to pH 0.2 Washing W A V 30 minutes APS 15% solid Samples of the L 1st neutralization S solution for assaying LEGEND Oxidation to 650 Fe, Mn and Co Leaching Leached A mV at pH 3.0
(30 min) residue W A Chemical analysis Caustic to W Weight measurement pH 3.0 V Volume measurement L APS S A Deplete W solution Co Precipitation L A V Fe+Mn Oxidation to 1000 S mV at pH 3.0 precipitate A 400 ml Water W
Loaded washing solution L A V Washing S 5 minutes A
W
Co precipitate FigureFigure 6. 6.ProcessProcess for for the the extraction of of cobalt cobalt from from the the ZPR. ZPR.
3.4.1. Precipitation of Fe-Mn 3.4.1. Precipitation of Fe-Mn Sodium hydroxide is firstly added to the pregnant liquor solution to bring the pH to 3.0. APS is thenSodium added hydroxide to the solution is firstly to increase added theto the Redox pregnant potential liquor from solution 90 to 650 to mV. bring During the thepH oxidation to 3.0. APS is thenprocess, added to the the pH solution of the solution to increase is maintained the Redox at 3.0 potential by a regular from addition 90 to 650 of mV. NaOH. During Samples the ofoxidation the process,solution the werepH of collected the solution at diff erentis maintained Eh values at and 3.0 analyzed by a regular for Fe, addition Mn, and of Co. NaOH. As anticipated Samples [1 of] the solutionFe and were Mn collected precipitate at priordifferent to Co Eh a behaviorvalues and confirmed analyzed by for the Fe, variations Mn, and of Co. the metalAs anticipated contents in [1] Fe and theMn solution precipitate as shown prior in to Figure Co a 7behavior. According confirmed to Figure 7by, the the optimal variations Eh for of the the precipitation metal contents of Fe in the solutionand Mnas shown without in precipitation Figure 7. According of Co is 650 to mVFigure and 7, the the minimum optimal Eh Eh for for the the precipitation precipitation of Coof Fe is and Mn 1000without mV. Theseprecipitation Eh values of are Co coherent is 650 mV with and those the reported minimum in [1]. Eh Table for6 thegives precipitation the proportions of ofCo the is 1000 mV.metals These inEh the values leached are solution coherent that with are removed those report by theed oxidation in [1]. toTable 650 mV.6 gives Iron the and proportions manganese are of the almost completely removed from the solution. Ideally, cobalt should not be precipitated at this stage; metals in the leached solution that are removed by the oxidation to 650 mV. Iron and manganese are however, it was observed that a close control of the Eh was not always possible as the Redox probe almost completely removed from the solution. Ideally, cobalt should not be precipitated at this stage; shows a significant measurement variability causing cobalt losses (~17%) in the Fe–Mn precipitate. however,The di ffiit cultywas inobserved the control that of thea close Eh is control attributed of tothe the Eh probe was and not to always its sensitivity possible to theas APSthe Redox addition. probe showsIndeed, a significant a small addition measurement of APS can variability make a large causing local stepcobalt in thelosses Redox (~17%) potential in the of the Fe–Mn solution. precipitate. It is The likelydifficulty that ain process the control solely basedof the on Eh the is use attribut of an APSed to dosage the probe [1,8] is and not recommendedto its sensitivity to achieve to the a APS addition.selective Indeed, separation. a small Cleary addition the oxidant of APS addition can make should a large be based local on step a reliable in the measurementRedox potential of the of the solution.ReDox It potentialis likely ofthat the a solutionprocess andsolely not based on the on dosage the use of the of oxidant.an APS dosage [1,8] is not recommended to achieveOnce a theselective Redox separation. potential reaches Cleary 650 the mV, oxidant the addition addition of APS should is stopped be andbased the on iron a andreliable measurementmanganese of precipitate the ReDox are potential removed of by the filtration solution (see and Figure not6 on). Thethe filtrationdosage of of the the oxidant. sludgy Fe-Mn precipitate is difficult and is believed to be one of the reasons for the loss of cobalt at this stage. EnsuringTable 6. Metal a rapid precipitation and adequate during filtration the oxidation of the solution for Fe-Mn after theremoval end ofof the the reaction leach solution seems critical(Average to limit(± standard cobalt losses. deviation) The results solution of from three that (3) tests solid /(Ehliquid = 90 separation to 650 mV, advances pH = 3.0, to 80 the °C, cobalt 15 min). precipitation step (Figure6). Results of Table6 show that Mn and Fe are e ffectively removed from the solution at the expenses of some cobalt losses.Species The large% Precipitated standard deviation observed for the fraction of Zn 0.3 ± 0.2 Cd 0.6 ± 0.2 Cu 1.1 ± 0.4 Ca 1.1 ± 0.6
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Metals 2020, 10, x FOR PEER REVIEW 9 of 15 cobalt precipitated is attributed to Eh measurement problems as discussed above and difficulties in the filtration of the precipitate from oneCo test to another17 ± one.8 Improving on the accuracy of the Eh Ni 0.6 ± 0.3 measurement method and of the solids/liquid separation of the Fe–Mn precipitate could significantly Fe 80 ± 20 improve the global recovery of cobalt. Mn 90 ± 10
4000 250 3500 Co 200 3000 2500 150 Mn 2000 Fe 1500 100 1000 50
Metal concentration (mg/L) Metal concentration 500
Mn Metal concentration (mg/L) Fe 0 0 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 Redox Potential (mV) Redox Potential (mV)
(a) (b)
FigureFigure 7. 7.Precipitation Precipitation of of Fe Fe and and Mn Mn with with increasing increasing the the Eh Eh while while maintaining maintaining a a pH pH of of 3.0 3.0 (a ()a Fe,) Fe, Mn, Mn, andand Co Co contents contents in thein the solution solution during during the oxidation the oxidation to 1000 to mV. 1000 (b )mV. Fe and (b) MnFe contentsand Mn incontents the solution in the duringsolution the during oxidation the belowoxidation 1000 below mV. 1000 mV.
Table 6. Metal precipitation during the oxidation for Fe-Mn removal of the leach solution (Average Once the Redox potential reaches 650 mV, the addition of APS is stopped and the iron and ( standard deviation) results of three (3) tests (Eh = 90 to 650 mV, pH = 3.0, 80 C, 15 min). manganese± precipitate are removed by filtration (see Figure 6). The filtration◦ of the sludgy Fe-Mn precipitate is difficult and is believedSpecies to be one % of Precipitated the reasons for the loss of cobalt at this stage. Ensuring a rapid and adequate filtrationZn of the soluti 0.3 on after0.2 the end of the reaction seems critical to ± limit cobalt losses. The solution from thatCd solid/liquid 0.6 separation0.2 advances to the cobalt precipitation ± step (Figure 6). Results of Table 6 showCu that Mn 1.1and Fe are0.4 effectively removed from the solution at ± Ca 1.1 0.6 the expenses of some cobalt losses. The large standard± deviation observed for the fraction of cobalt Co 17 8 precipitated is attributed to Eh measurement problems± as discussed above and difficulties in the Ni 0.6 0.3 ± filtration of the precipitate from one Fetest to an 80other one.20 Improving on the accuracy of the Eh ± measurement method and of the solids/liquidMn separa 90 tion of 10the Fe–Mn precipitate could significantly ± improve the global recovery of cobalt. 3.4.2. Precipitation of Co 3.4.2. Precipitation of Co The cobalt precipitation is carried out by adding APS to bring the Redox to 1000 mV and keeping The cobalt precipitation is carried out by adding APS to bring the Redox to 1000 mV and keeping the solution at 80 ◦C to accelerate the precipitation [1]. The precipitation of cobalt under the form of CoOOHthe solution is slow at as80 shown°C to accelerate in Figure8 thea.This precipitation observed [1]. behavior The precipitation is consistent of with cobalt previously under the reported form of resultsCoOOH [1] is reproduced slow as shown in Figure in Figure8b. Table 8a. This7 gives observed the variation behavior of is the consistent solution with composition previously from reported the leachresults step [1] to reproduced the Co precipitation in Figure one. 8b. Table Zn, Cd, 7 gives and Cu the remain variation in the of the solution solution while composition cobalt precipitates from the withleach some step of to the theiron Co precipitation and manganese one. remaining Zn, Cd, and in Cu the remain solution in after the solution the first while oxidation cobalt step. precipitates Table8 giveswith the some proportions of the iron of and the dimanganesefferent metals remaining precipitated in the during solution the after cobalt the precipitation first oxidation step. step. About Table 2% 8 ofgives the copper the proportions precipitates of withthe different the cobalt metals and represents precipitated a critical during impurity the cobalt as precipitation discussed later. step. The About fact that2% theof the observed copper precipitates Co precipitation with the rate cobalt in Figure and 8re ispresents significantly a critical less impurity than that as discussed reported inlater. [ 1] The is anfact indication that the observed that there Co is roomprecipitation to improve rate thein Figure proposed 8 is significantly processing scheme less than in that order reported to reduce in [1] the is precipitationan indication time that and there subsequently is room to theimprove volume the of pr theoposed precipitation processing vessels scheme for ain continuous order to reduce process. the precipitation time and subsequently the volume of the precipitation vessels for a continuous process.
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100
80
60
40 Co removal, % Co removal,
20
0 0 306090120 Precipitation duration, min.
(a) (b)
Figure Figure8. Cobalt 8. Cobalt precipitation precipitation rates. rates. (a) (Observeda) Observed rate rate of of Co Co precipitation precipitation (pH (pH= =3.0, 3.0, Eh Eh= 1000= 1000 mV, mV, 80 °C, 1 g/L80 Co).◦C, 1 ( gb/L) Co).Rates (b of) Rates Co precipitation of Co precipitation reported reported in in[1] [1 .] B-0. Table 7. Composition of the solutions from the ZPR leach, Fe-Mn and Co precipitation steps. Table 7. Composition of the solutions from the ZPR leach, Fe-Mn and Co precipitation steps. All Concentrations in mg/L Solution Volume (mL) Zn Cd All Concentrations Cu Co in Fe mg/L Mn LeachSolution solution Volume 841 (mL) 11,800 Zn 370Cd 3600Cu 1170Co 270Fe 26Mn LeachAfter Fe-Mn solution ppt 944 841 10,511 11,800 327 370 3200 3600 860 1170 46 270 1.3 26 After Co ppt 975 10,111 318 3000 84 16 0.4 After Fe-Mn ppt 944 10,511 327 3200 860 46 1.3
TableAfter 8. Metal Co ppt precipitation during 975 the cobalt 10,111 precipitation 318 stage (Average3000 standard 84 deviation) 16 0.4 for ± three (3) tests (Eh = 1000 mV, pH = 3.0, 80 ◦C, 120 min). Table 8. Metal precipitation during the cobalt precipitation stage (Average ± standard deviation) for Species % Precipitated three (3) tests (Eh = 1000 mV, pH = 3.0, 80 °C, 120 min). Zn 0.4 0.1 ± Cd 1 0.3 Species % Precipitated± Cu 2 1 ± ZnCa 0.4 1 ± 0.10.4 ± Co 89 3 Cd 1 ±± 0.3 Cu 2 ± 1 3.4.3. Washing and Composition of theCa Cobalt Precipitate1 ± 0.4
The cobalt precipitate is washed withCo water at89 80 ◦C± at 1% solids3 for 5 min, filtered, dried overnight, weighed, and assayed. Table9 gives the composition of the cobalt product obtained from 3 independent 3.4.3. Washingtests. Table and9 compares Composition the average of the composition Cobalt Precipitate of the cobalt product to a target cobalt hydroxide feed for a cobalt refinery [18]. Results show that some tunings (likely at the cobalt precipitation stage) of the Theprocess cobalt are precipitate still required is to washed lower the with zinc andwater copper at 80 contents °C at to 1% meet solids the composition for 5 min, of filtered, the feed dried overnight,material weighed, for the Co-refinery.and assayed. Table 9 gives the composition of the cobalt product obtained from 3 independent tests. Table 9 compares the average composition of the cobalt product to a target cobalt Table 9. Composition (%) of the produced cobalt hydroxide and specifications for the feed of a custom hydroxide feed for a cobalt refinery [18]. Results show that some tunings (likely at the cobalt cobalt refinery. precipitation stage) of the process are still required to lower the zinc and copper contents to meet the composition ofMaterial the feed mate Corial for the Cd Co-refinery. Cu Zn Fe Mn Ni Pb Produced Co 45 4 0.120 0.004 4.4 0.7 0.8 0.2 2 1 <0.1 <0.1 0.3 0.1 hydroxide * ± ± ± ± ± ± TableCustom 9. Composition refinery Co (%) of the produced cobalt hydroxide and specifications for the feed of a custom 23.2 n.a. 1.61 0.19 2.39 3.27 0.39 n.a. cobalthydroxide refinery. feed [19] *: Average standard deviation of three (3) replicated tests. ± Material Co Cd Cu Zn Fe Mn Ni Pb Produced Co hydroxide * 45 ± 4 0.120 ± 0.004 4.4 ± 0.7 0.8 ± 0.2 2 ± 1 <0.1 <0.1 0.3 ± 0.1 Custom refinery Co 23.2 n.a. 1.61 0.19 2.39 3.27 0.39 n.a. hydroxide feed [19] *: Average ± standard deviation of three (3) replicated tests.
The XRD powder pattern of the cobalt product is shown in Figure 9. Results confirm that the produced material is under the form of CoO(OH) and Co(OH)2. The presence of these compounds was expected based on observations from other researchers [1].
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The XRD powder pattern of the cobalt product is shown in Figure9. Results confirm that the produced material is under the form of CoO(OH) and Co(OH)2. The presence of these compounds Metals 2020was, expected10, x FOR basedPEER REVIEW on observations from other researchers [1]. 11 of 15
Metals 2020, 10, x FOR PEER REVIEW 11 of 15
FigureFigure 9. 9.XRD XRD powder powder powder patternpattern of of of the the the cobalt cobalt cobalt product. product. product. 3.5. Overall Process Performance and Quality of the Product 3.5. Overall3.5. Overall Process Process Performance Performance and and Quality Quality ofof the ProductProduct The process flow sheet for the extraction of cobalt from the ZPR is shown in Figure 10. The flowsheet The process flow sheet for the extraction of cobalt from the ZPR is shown in Figure 10. The Thehas similaritiesprocess flow with sheet the flowsheet for the presented extraction in [1 ].of The co similaritybalt from between the ZPR the two is flowsheetsshown in developed Figure 10. The flowsheet has similarities with the flowsheet presented in [1]. The similarity between the two flowsheetindependentlyflowsheets has similarities developed is an indication independently with thatthe thisflowsheet is an processing indication pr schemeesented that this is ainprocessing viable [1]. routeThe scheme forsimilarity the is a recovery viable between route of cobalt for the two flowsheetsfromthe recovery adeveloped ZPR produced of cobalt independently byfrom cementation a ZPR produced is onan zincindication by dust. cementation that this on zinc processing dust. scheme is a viable route for the recovery of cobalt from a ZPR produced by cementation on zinc dust.
FigureFigure 10. ZPR processing circuit to to extract extract cobalt cobalt (See (See Table Table 1010 for for the the composition composition of of streams streams 1–6). 1–6).
InIn orderorder toto assessassess thethe robustnessrobustness of the propos proposeded process, process, the the whole whole processing processing sequence sequence of of FigureFigure 1010 was repeated with with three three ZPR ZPR feed feed sample samples.s. The The previous previous sections sections gave gave the results the results of the of the repetitions obtained for the various stages of the process. Table 10 summarizes these results in Figurerepetitions 10. ZPR obtained processing for the circuit various to stages extract of cobalt the process. (See Table Table 10 10 forsummarizes the composition these results of streams in terms 1–6). termsof cobalt of cobalt distribution. distribution. The repeti The repetitionstions show show that thatthe thecritical critical step step in inthe the proposed proposed process process is isthe the oxidationoxidation/precipitation/precipitation of the iron and manganese pr priorior to to the the cobalt cobalt precipitation. precipitation. The The main main cobalt cobalt In order to assess the robustness of the proposed process, the whole processing sequence of losses occur at the Fe-Mn precipitation step. A problem or inaccuracy in the measurement of the Figure Redox10 was potential repeated and withthe difficulties three ZPR in the feed filtration sample of thes. The Fe-Mn previous sludge cause sections some gave Co precipitation the results of the repetitionsand an obtained incomplete for removalthe various of impurities stages of that the will process. subsequently Table 10contaminate summarizes the cobaltthese product.results in terms of cobaltCobalt distribution. losses also occur The atrepeti the finaltions precipitatio show thatn step, the where critical recovery step could in the be improved proposed by processusing a is the oxidation/precipitationpH above 3.0 and allowing of the ironmore and time manganesefor precipitation prior [1]. to However, the cobalt the useprecipitation. of a higher pH The and main of cobalt losses anoccur increase at the precipitation Fe-Mn precipitation time may cause step. more A pr impuritiesoblem or to inaccuracy precipitate inwith the the measurement cobalt. This of the optimization problem of maximizing Co recovery under the constraint of an acceptable product Redox potential and the difficulties in the filtration of the Fe-Mn sludge cause some Co precipitation purity can be adequately tackled down using a factorial Design Of Experiments (DOE) [19] if the and an incomplete removal of impurities that will subsequently contaminate the cobalt product.
Cobalt losses also occur at the final precipitation step, where recovery could be improved by using a pH above 3.0 and allowing more time for precipitation [1]. However, the use of a higher pH and of an increase precipitation time may cause more impurities to precipitate with the cobalt. This optimization problem of maximizing Co recovery under the constraint of an acceptable product purity can be adequately tackled down using a factorial Design Of Experiments (DOE) [19] if the
Metals 2020, 10, 1553 12 of 15
losses occur at the Fe-Mn precipitation step. A problem or inaccuracy in the measurement of the Redox potential and the difficulties in the filtration of the Fe-Mn sludge cause some Co precipitation and an Metalsincomplete 2020, 10, x FOR removal PEER ofREVIEW impurities that will subsequently contaminate the cobalt product. Cobalt losses12 of 15 also occur at the final precipitation step, where recovery could be improved by using a pH above 3.0 processand is allowing to be morecontinued time forwith precipitation an optimization [1]. However, phase. the The use aspect of a higher of the pH process and of an sensitivity increase to operatingprecipitation conditions, time may particularly cause more impuritiesthe control to precipitateof the Redox with thepotential, cobalt. This addressed optimization here problem is seldom discussedof maximizing in the literature Co recovery dealing under with the the constraint recovery of an ofacceptable cobalt from product a ZPR, purity while can it is be as adequately important as tackled down using a factorial Design Of Experiments (DOE) [19] if the process is to be continued with the process itself, especially if it is to go on to piloting. an optimization phase. The aspect of the process sensitivity to operating conditions, particularly the control of the Redox potential, addressed here is seldom discussed in the literature dealing with the Table 10. Distribution (%) of the cobalt in the process streams of the circuit of Figure 9. recovery of cobalt from a ZPR, while it is as important as the process itself, especially if it is to go on to piloting. Stream Number as Process Stream Avg. Std-Dev in Figure 10 Table 10. Distribution (%) of the cobalt in the process streams of the circuit of Figure9. ZPR 1 100 100 ProcessWashed Stream solids ZPR Stream Number as in 2 Figure 10 Avg. 86 Std-Dev 3 LeachZPR solution 1 3 100 84 100 4 WashedCo ppt solids feed ZPR solution 2 4 7086 10 3 Leach solution 3 84 4 SpentCo ppt solution feed solution from Co ppt 4 5 70 7 10 1 Spent solutionCobalt from hydroxide Co ppt 5 6 62 7 14 1 Cobalt hydroxide 6 62 14 In summary, the proposed processing flow sheet yields a Co product assaying 45 ± 4% Co at a recovery Inof summary,62%. Improvements the proposed to processing the reproducibility flow sheet yields of the a method Co product can assaying be achieved 45 4% by Coimproving at a ± the controlrecovery of of the 62%. Redox Improvements potential and to the the reproducibility solids/liquid of separation the method of can the be precipitated achieved by improvingFe-Mn sludge. Thesethe results control cannot of the Redoxbe compared potential to and those the of solids other/liquid processes separation used of for the processing precipitated a ZPR Fe-Mn produced sludge. by cementationThese results on zinc cannot dust, be comparedas it was not to those possible of other to find processes such useddata forin processingthe literature. a ZPR produced by cementation on zinc dust, as it was not possible to find such data in the literature. Ideally, to avoid paying refining charges from the selling of the cobalt hydroxide product to a Ideally, to avoid paying refining charges from the selling of the cobalt hydroxide product to a customcustom Co refinery, Co refinery, one one should should aim aim at at producing producing electrolysis-gradeelectrolysis-grade cobalt cobalt at theat the end end of the of ZPR-Cothe ZPR-Co process.process. The Theconcentration concentration of ofcobalt cobalt in in the the solu solutiontion releasedreleasedby by the the S /LS/L separation separation of theof Fe-Mnthe Fe-Mn precipitateprecipitate is low is low at 3 at g/L, 3 g/ L,and and should should be be increased increased to atat leastleast 45 45 g g/L/L[20 [20]] to to allow allow Co Co electro-winning. electro-winning. SolventSolvent Extraction Extraction (SX) (SX) is an is an approach approach to to increase increase the solutionsolution concentration concentration and and remove remove some some of the of the accompanyingaccompanying impurities impurities (See (See Table Table 9).9). The The other optionoption is is to to stock stock pile pile the the produced produced Co hydroxide Co hydroxide and andto dissolve to dissolve it under it under reducing reducing conditions conditions toto convert convert Co(III) Co(III) into into Co(II) Co(II) with with a controlled a controlled amount amount of of sulfuricsulfuric acid acid to to generate generate thethe solutionsolution to to feed feed the the electrolytic electrolytic cells. cells. Figure Figure 11 shows 11 shows the two the options. two options.
. Figure 11. Processing options to produce electrolytic cobalt. Figure 11. Processing options to produce electrolytic cobalt.
3.6. Economics of the Process The analysis presented in this section focuses only on the operating costs of the process of Figure 10. It is a preliminary analysis and the calculated costs should be considered as Class 5 estimates (AACE International). Results are used to anticipate the economic viability of the process and decide on the continuation of the laboratory test work to optimize the proposed ZPR-Co process. A cobalt price of 30.20 USD/kg (LME Price August 2020) is used to estimate the revenues that can be generated by processing the ZPR. The Net Smelter Return, if the cobalt hydroxide is to be sold to a refinery, is estimated by assuming that 90% of the cobalt in the hydroxide is payable and that the refining charges are 3.00 CAD/kg of Cobalt hydroxide. If the cobalt content of the produced Co
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3.6. Economics of the Process The analysis presented in this section focuses only on the operating costs of the process of Figure 10. It is a preliminary analysis and the calculated costs should be considered as Class 5 estimates (AACE International). Results are used to anticipate the economic viability of the process and decide on the continuation of the laboratory test work to optimize the proposed ZPR-Co process. A cobalt price of 30.20 USD/kg (LME Price August 2020) is used to estimate the revenues that can be generated by processing the ZPR. The Net Smelter Return, if the cobalt hydroxide is to be sold to a refinery, is estimated by assuming that 90% of the cobalt in the hydroxide is payable and that the refining charges are 3.00 CAD/kg of Cobalt hydroxide. If the cobalt content of the produced Co hydroxide is 45% (see Table9) then the revenues (assuming an exchange rate of 1.25 CAD /USD) generated per kg of cobalt hydroxide are:
12.28 CAD$ NSR = 0.9 0.45 30.2 1.25 3 = , (3) × × × − kg of Co hydroxide
Since the test work has shown that the processing of 75 g of ZPR yields 1.2 g of Co hydroxide, the potential revenues per kg of ZPR are 0.20 CAD/kg ZPR. The operating costs considered here are due to the reagents and exclude the energy costs for heating the solution at the precipitation stage. Table 11 gives the calculated operating costs per kg of ZPR. The reagent costs are probably over-estimated as purchasing the sulfuric acid may not be required. Indeed some spent zinc electrolyte could be used for the Co process (see Figure1). Additionally, it is very likely that recycling some of the solution streams of the ZPR-Co process could allow a reduction in the consumption of caustic and APS. This preliminary evaluation shows that the proposed process is not viable but the process certainly deserves to be optimized to increase the overall cobalt recovery and to reduce the consumption of APS that is the main contributor to the operating costs. The option of producing metallic cobalt rather than cobalt hydroxide should also be investigated.
Table 11. Preliminary estimation of the operating costs for the ZPR-Co process of Figure9. (The reagent prices were obtained from quotations dating of 2019 and are used here only to give an order of magnitude).
Reagent Consumption (kg/kg of ZPR) Price CAD$/kg CAD$/kg ZPR
H2SO4 1.9 0.170 0.032 NaOH 0.07 0.80 0.056 APS 0.25 0.85 0.22 Total costs - - 0.30 Gross revenues - - 0.20 Benefits (losses) - - (0.10)
3.7. Residues of the ZPR-Co Process The proposed process generates two solid residues (the ZPR leached residue, the Fe–Mn precipitate) and two liquid effluents (the solutions from the ZPR washing and from the Co precipitation as indicated in Figure 12. Table 12 gives the compositions of the liquid and solid residues. The copper content of the ZPR leached solids is 36%, which is above the copper content of the ZPR (see Table2). If the metals of the reject solutions were to be precipitated under the form of hydroxides by increasing the pH above 9.0, the precipitate would assay 13% Cu, which is still in the range of the ZPR copper content. Thus, two of the residues of the ZPR-Co process could possibly be used as copper sources and possibly be sold to a Cu smelter. Metals 2020, 10, x FOR PEER REVIEW 13 of 15 hydroxide is 45% (see Table 9) then the revenues (assuming an exchange rate of 1.25 CAD/USD) generated per kg of cobalt hydroxide are: