Copper Recovery from Silicate-Containing Low-Grade Copper Ore Using Flotation Followed by High-Pressure Oxidative Leaching

Resources Processing 64 : 3–14 (2017) RESOURCES PROCESSING Original Paper

Baisui HAN1*, Batnasan ALTANSUKH2, Kazutoshi HAGA3, Yasushi TAKASAKI2 and Atsushi SHIBAYAMA2

1Graduate School of Engineering and Resource Science, Akita University, 1-1Tegata Gakuen-machi, Akita 010-8502, Japan 2Graduate School of International Resource Sciences, Akita University, 1-1Tegata Gakuen-machi, Akita 010-8502, Japan 3Graduate School of Engineering Science, Akita University, 1-1Tegata Gakuen-machi, Akita 010-8502, Japan

Abstract In this paper, we present the results for recovery of copper from silicate-containing low-grade copper ore using flotation followed by atmospheric and high-pressure oxidative leaching. The effects of various flotation parameters, such as flotation time, PAX dosage, slurry pH and air injection rate on beneficiation of copper from the low-grade copper ore were studied. The recovery of copper reached 93.1% and the grade of copper improved to 18.2 mass% from 0.4 mass% under collector-less optimum flotation conditions. The enrichment ratio of copper in the concentrate was 45. The copper concentrate obtained from flotation of the low-grade copper ore was treated by atmospheric leaching and high-pressure oxidative leaching processes. A maximum recovery (>93.0%) of copper was obtained by high-pressure oxidative leaching in the water, whereas the copper recovery was lower than 12% in the sulfuric acid solution under atmospheric leaching conditions. The copper concentration in a pregnant leach solution enriched up to 15.0 g/L under the optimized leaching conditions. The effect of impurity such as iron in the sample on the froth flotation and both leaching processes is also con- sidered. A process flow for the recovery of copper from silicate-containing low-grade copper ores is proposed as a result of these studies.

Key words: Low-grade copper ore, Chalcopyrite, Flotation, High-pressure oxidative leaching

1. Introduction tion of high-grade copper ore have gradually been happened during last several decades6,7. With the Copper is one of the important non-ferrous decreasing of copper grade, the content of the metals which has a lot of commercial applications copper ores becomes complex due to the increas- in a wide range of industries such as medicine, ing of impurities such as clay minerals and refrac- construction, machinery, electrical and electronics tory ores. For those reasons, it becomes more dif- 1 and telecommunication . Chalcopyrite (CuFeS2) ficult to apply the conventional pyrometallurgical is the most abundant and the most refractory cop- method to treatment of the low-grade copper per sulfide mineral which accounts for about 70% ores6,7. On the other hand, the copper market con- of the copper reserves in the world2. At present, a tinually constrained on the stable supply because large portion of the copper is produced from chal- the increasing of demand of copper in recent copyrite ore via flotation followed by the pyrome- years and the low-grade ores and tailings can ac- tallurgical method3–5. Due to the long-term mining count for up to three-quarters of the total reserves activity and economic growth of countries in the of certain metals8–10. Hence, it becomes important world, the reduction of copper grade and deple- to address the utilization of common secondary resources especially low-grade ores by the devel- Accepted 24 April, 2017 opment of the processing technology. Recently, *e-mail: [email protected] much attention has been focused on the hydro

Vol. 64, No. 1 (2017) 3 HAN, ALTANSUKH, HAGA, TAKASAKI and SHIBAYAMA metallurgical route, particularly acid leaching, leaching and high-pressure oxidative leaching which suggests a viable alternative approach to processes, respectively. As a result of this study, extract copper from low-grade sulfide ores, com- an efficient method has been proposed to recover plex ores and tailings11–14. There are many studies copper from a silicate-containing low-grade cop- related to the development of hydrometallurgical per ore by flotation followed by high-pressure ox- routes to extract copper from low-grade sulfide idative leaching. ores by atmospheric leaching and biological leaching processes15–18. Authors have concluded 2. Experimental that both direct atmospheric and biological leaching of copper from its ore are extremely slow due 2.1 Sample to the formation of an elemental sulfur passiva- A silicate-containing low-grade copper ore from tion layer on the surface of unreacted particles3. Chile was used in this study. The sample was firstly On the other hand, huge amounts of acid are re- prepared by a jaw crusher (P-1, Fritsch Japan Co., quired in the copper leaching from the low-grade Ltd) and a disc mill (P-13, Fritsch Japan Co., Ltd) copper ore, especially it contains large amounts of followed by a Sieve Shaker (AS200 digit, Retsch carbonates or siliceous gangue minerals as impu- Japan Co., Ltd) to a size fraction of under 160 μm. rities which make difficulty on the separation The average particle size of the sample was 27 μm process and produce many residues. Additionally, (D50) which estimated by a wet type size distribu- copper concentration in the leach liquor/pregnant tion analyzer (Microtrac MT3300II). The chemical leach solution (PLS) is much lower when acid composition of the silicate-containing low-grade leaching is applied for copper extraction from copper ore was determined by inductively coupled low-grade ores and it causes difficulty in subse- plasma-optical emission spectrometry (ICP-OES, quent processes such as solvent extraction (SX) SPS5510 SII Hitachi High-Tech Science Corpora- and electrowinning (EW). Hence, it is essential to tion, Chiba, Japan) and X-Ray Fluorescence Spec- develop the hydrometallurgical processes for the trometer (XRF, ZSX Primus II Rigaku). As shown extraction of copper from low-grade copper ores in Table 1, the sample consists of 0.4 mass% cop- in combination with flotation which is widely per (Cu), 4.6 mass% iron (Fe) and 38.3 mass% sil- used to beneficiate and selectively separate cop- icon (Si), and small amounts of a number of other per ores as a pre-treatment process for upgrading elements, including aluminium (Al), calcium (Ca), the copper grade and reducing the impurity con- potassium (K), magnesium (Mg) and sulfur (S). tent. However, low-grade copper ores cannot be The mineral composition of the ore was identified economically beneficiated by the conventional by X-Ray Diffractometer (XRD, RINT-2200V flotation process due to the fine particle size dis- Rigaku) and XRD pattern of the sample is shown tribution of the ores. Therefore, an efficient ap- in Fig. 1. The main mineralogical constituents of proach, which could give reasonably high copper the sample are gypsum (CaSO4·2H2O), chlorite recovery, low impurity content and low reagent ((Mg, Fe)6(Si, Al)4O10(OH)8) and quartz (SiO2). consumption, has been attracting much attention 2.2 Procedure from researchers in this field. For these reasons, 2.2.1 Flotation of silicate-containing low-grade this study aims to develop an efficient method for copper ore sample the recovery of copper from low-grade copper ore All the flotation tests were carried out using a by flotation followed by the leaching process in laboratory mechanical flotation machine (237FL, which a high-pressure leaching was selected as a Mekhanobr-tekhnika Corp.) equipped with a plas- major approach. In addition, there are few refer- tic flotation cell (500 mL) as shown in Fig. 2. The ences related to the copper leaching from the low- most common reagents for traditional copper flo- grade copper resources using high-pressure oxi- tation such as collector of potassium amyl xan- dative leaching as an alternative approach for thate (PAX, C5H11OCSSK), frother of methyl efficient copper recovering. isobutyl carbinol (MIBC, C6H14O) and pH regula- In this study, the effects of flotation parameters, tor of calcium hydroxide (Ca(OH)2) and sulfuric particularly collector (PAX) dosage, frother (MIBC), pH, contact time on the flotation perfor- Table 1 Chemical compositions of a silicate-containing mance of a silicate-containing low-grade copper low-grade copper ore ore were investigated. Leaching efficiency of cop- Contents per from the froth concentrate obtained by the flotation of the silicate-containing low-grade Al Ca Cu Fe K Mg Si S copper ore was investigated using atmospheric Grade, mass% 1.8 1.4 0.4 4.6 1.3 2.7 38.3 4.5

4 RESOURCES PROCESSING Copper Recovery from Silicate-Containing Low-Grade Copper Ore Using Flotation Followed by High-Pressure Oxidative Leaching

OES, XRF and XRD. The yield (Y), enrichment ratio (I) and recovery (R) of each metal from the low-grade copper ore were evaluated by Eqs. (1)– (3). C Yield : Y (%)=× 100 F (1) c Enrichment ratio : I = (2) f cf()− t Recovery : R (%)=× 100 (3) fc()− t

Where Y is the yield, I is an enrichment ratio, and R is the recovery of each metal. F and C are Fig. 1 XRD pattern of a silicate-containing low-grade cop- the weight of the feed and concentrate, f, c, and t per ore. are the metals grade in feed, concentrate and tailing, respectively. 2.2.2 Atmospheric acid leaching of froth concentrate Froth concentrate from the flotation of silicate- containing low-grade copper ore sample was dissolved in sulfuric acid (H2SO4) solution placed in a beaker. All experiments were carried out using various acid concentrations (0–10 M (mol/L)) at a fixed temperature (90°C), fixed leaching time (60 min) and adjusted stirring speed (700 rpm). After experiment, solid residue and pregnant leach solution (PLS) were separated by filtration. Concentrations of metals in PLS were measured by ICP-OES and the leaching rate of metals from the froth concentrate into the sulfuric acid solution was calculated by Eq. (4) as shown bellow. Fig. 2 Laboratory mechanical flotation machine. CV⋅ R = LL× Metal leaching rate : M (%) 100 CmFF⋅ acid (H2SO4) were used in this study. The silicate- containing low-grade copper ore samples were (4) first introduced into the flotation cell with 500 mL water under constant stripping (1000 rpm) to pre- Where CL and CF is the concentration of metal pare a slurry of 10% pulp density. pH of the slurry in PLS (mg/L) and feed (mg/kg), respectively. VL was then adjusted to target pH ranging from pH 4 is the volume of PLS (L) and mF is the dry mass to pH 10 using 1 M H2SO4 and 1 M Ca(OH)2 solu- of the feed (kg). tions and conditioning continued for 5 min. After 2.2.3 High-pressure oxidative leaching of froth adjusted the pH of the slurry, appropriate amounts concentrate of PAX as a collector and MIBC as frother were High-pressure oxidative leaching experiments added to the cell and stirring continued for 5 min. were conducted in an autoclave (Nitto Koastu, Then, a certain amount (0.8–1.7 L/min) of air was Japan) using the froth concentrate obtained from injected into the cell throughout the flotation. the flotation of the silicate-containing low-grade Froth layer at the top of the cell during the flota- copper ore. The autoclave is equipped with 0.2 L tion was gathered in the collection chamber by an of a Teflon vessel, inlet/discharge lines for oxygen auto-scraper. The collected froth concentrate and and excess gasses, pressure and temperature sen- tailing of flotation were dried at 70°C into an elec- sors as well as an electrical heating system. A tric drying oven for 1 day and analyzed by ICP- schematic illustration of the autoclave is shown in

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HAN, ALTANSUKH, HAGA, TAKASAKI and SHIBAYAMA

Fig. 3 A schematic illustration of an autoclave for high- Fig. 4 The copper grade and recovery as a function of flota- pressure oxidative leaching. tion time. (Flotation time: 5–20 min, pulp density: 10%, slurry pH: 8, PAX: 50 g/t-ore, MIBC: 200 g/t- ore and air injection rate: 0.8 L/min) Fig. 3. A 5 g sample of the froth concentrate was transferred into a vessel with 50 mL distilled Table 2 Effect of flotation time on copper enrichment ratio and yield water or sulfuric acid solution with concentration ranging from 0.5 to 1.0 M. The vessel containing Flotation time, Contents slurry was placed into the autoclave and experi- min Cu enrichment ratio Yield, % mental parameters were set to their nominal val- 5 18.31 3.19 ues throughout the leaching experiments as fol- 10 22.56 3.81 lows: when temperature reached at desired value 15 16.24 4.66 (100°C, 160°C, 170°C and 180°C), oxygen gas 20 10.41 6.22 was injected to the slurry into the vessel with con- trolling the total pressure (0.8 MPa, 1.5 MPa, 2.0 MPa and 2.5 MPa), and allowed the oxidation pulp density, slurry pH, collector (PAX) dosage, reaction at varying times from 60 to 180 min. Af- frother (MIBC) dosage and air injection rate, were ter the oxidation reaction is completed, turn off kept constant at 10%, pH of 8, 50 g/t-ore, 200 g/t- the oxygen gas supply followed by cooling down ore and 0.8 L/min. The experimental results pre- the slurry to below 50°C and purge the autoclave sented in Fig. 4 and Table 2 showed that the with excess oxygen gas and vapor through the grade, recovery and enrichment ratio of copper discharge line. The oxidized slurry was filtrated to are increased with increasing of flotation time separate solid residue and pregnant leach solution from 5 to 10 min. The maximum grade, recovery and both phases were analyzed by using ICP- and enrichment ratio of copper from the low- OES, XRF and XRD. The dissolution rate of each grade copper ore were 7.1 mass%, 87.8% and metal from the froth concentrate in distilled water 22.6 after 10 min of flotation. An increase in the and in H2SO4 solution under high-pressure oxida- flotation time further up to 20 min resulted in a tive leaching condition was calculated using Eq. 3.2% and 10.4 decreases in the copper grade and (4). enrichment ratio, while the copper recovery did not change obviously. The yield of the low-grade 3. Results and Discussion ore sample is increased with increasing the flotation time while there is no change like other flota- 3.1 Flotation of silicate-containing low-grade tion results (Table 2). The increment of the yield copper ore may be related to the flotation of impurity miner- 3.1.1 Effect of flotation time als such as clay minerals and silicates present in The effect of flotation time on the copper bene- a silicate-containing low-grade copper ore. As a ficiation and recovery from a silicate-containing result, flotation time was selected to be 10 min as low-grade copper ore was investigated by varying an optimum condition for further flotation experi- times from 5 to 20 min. Other conditions, namely ments.

6 RESOURCES PROCESSING Copper Recovery from Silicate-Containing Low-Grade Copper Ore Using Flotation Followed by High-Pressure Oxidative Leaching

Fig. 6 The grade and recovery of copper and iron as a func- Fig. 5 The grade and recovery of copper and iron as a function of slurry pH. (Slurry pH: 4–10, PAX: 0 g/t-ore, tion of PAX dosage. (PAX dosage: 0–50 g/t-ore, flota- flotation time: 10 min, pulp density: 10%, MIBC: tion time: 10 min, pulp density: 10%, slurry pH: 8, 200 g/t-ore and air injection rate: 0.8 L/min) MIBC: 200 g/t-ore and air injection rate: 0.8 L/min)

Table 4 Effect of slurry pH on copper enrichment ratio Table 3 Effect of PAX dosage on copper enrichment and yield ratio and yield Contents PAX dosage, Contents Slurry pH Cu enrichment ratio Yield, % g/t-ore Cu enrichment ratio Yield, % pH 4 19.25 3.94 0 24.74 2.77 pH 6 23.09 3.28 25 21.47 2.34 pH 8 (natural pH) 22.69 2.77 50 14.12 5.52 pH 10 34.09 2.33

3.1.2 Effect of PAX and its dosage particle attachment that represents an increase of The flotation experiments were conducted un- yield20. The tendency of the grade of iron in con- der the fixed conditions as follows: pulp density centrate and the recovery of iron from the low- of 10%, frother (MIBC) dosage of 200 g/t-ore, air grade copper ore were quite similar to the both of injection rate of 0.8 L/min at natural pH (pH 8) grade and recovery of copper. and flotation time of 10 min when collector (PAX) 3.1.3 Effect of slurry pH dosage varied from 0 to 50 g/t-ore. The results of The flotation tests were performed under vary- the flotation experiments shown in Fig. 5 indicat- ing slurry pH ranging from pH 4 to pH 10 when ed the maximum copper grade of 11.1 mass%, other fixed parameters were: pulp density of 10%, maximum copper enrichment ratio of 24.7 and collector (PAX) dosage of 0 g/t-ore, frother copper recovery of 78.6% when performed the (MIBC) dosage of 200 g/t-ore, air injecting rate of flotation without PAX. These findings are in good 0.8 L/min and flotation time of 10 min. 1 M 19 agreement with other published result : low- H2SO4 and 1 M Ca(OH)2 were used as a pH regu- grade ore i.e. chalcopyrite is floatable without a lator. The results obtained were presented in collector due to its self-induced/ collector-less Fig. 6 and Table 4, respectively. When the slurry flotation ability in the pulp potential ranges. pH increased from pH 4 to pH 10, the grade of Self-induced/collector-less flotation is an influen- copper reached to 16.7 mass% from 9.4 mass%, tial property of this type ore to promote the cop- whereas the recovery of copper and yield were per recovery and reduce the reagent consumption. decreased to 74.7% and 2.3, from 83.1% and 3.9, With the increase of PAX dosage to a 50 g/t-ore, respectively. The grade of iron decreased with in- copper grade in the froth concentrate decreased to creasing the pH of slurry from pH 4 to pH 6 and 6.4 mass% and copper recovery and yield in- increased further with increasing the pH of the creased to 90.5% and 5.5% (Fig. 5 and Table 3). slurry. The results obtained are verified that high It is expected that the availability of extra collec- pH value of the slurry has a positive effect on tor molecules in the feed enhances the hydro copper beneficiation due to the depression effect phobicity of particles promoting the bubble- of pyrite (FeS2) and gangue minerals including

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Table 6 The optimum conditions for recovery of copper from a silicate-containing low-grade copper ore Cu recovery, Cu grade, Parameters Conditions % mass% Pulp density 10% PAX dosage 0 g/t-ore MIBC dosage 200 g/t-ore 93.1 18.2 pH 10 Air injection rate 1.7 L/min Flotation time 10 min

Table 7 Chemical compositions of silicate-containing low-grade copper ore and froth concentrate Grade, mass% Sample Al Ca Cu Fe K Mg Initial sample grade 1.8 1.4 0.4 4.6 1.3 2.7 Froth concentrate 2.2 0.8 18.2 19.6 0.8 2.2 Fig. 7 The grade and recovery of copper and iron as a function of air injection. (Air injection rate: 0.8–1.7 L/ min, slurry pH: 10, PAX: 0 g/t-ore, flotation time: 10 min, pulp density: 10% and MIBC: 200 g/t-ore) to the formation of froth and its relatively faster Table 5 Effect of air injection rate on copper enrichment flow as well as lightly loaded stabilized hydro- 24 ratio and yield phobic particles . However, the air injection rate increased further until 1.7 L/min, the copper grade Air injection rate, Contents L/min and its enrichment ratio increased to 18.2 mass% Cu enrichment ratio Yield, % and 45.5, while the recovery of copper did not 0.8 41.79 2.04 change much (>93%) than that value with air in- 1.0 31.38 2.13 jection of 1.0 L/min. It seems that this observation 1.3 34.63 2.33 1.7 45.51 1.52 may be related to the greater froth depth and stability of bubbles formed through sufficient particle attachment. 3.1.5 The beneficiation of copper from the 21–23 silicate (SiO2) . Result showed that collector- high-silicate containing low-copper grade ore less flotation is possible to recover copper from under optimum flotation conditions the low-grade copper ore containing silicate. As a result of a batch flotation experiments 3.1.4 Effect of air injection rate described in Chapter 3.1.1–3.1.4, the optimum The flotation experiments was carried out at air flotation conditions were determined and results injection rates varied from 0.8 to 1.7 L/min with are summarized in Table 6. Under the optimum other variables like pulp density, collector (PAX) flotation conditions, flotation experiments were dosage, frother (MIBC) dosage, pH range and performed and the results obtained are presented flotation time were fixed at 10%,0 g/t-ore, in Table 7. It was revealed that the grade of 200 g/t-ore, pH 10 and 10 min. The results of copper achieved 18.2% with enrichment ratio these flotation experiments were summarized in of 45.5 when the recovery of copper was 93.1%. Fig. 7 and Table 5. The copper grade and enrich- The XRD patterns of silicate-containing low- ment ratio of copper decreased to 12.6 mass% grade copper ore and its froth concentrate ob- from 16.7 mass% and to 31 from 42, whereas its tained from flotation under the optimum condition recovery increased up to 91.0% from 74.7%, re- are shown in Fig. 8. Main mineral constituents in spectively, when the air injection rate is increased the froth concentrate are chalcopyrite (CuFeS2), from 0.8 to 1.0 L/min. This result revealed that quartz (SiO2), gypsum (CaSO4·2H2O), chlorite the formation of froth is lower due to the burst of (Mg, Fe)6(Si, Al)4O10(OH)8 and pyrite (FeS2). The bubbles before they reach the overflowing lip, peak of chalcopyrite (CuFeS2) and pyrite (FeS2) when an air injection rate is 0.8 L/min resulting in were not detected by XRD of silicate-containing a low copper recovery (74.7%)24. The high copper low-grade copper ore because of their low-grade recovery of 91.0% and a low copper grade of 12.6 and detection limitation of the equipment. mass% with the air injection of 1.0 L/min are due The froth concentrate containing 18.2 mass%

8 RESOURCES PROCESSING Copper Recovery from Silicate-Containing Low-Grade Copper Ore Using Flotation Followed by High-Pressure Oxidative Leaching

Fig. 10 SEM-EDS image of the solid residue obtained from atmospheric sulfuric acid leaching at 90°C for 60 min leaching. (Whitish areas show the sulfur (S0) passivation layer)

Fig. 8 XRD pattern of silicate-containing low-grade copper ore and its froth concentrate. (H2SO4) solutions at the temperature of 90°C, and agitation speed of 700 rpm for 1 hour. As shown in Fig. 9, the leaching efficiencies of copper and iron dissolution from the copper concentrate did not exceed 12% and 18% respectively with sul- phuric acid solutions. The solid residue obtained from the atmospheric leaching with sulfuric acid was analyzed by SEM-EDS. It can be seen from Fig. 10 that the surface of minerals in the froth concentrate was coated by sulfur-passivating layer. For that reason, it could be concluded that some of the chalcopyrite (CuFeS2) dissolved into the pregnant leach solution at an early stage and after that the formation of passivation layers of elemental sulfur (S0) on the chalcopyrite may im- pede the leaching kinetics of copper from its concentrate. 3.2.2 High-pressure oxidative leaching 3.2.2.1 Effect of total pressure Fig. 9 Leaching efficiencies of copper and iron as a function The effect of total pressure (oxygen and vapor of acid concentration. (H2SO4 concentration: 0–10 M, pulp density: 10%, temperature: 90°C, leaching time: pressure) on the copper and iron dissolution were 60 min and agitation speed: 700 rpm at atmospheric conducted in the range of 0.8 (without oxygen pressure) supply)–2.5 MPa. The other conditions were fixed at a pulp density of 10%, leaching temperature of 170°C, leaching time of 60 min, agitation speed Cu and 19.6 mass% Fe was further used for ex- of 700 rpm and sulfuric acid concentration of traction of copper with distilled water and sulfuric 1 M. It can be seen from the results shown in acid solutions using atmospheric and high- Fig. 11 that both copper and iron dissolution de- pressure acid/oxidative leaching. pend significantly on the total pressure. The cop- 3.2 Atmospheric and high-pressure oxidative per and iron leaching rate reached 52.2% and leaching of copper from the froth concentrate 40.7%, respectively at the total pressure of 1.5 3.2.1 Atmospheric acid leaching MPa, whereas their rates were 7.8% and 5.5%, Under the atmospheric oxidative condition, a which is similar to that rates under atmospheric sample of the froth concentrate was dissolved in leaching condition (Fig. 9), when the total pres- various concentrations (0–12 M) of sulfuric acid sure was 0.8 MPa (without oxygen gas addition in

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Fig. 11 The leaching efficiency of copper and iron as a func- Fig. 12 The leaching efficiency of copper and iron as a function of total pressure. (Total pressure: 0.8 (without tion of temperature. (Temperature: 100–180°C, oxygen supply)–2.0 MPa, H2SO4 concentration: H2SO4 concentration: 1.0 M, leaching time: 60 min, 1.0 M, leaching time: 60 min, pulp density: 10%, total pressure: 2.0 MPa, pulp density: 10%, and agi- temperature: 170°C and agitation speed: 700 rpm) tation speed: 700 rpm)

the system). It was observed that as a result of chalcopyrite (CuFeS2) is a highly temperature de- oxygen gas supply, sulfide minerals entirely oxi- pendent process. dized to sulfate under the high-pressure oxidation 3.2.2.3 Effect of sulfuric acid concentration condition at 170°C promoting the dissolution of To determine the efficiency of copper dissolu- copper and resulted in the higher copper leaching tion, the copper concentrate was leached with dis- rate. However, a further increase in the total pres- tilled water and different concentrations of sulfu- sure from 1.5 to 2.5 MPa does not produce a sub- ric acid solutions ranging from 0.5 to 1.0 M. The stantial increase in the dissolution rate of copper leaching experiments were conducted under dif- and iron. These results are in agreement with the ferent leaching times from 0 to 180 min while observation made by some researchers they con- other parameters like pulp density, total pressure cluded that due to surface saturation by oxygen, in an autoclave, agitation speed and temperature leaching is not affected by oxygen partial pressure were fixed at 10%, 2.0 MPa, 700 rpm and 180°C. after a certain pressure21,25. When the total pres- The results of copper dissolution with distilled sure was 2.0 MPa, a maximum copper dissolution water and with different concentrations of sulfuric rate of 59.7% was achieved within 60 min leach- acid solutions at varying leaching times are shown ing. Thus total pressure of 2.0 MPa was selected in Fig. 13 (a). It is interesting that an excellent in subsequent experiments. copper leaching rate (93.4%) achieved after 120 3.2.2.2 Effect of temperature min and the maximum (98.3%) copper dissolution The influence of the temperature ranging be- obtained after 180 min leaching with distilled wa- tween 100–180°C on the copper and iron leaching ter, while the iron dissolution did not exceed 5.4% rate was studied under certain conditions while (Fig. 13 (b)). The efficiency of copper dissolution the sulfuric acid concentration of 1 M, pulp densi- decreased with increasing the sulfuric acid con- ty of 10%, leaching time of 60 min, agitation centration and reached the minimum dissolution speed of 700 rpm and a total pressure of 2.0 MPa. of 70.8% with 1.0 M H2SO4 solution under the As shown in Fig. 12, the temperature has a large leaching condition. It was observed that the sulfu- effect on the leaching of copper and iron. A linear ric acid concentration gave an opposite effect on relationship between temperature and copper the dissolution of copper from the copper concen- leaching rate was found. With increasing the tem- trate. On the contrary, the efficiency of iron disso- perature from 100°C to 180°C, the copper leach- lution from the concentrate increased with in- ing rate was raised from 17.7% to 61.1%. It sug- creasing the sulfuric acid concentration and the gested that the dissolution of copper from iron dissolution rate reached 67.7% and 70.0%

10 RESOURCES PROCESSING Copper Recovery from Silicate-Containing Low-Grade Copper Ore Using Flotation Followed by High-Pressure Oxidative Leaching

Fig. 13 Leaching efficiencies of (a) copper and (b) iron as functions of leaching media and leaching time. (Dissolution: distilled water and H2SO4 solution (0.5–1.0 M), leaching time: 0–180 min, total pressure: 2.0 MPa, pulp density: 10%, temperature: 180°C and agitation speed: 700 rpm) after 120 min and 180 min leaching, respectively, when sulfuric acid concentration was 1.0 M (Fig. 13 (b)). The reduction of copper dissolution from the concentrate in the sulfuric acid solution may be induced by the solubility of oxygen in sulfuric acid solution containing copper and iron sulfates and the salting-out effect of the sulfuric acid solution26. It was observed that after high-pressure oxidative leaching with distilled water under the optimum condition, pH of slurry decreased up to pH of 1.47 while an initial slurry pH was 6.1. The major mineral phase in the solid residue is hematite (Fe2O3) as identified by XRD analysis (Fig. 14). When using 0.5 M and 1.0 M sulfuric acid solutions in the leaching, the initial pH val- ues of slurries of pH 0.72 and pH 0.49 were increased to pH 0.9 and pH 0.8, respectively. The XRD patterns of solid residues from 0.5 M and 1.0 M sulfuric acid leaching showed the quartz, Fig. 14 XRD patterns of the froth concentrate and residues chalcopyrite and hydronium jarosite ((H3O) after high-pressure oxidative leaching. Fe3(SO4)2(OH)6) peaks (Fig. 14). This result is consistent with previous data reported in the liter- ature27,28. It was seen that hematite is the favorite crease of the efficiency of copper dissolution in phase around pH 1.5 at 180°C and the major iron the acid solution under the high-pressure oxida- phase of the solid residue is transported to jarosite tive conditions. As a result, it is concluded that the when decreasing the pH to 1 from 1.5. On the oth- higher extraction (98.4%) of copper from the er hand, the complete copper dissolution (>98%) froth concentrate of a silicate-containing low- was obtained with water (0 M H2SO4), about 70% grade copper ore with water (without sulfuric copper leaching rate was achieved with 0.5 and acid) can be achieved by using high pressure oxi- 1.0 M sulfuric acid conditions due to the stability dative leaching process. areas of copper in an aqueous solution under the 3.2.2.4 Application of optimum high-pressure oxi- conditions was favorited as cupric ions (Cu2+)29. dative leaching for copper extraction However, it is evident from the Figs. 13–14 that The results of numerical experiments as de- chalcopyrite was remained in the solid residues of scribed above sections suggested that the opti- the sulfuric acid leaching and confirmed the de- mum high-pressure oxidative leaching conditions

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Table 8 The optimum high-pressure oxidative leaching Fe2(SO4)3 + 3H2O → Fe2O3 + 3H2SO4 (7) conditions for recovery of copper from froth concentrate 3Fe2(SO4)3 + 14H2O Cu leaching Cu concen- Parameters Conditions → 2(H3O)Fe3(SO4)2(OH)6 + 5H2SO4 (8) rate, % trate, g/L

H2SO4 concentration 0 M (Water) It is observed that the formation of hematite Temperature 180°C Total pressure 2.0 MPa 98.4 15.0 and jarosite by high-pressure oxidative leaching Pulp density 10% of the chalcopyrite with distilled water and sulfu- Leaching time 180 min ric acid solution depends not only on the leaching medium, but also on the pH value of slurry. Based on the experimental results, a flowchart for copper extraction from the froth concentrate is proposed for the recovery of copper from silica- were found to be as shown in Table 8. Under the containing low-grade copper ore, as seen in optimum high-pressure oxidation conditions, Fig. 15. The copper grade in the froth concentrate 98.4% of copper was leached with distilled water reached 18.1 mass% from 0.4 mass% under the and the obtained pregnant leach solution con- optimum flotation conditions, when the recovery tained 15.0 g/L copper (Table 8). of copper was 93.1%. The vast majority of copper The mechanism of copper dissolution from the (>98.4% Cu) was leached from the froth concen- froth concentrate, which contains chalcopyrite, trate in distilled water under the high-pressure can be described by Eq. (5). It means that sulfur in oxidation leaching conditions after flotation of the chalcopyrite entirely oxidized to sulfate under the silicate-containing low-grade copper ore. The high-pressure (2.0 MPa) oxidation condition at concentration of copper in the pregnant leach 180°C in the presence of oxygen gas, resulting an solution from high-pressure oxidative leaching excellent copper dissolution (>90%). reached to 15.0 g/L. On the other hand, more than 98% of iron was removed from the copper con- 2CuFeS2 + 8.5O2 + H2SO4 centrate by the high-pressure oxidation leaching → 2CuSO4 + Fe2(SO4)3 + H2O (5) process, and it precipitated as hematite in the solid residue. As a result, a selective and the highest It can be seen from Eq. (5) that the oxidation of copper recovery (>91%) was achieved by the com chalcopyrite is an acid consuming reaction, and bined process with flotation and high-pressure complete dissolution (>98%) of copper from the oxidation leaching. froth concentrate in distilled water was achieved under the high-pressure oxidative condition. It is 4. Conclusions obvious that pyrite in the froth concentrate can be a source of sulfuric acid as presented below: This study focused on the development of a hydrometallurgical process for recovering copper 2FeS2 + 7.5O2 + H2O → Fe2(SO4)3 + H2SO4 (6) from a silicate-containing low-grade copper ore by combined process of flotation and leaching. Oxygen gas (O2) supply in the slurry promotes Experimental results obtained can be concluded the oxidation of minerals such as pyrite and as bellow. chalcopyrite and the generation of sulfuric acid (1) Flotation process: A collector-less flotation which can dissolve copper efficiently from the was suggested for beneficiation of copper concentrate under the high-pressure and high- from the silicate-containing low-grade copper temperature condition. Hence, it can be said that ore due to its high self-induced ability under the presence of pyrite in the sample has a signifi- the flotation condition. The copper grade in cant for the production of sulfuric acid and disso- the froth concentrate reached 18.1 mass% lution of copper from the sample. from 0.4 mass% with an enrichment ratio of Furthermore, XRD measurement confirmed the 45 under the optimum flotation conditions, formation of hematite (Fe2O3) and hydronium while the recovery of copper was 93.1%. jarosite ((H3O)Fe3(SO4)2(OH)6) in the solid resi- (2) Leaching process: Experimental results re- dues after the high-pressure oxidative leaching vealed that the copper dissolution from the with distilled water and with sulfuric acid solution froth concentrate did not exceed 11.9% under due to undergo hydrolysis of ferric ions (Fe3+) in atmospheric leaching with sulfuric acid solu- aqueous solution (Fig. 14) by the following reaction. However, about 98.4% of copper could tions (Eqs. 7 & 8). be dissolved from the froth concentrate in dis-

12 RESOURCES PROCESSING Copper Recovery from Silicate-Containing Low-Grade Copper Ore Using Flotation Followed by High-Pressure Oxidative Leaching

Fig. 15 A flowchart for the copper recovery from the silicate-containing low-grade copper ore.

tilled water by high-pressure oxidation leach- Acknowledgements ing. The copper concentration in the pregnant leach solution reached to 15.0 g/L. Moreover, This research was supported by the Program over 98% of iron in copper concentrate was for Leading Graduate Schools, “New Frontier removed and precipitated as hematite in the Leader Program for Rare Metals and Resources” residue under the optimum conditions. of Japan Society for the Promotion of Science (3) The presence of pyrite (FeS2) in the sample (JSPS). We gratefully acknowledge their financial has a positive effect on copper leaching pro- support. cess, and it can be a source of sulfuric acid (H2SO4) production under the high-pressure References oxidative leaching condition promoting the copper dissolution. 1. M. Sethurajan, D. Huguenot, P.N. Lens, H.A. Horn, L.H. Figueiredo, E.D. Van Hullebusch: (4) It can be concluded that the leaching media, Journal of Environmental Management, 177, pressure (oxygen supply), temperature and pp. 26–35 (2016) acidity of the slurry are the important valu 2. S. Wang: JOM, 57, 7, pp. 48–51 (2005) ables for the dissolution of copper, and the 3. A.A. Baba, K.I. Ayinla, F.A. Adekola, M.K. formation of hematite and jarosite throughout Ghosh, O.S. Ayanda, R.B. Bale, S.R. Pradhan: In- the copper leaching from chalcopyrite ternational Journal of Mining Engineering and (silicate-containing low-grade copper ore) in Mineral Processing, 1, 1, pp. 1–16 (2012) the presence of aqueous solutions under 4. A.A. Baba, M.K. Ghosh, S.R. Pradhan, D.S. Rao, high-pressure oxidative leaching conditions. A. Baral, F.A. Adekola: Transactions of Nonfer- A selective (iron removal rate >98%) and effi- rous Metals Society of China, 24, 5, pp. 1587– 1595 (2014) cient copper extraction (Cu leaching rate >98%) 5. E.M. Córdoba, J.A. Muñoz, M.L. Blázquez, F. method is proposed for the recovery of copper González, A. Ballester: Hydrometallurgy, 93, 3, (total Cu recovery >91%) from the silicate- pp. 81–87 (2008) containing low-grade copper ore using flotation 6. D. Dreisinger: Developments in Mineral Process- followed by high-pressure oxidative leaching. ing, 15, pp. 825–848 (2005)

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