The Direct Leaching of Nickel Sulfide Flotation Concentrates - A Historic and State-of-the-Art Review Part II: Laboratory Investigations into Pressure Leaching

Figures & data

Table 1. Summary of laboratory investigations carried out on ammoniacal pressure leaching of nickel sulfide concentrates.

Reference	Nickel sulfide mineralogy	Nickel grade (wt% Ni)	Concentrate feed particle size	Mode of leaching	Leaching conditions
Totlani and Balachandra (1969)	Millerite, pentlandite, violarite & bravoite^a	6.8%	P50 – 55 74 µm (as received) P90 74 µm (reground)	Pressure leaching	4 – 10-h residence time, 190°F (87.7°C) − 250°F (121.1°C), 5 & 7% NH3, 150 psi (1034.2 kPa) to 500 psi (3447.4 kPa) total pressure, solids conc. 250 g/L
Tandon et al. (1971)	Pentlandite, millerite & violarite	1.56%	−149 µm	Pressure leaching	75 - 250°C 5–20% NH4OH, 0–35 g/L (NH4)2SO4, 588.4–2451.7 kPa O2 partial pressure, solids conc. 1.118–1.535 g/mL, 120 min residence time

^aNickel-rich variety of pyrite [(Fe, Ni, Co)S₂.

Table 2. Summary of general laboratory investigations into aqueous pressure oxidation of nickel sulfide concentrates in sulfate media.

Reference	Nickel sulfide mineralogy	Nickel grade (wt% Ni)	Concentrate feed particle size	Leaching conditions
Downes and Bruce (1955)	Nickeliferous pyrrhotite*	0.85%	Not specified	Pulp (S:L ratio from 2:1 to 4:1) leached at 110°C using either air or oxygen at 30–400 psig for 1–2 h at 480 rev/min followed by sulfur agglomeration at 125°C for 15 min
Vezina (1973)	Pentlandite	6.85%	Approximately 55% below 530 µm, reground in a ball mill for 0.5–2.5 h prior to leaching	30% solids, 110°C 551.6 kPa O2 (gauge), 8-h leaching time, sulfuric acid addition of 87.5 kg/tonne concentrate
Kuzeci, Li and Kammel (1989)	Pentlandite	6.3%	Not specified	80–110°C 120 min leaching time, 0.1–1 mol/L H2SO4 1 MPa O2 pressure, solids conc. 50–300 g/L, 0.5 g/L quebracho extract (sulfur dispersant)
Wang and Zhao (1993)	Pyrrhotite, other possible phase(s)	1.2%	Proportion below 36 µm varied from 20 to 100%	110–160°C 400–1500 kPa O2 pressure, acid: ore ratio of 2 to 3%, 500–900 rpm, 0.1 to 0.5% of an unidentified leach additive, L:S ratio from 1 to 10, up to 4.5 h leaching time
Kosyakov et al. (1995)	Pentlandite	0.72–2.44%	Not specified	Ambient pressure pre-leach – pH 0.6–4, 40–80°C 0.25 – 4-h leaching time, 250 g/L solids conc., 1000 rev/min, inert gas atmosphere (carbon dioxide) Autoclave leach − 80 (1.01 bar O2 pressure) & 140°C (6 bar O2 pressure), 1000 rev/min, up to 8 h @ 140°C up to 40 h at 80°C
Honey, Muir and Hunt (1997)	Pentlandite	14.5%	P80 80 µm	Oxygen pressure leaching for 2 h at 200°C (1000 kPa), 150°C (1000 kPa) and 110°C (250, 500 & 1000 kPa)
Pandey et al. (2002)	Seigenite^3, violarite, pentlandite & millerite	10.85%	P100 211 µm	1.085–5.42 MPa O2 pressure, 10–50 g/L H2SO4, 100 - 145°C 0.5–5 h, 500 rev/min, solids conc. 200 g/L
Magwaneng et al. (2017)	Pentlandite, pyrrhotite	0.99%	D50 70 µm	0–3 M H2SO4, 100 - 180°C 30–150 min, 1–3 MPa total pressure, S:L ratio 1:10, 400 rpm stirring speed
McDonald, Li and Austin (2020)	Concentrate #1 (fresh) & #2 (aged −4.5 years) - pentlandite, violarite	Concentrate #1–8.11% Concentrate #2–6.77%	P100 75 µm	Concentrate #1 - 250°C 750 rev/min, 10% w/w solids in deionized water or site process water†, 90 min, 700 kPa O2 pressure or 100 kPa O2 with O2 flow controlled at 2.4 L/min Concentrate #2 - 250°C 750 rev/min, 10% w/w solids in site process water* or 1 molal MgSO4 solution, 100 kPa O2, 150 min

*Nickel may be present in pyrrhotite either as exsolved lamellae of pentlandite or in solid solution within the pyrrhotite.

^bSiegenite – (Ni,Co)₃S₄.

†Site process water composition − 330 mg/L Mg²⁺, 3060 mg/L Na+, 110 mg/L Ca²⁺, 80 mg/L K+, 890 mg/L SO₄^2-, 6200 mg/L Cl-

Table 3. Summary of investigations into the influence of various leaching additives on aqueous pressure oxidation of nickel sulfide concentrates in sulfate media.

Reference	Nickel sulfide mineralogy	Nickel grade (wt% Ni)	Concentrate feed particle size	Additive	Leaching conditions
Chloride and copper (II) ions
Subramanian and Ferrajuolo (1976)	Concentrate #1 – pyrrhotite Concentrate #2 – pentlandite Concentrate #3 – pentlandite	Concentrate #1–1.27% Concentrate #2–7.05% Concentrate #3–4.38%	Concentrate #1 – P91 74 µm Concentrate #2 – P95 45 µm Concentrate #3 – P95 45 µm	0–10 g/L Cl- (as NaCl or CaCl2)	Concentrate #1 – 20% solids, 110°C 0.55 & 1.05 MPa O2 pressure, 1 & 2 h, 800 rev/min Concentrate #2 – 20% solids, 110°C 2.1 MPa O2 pressure, 1 & 2 h 0, 10 & 100 g/L H2SO4, 800 rev/min Concentrate #3 – 20% solids, 110°C 2.1 MPa O2 pressure, 2 h, 0 & 10 g/L H2SO4, 800 rev/min
Crossley et al. (1998)	Concentrate #1 – not specified Concentrate #2 – not specified	Concentrate #1–14.7% Concentrate #2–10.8%	Concentrate #1 – P80 106, 28 & 14 µm Concentrate #2 – P80 28 µm	1–20 g/L Cu^2+ plus 2–50 g/L Cl-	Concentrate #1 – 8.9% solids, 110°C 500 kPa O2 pressure, up to 3 h Concentrate #2 – 8.9% solids, 110 & 140°C 500–2000 kPa O2 pressure, up to 4 h
Sulfur dispersing agents
Ryan (1972)	Concentrate #1 – pyrrhotite Ni-sulfide concentrate(s) – Not specified	Concentrate #1–1.3% Ni-sulfide concentrate(s); 4.7–7%	Concentrate #1 – P100 44 µm Ni-sulfide concentrate(s) – 57 to 100% passing 44 µm	Mixture of primary, secondary, and tertiary amines 0–2.5 kg/tonne conc.	Concentrate #1 – 30% solids, 110 - 160°C 2026.5 kPa O2 pressure, 0.5 or 1 h Ni-sulfide concentrate(s) – 30% solids, 110 - 160°C 2026.5 kPa O2 pressure, 1 or 2 h
Jones, Krause and O’Neill (1974)	Pyrrhotite concentrates	0.91–1%	98–99.2% passing 149 µm	0–2.5 kg tall oil pitch/tonne conc.	20–30% solids, 130 - 150°C 1700 kPa O2 pressure, up to 120 min, 800 rpm agitation rate
Tarasov and Timoshenko (2004)	Pyrrhotite concentrates	3.5–3.6%	Not specified	2–7 kg lignosulfonate/tonne conc.	L:S ratio 1.5 to 1.6, 125 - 130°C 1000 kPa O2 pressure, 60 min leaching time
Tong (2009)	Concentrate #1 – pentlandite Concentrate #2 – pentlandite Concentrate #3 – pentlandite Concentrate #4 (Voisey’s Bay) - pentlandite	Concentrate #1–16.0% Concentrate #2–13.8% Concentrate #3–11.6% Concentrate #4 (Voisey’s Bay) − 20.8%	Concentrate #1 – P80 48 µm Concentrate #2 – P80 10 µm Concentrate #3 – P100 44 µm Concentrate #4 (Voisey’s Bay) – P80 50 µm	0–50 kg/tonne of either lignosulfonate, humic acid, quebracho or ortho-phenylene diamine (OPD) 0–10 g/L Cl- (as NaCl)	Concentrate #1 & #2 – 20 or 250 g/L solids, 0.5 mol/L H2SO4, 140°C 690 kPa O2 pressure, 2-h leaching time, 700 rpm agitation rate Concentrate #3 & #4 – 250 g/L solids, 40–49 g/L H2SO4, 150°C 690 kPa O2 pressure, 1.5 or 2-h leaching time, 800 rpm agitation rate
Guerra and Shepherd (2011)	Pyrrhotite	0.628%	D80 100 µm	10 g/L PTFE (Dyneon 1641)	10 g/L solids, 50 g/L H2SO4, 2.25 g/L Fe2(SO4)3, 300–1200 rpm, 690 kPa O2 pressure, 140°C up to 100 min leaching time
Naftal et al. (2015)	Concentrates #1 & #2 – pyrrhotite & pentlandite	Concentrates #1 & #2–2.17% & 4.22%	Concentrates #1 & #2–79.7% & 80.3% passing 44 µm	Concentrate #1–2.7–18.9 kg lignosulfonate/tonne conc., 246–448 kg tailings/tonne conc. Concentrate #2–12.4–44.8 kg lignosulfonate/tonne conc., 80–706 kg tailings/tonne conc.	Initial L:S ratio of 1.75 mL/g, 150°C 1000–3000 rpm, 900 kPa O2 pressure, 90 min leaching time
Nitrogen species catalyzed (NSC) leaching
Anderson (2004)	PGM concentrate & ore – pentlandite Ni concentrate – pentlandite	PGM concentrate & ore − 19.5% & 0.74% Ni concentrate − 16.3%	PGM concentrate & ore – P80 40 µm Ni concentrate – P80 10 µm	2 g/L (presumably as nitrite, NO2-)	PGM concentrate & ore − 140°C 50 g/L H2SO4, 100 g/L solids, 45 min leaching time, 620 kPa (gauge) pressure PGM concentrate & ore − 125°C 10 g/L H2SO4, 100 g/L solids, 20 min leaching time, 620 kPa (gauge) pressure

Table 4. Summary of laboratory investigations carried out to date on mechanochemical treatment of nickel sulfide concentrates prior to oxygen pressure leaching in aqueous sulfate media.

Reference	Nickel sulfide mineralogy	Nickel grade (wt% Ni)	Milling equipment	Milling conditions	Leaching media	Leaching conditions
Pawlek (1976)	Pentlandite in both concentrates	7.55% & 3.03%	Attritor	15 min grinding time	Water or sulfuric acid	100 - 180°C 30- or 45-min leaching time, 202.65 or 979.1 kPa O2 pressure, Ag+ or Cl- additions (concentrations not specified)
Kulebakin et al. (1981)	Nickeliferous Pyrrhotite	5.47%	Planetary mill, 500 g of 3 mm steel beads	25 g concentrate, 125 mL water (wet milling only), 7–420 sec milling time	Pressure oxidation in dilute H2SO4	108°C 60 min, 506.6 kPa O2 pressure, L:S ratio of 5:1 & 1.5:1
Kulebakin and Sirkis (1983)	Pentlandite	26.4%	Planetary mill, 500 g of 3 mm steel beads	25 g concentrate, 37.5 mL water, 0.5–30 min milling time	High & ambient pressure oxidation in water	Pressure oxidation − 108°C & 130°C 120 min, 506.6 kPa O2 pressure, solids conc. 20 g/L Ambient pressure – 90°C 120 min, 101.3 kPa O2 pressure, solids conc. 20 g/L

*Mass of grinding media to mass of concentrate.

Figure 1. (a) Simplified HPAL flowsheet for Ni and Co extraction from nickel laterites and (b) co-processing of nickel laterite and nickel sulfide concentrate(s) via pressure oxidation. Adapted from Ferron and Fleming, Crossley et al., 1998.

Table 5. Summary of laboratory investigations carried out to date on co-processing of nickel laterite ores and nickeliferous sulfide concentrates via oxygen pressure leaching.

Reference	Nickel laterite feed grade	Nickel-bearing sulfide feed	Nickel grade in sulfide feed	Feed particle size (a) laterite (b) sulfidic feed	Leaching conditions
Ferron and Fleming (2004)	1.36%	A – Nickeliferous pyrrhotite tailings B – Cu-Ni-PGM bearing concentrate	A − 0.94% B − 3.05%	Not specified	250°C 26 or 27% solids, 690 kPa O2 overpressure, 120 min residence time, 0 or 10 g/L NaCl, equivalent H2SO4 addition of 256–560 kg/tonne, 600 rpm agitation rate
Quinn, Turner and van der Muelen (2009)	70:30 laterite sulfide blend grading 1.24 or 1.63% Ni	Nickel sulfide ore, grade & mineralogy not specified	Not specified	−6 mm for feed blend, milled further in PLS prior to pressure leaching	220 – 240°C 28 wt% solids in heap leach liquor containing dissolved metal sulfates and residual H2SO4 ± chlorides, 800–1100 kPa O2 overpressure, 250 min residence time, supplementary H2SO4 addition of 0 or 150 kg/tonne, 350 rpm agitation rate
McDonald et al. (2012), McDonald and Li (2020)	High grade laterite blend grading 1.90% Ni	Low-grade Ni-sulfide concentrate	8.11%	(a) −500 µm (b) −75 µm	250°C 30 or 40 wt% sulfide-laterite blend in site process water*, 100–250 kPa O2 overpressure, 90 (laterite only) or 120 min (laterite-sulfide blend) residence time, equivalent H2SO4 addition of 247–437 kg/tonne, 750 rpm agitation rate

*Site process water composition − 210 mg/L Mg²⁺, 1920 mg/L Na+, 70 mg/L Ca²⁺, 50 mg/L K+, 560 mg/L SO₄^2- & 3300 mg/L Cl⁻

Table 6. Summary of laboratory investigations carried out on pressure leaching of nickel sulfide concentrates in lixiviant systems other than ammoniacal solutions and sulfuric acid/sulfate-based systems.

Reference	Nickel sulfide mineralogy	Nickel grade (wt% Ni)	Concentrate feed particle size	Leaching conditions
Chloride-based systems
Hubli, Mukherjee and Gupta (1983)	Concentrate #1 & #2 – Pentlandite, bravoite, millerite & violarite	Concentrate #1–8% Concentrate #2–7.6%	P50 150 µm	pH 2 HCl solution, 90 - 120°C ferrous chloride addition of 60–100% stoichiometry, 2–10 h leaching time, 308–515 kPa O2 pressure (gauge), solids conc. 500 g/L
Mukherjee, Hubli and Gupta (1985)	Pentlandite, millerite, minor bravoite and violarite	Concentrate #1–8% Concentrate #2–3.9%	Concentrate #1 – P100 150 µm Concentrate #2 – P100 45 µm	1–2 M HCl, 4 M NaCl, 0–1 M Cu(II), 90 - 120°C 100–510 kPa O2 pressure, up to 7 h leaching time, 10–50% w/v pulp density
Smyres, Lei and Carnahan (1985)	Pentlandite	1.6%	P97 74 µm*	100 – 120°C 450 rpm, 344.7 kPa (gauge) oxygen pressure, 18.2%w/v pulp density, 0.186–0.285 g HCl per g concentrate, up to 5.5 h leaching time
Hubli et al. (1995)	Millerite	4.5%	− 45 + 23 µm	0.03–1 M HCl, 3 M NaCl, 0–1 M Cu(II), 60 - 90°C up to 240 min leaching time, 500 rev/min, inert atmosphere (gas not specified), pulp density not specified
Nitric acid-based systems
Habashi (1973)	Pentlandite	1.2%	P100 37 µm	Pressure leaching − 110°C 7.5–45% HNO3, 4-h leach time, L:S ratio 4:1 O2 pressure leaching − 110°C 7.5% HNO3, 689.5 kPa O2 overpressure, 4-h leach time, L:S ratio 4:1 Atmospheric pressure leaching − 100°C 30% HNO3, 8-h leach time, L:S ratio 4:1

*Concentrate had agglomerated during storage; agglomerates were crushed to below 149 µm in small scale experiments and concentrate used as received in large scale experiments.

Totlani, M., and J. Balachandra. 1969. Studies in leaching of sulphide concentrates of copper and nickel from indigenous source. Symposium on Non-Ferrous Metals Technology 23–26.

 Google Scholar

Tandon, D., A. Saha, M. Mahanty, and P. Bhatnagar. 1971. Studies on ammoniacal pressure leaching of bulk Ni-Cu-Mo sulphide concentrate. NML Technical Journal 13 (4):87–92.

 Google Scholar

Downes, K. W., and R. W. Bruce. 1955. The recovery of elemental sulfur from pyrite and pyrrhotite. Transactions of the Canadian Institute of Mining, Metallurgy and Petroleum 58:77–82.

 Google Scholar

Vezina, J. A. 1973. Further studies on acid pressure leaching a chalcopyrite-pentlandite-pyrrhotite concentrate. CIM Transactions LXXVI:89–92.

 Google Scholar

Kuzeci, E., X.-M. Li, and R. Kammel. 1989. Acid leaching of a nickel sulphide concentrate (in German). Metall 43 (5):434–39.

 Google Scholar

Wang, Z., and L. Zhao, 1993. The acidic oxidizing pressure leaching of Jinchuan pyrrhotite concentrate. In Proceedings of the International Conference on Mining and Metallurgy of Complex Nickel Ores. Beijing: International Academic Publishers, September 5 - 8, Jinchang, China Fu, C., 267–73.

 Google Scholar

Kosyakov, A., M. Hamalainen, P. Gromov, A. Kasikov, V. Masloboev, and Y. Neradovsky. 1995. Autoclave processing of low grade copper-nickel concentrates. Hydrometallurgy 39 (1–3):223–34. doi:10.1016/0304-386X(95)00037-H.

 Web of Science ®Google Scholar

Honey, D. J., D. M. Muir, and P. R. Hunt. 1997. Hydrometallurgical processing options for nickel sulphide concentrates. In Proceedings of the nickel-cobalt 97 international symposium, ed. W. C. Cooper, I. Mihaylov, pp. 15–29. Sudbury, ON, Canada: The Metallurgical Society of CIM.

 Google Scholar

Pandey, B., D. Bagchi, V. Kumar, A. Agrawal, and Premchand. 2002. Pressure sulpuric acid leaching of a sulphide concentrate to recover copper, nickel and cobalt. Mineral Processing and Extractive Metallurgy 111 (2):106–09. doi:10.1179/mpm.2002.111.2.106.

 Google Scholar

Magwaneng, R. S., A. Batnasan, K. Haga, and A. Shibayama. 2017. Development of a combined Flotation and high pressure leaching process for copper and nickel recovery from mine tailing. Journal of Minerals and Materials Characterization and Engineering 5 (03):118. doi:10.4236/jmmce.2017.53010.

 Google Scholar

McDonald, R. G., J. Li, and P. J. Austin. 2020. High temperature pressure oxidation of a low-grade nickel sulfide concentrate with control of the residue composition. Minerals 10 (3):249. doi:10.3390/min10030249.

 Web of Science ®Google Scholar

Subramanian, K., and R. Ferrajuolo. 1976. Oxygen pressure leaching of Fe-Ni-Cu sulfide concentrates at 110°C—effect of low chloride addition. Hydrometallurgy 2 (2):117–25. doi:10.1016/0304-386X(76)90022-0.

 Google Scholar

Crossley, B., T. Gerrans, D. Honey, and D. Muir, inventor(s); WMC Resources Ltd, assignee. 1998. Leaching of a nickel sulphide concentrate. AU Patent 8077698A. Filed August 18.

 Google Scholar

Ryan, P. J. M., inventor; Inco Ltd, assignee. 1972. Leaching Sulfide Concentrates. CA Patent 964867A. Filed May 8.

 Google Scholar

Jones, W. E., E. Krause, and C. E. O’-Neill, inventor(s); Inco Ltd, assignee. 1974. High Temperature Leaching of Sulfide Ores. CA Patent 1024355A. Filed November 5.

 Google Scholar

Tarasov, A. V., and E. M. Timoshenko. 2004. Investigation of method of addition of surfactant to the pressure leaching of nickel-containing pyrrhotite concentrates. In Proceedings of the international conference on the use of pressure vessels for metal extraction and recovery, ed. M. J. Collins and V. G. Papangelakis, 295–304. Banff, Alberta, Canada: Canadian Institute of Mining, Metallurgy and Petroleum.

 Google Scholar

Tong, L. 2009. Sulfur dispersing agents for nickel sulfide leaching above the melting point of sulfur. Doctoral diss., University of British Columbia, 258 pp.

 Google Scholar

Guerra, E., and J. L. Shepherd. 2011. The effect of polytetrafluoroethylene on pressure oxidation of nickeliferous pyrrhotite. Hydrometallurgy 106 (3–4):179–82. doi:10.1016/j.hydromet.2011.01.004.

 Web of Science ®Google Scholar

Naftal, M. N., S. S. Naboichenko, E. V. Salimzhanova, O. V. Bol’-Shakova, and T. P. Saverskaya. 2015. Influence of various stabilizing factors on an elemental sulfur emulsion during high-temperature leaching of nickel-pyrrhotine concentrates. Russian Metallurgy (Metally) 2015 (3):171–84. doi:10.1134/S0036029515030052.

 Google Scholar

Anderson, C. G. 2004. Applications of NSC pressure leaching. In Proceedings of the international conference on the use of pressure vessels for metal extraction and recovery, ed. M. J. Collins and V. G. Papangelakis, 855–86. Banff, Alberta, Canada: Canadian Institute of Mining, Metallurgy and Petroleum.

 Google Scholar

Pawlek, F. E. 1976. The influence of grain size and mineralogical composition on the leachability of copper concentrates. In Extractive metallugy of copper - hydrometallurgy and electrowinning, ed. J. C. Yannopoulos and J. C. Agarwal, 691–705. Nevada, USA: American Institute of Mining, Metallurgical and Petroleum Engineers, Las Vegas.

 Google Scholar

Kulebakin, V. G., V. N. Fedorov, A. L. Sirkis, Y. T. Mazurov, V. A. Mikhin, and T. S. Yusupov. 1981. Autoclave leaching of activated pyrhotite concentrate. Soviet Mining 17 (6):566–69. doi:10.1007/BF02497998.

 Google Scholar

Kulebakin, V., and A. Sirkis. 1983. Mechanism of autoclave-oxidative leaching of activated pentlandite. Soviet Mining 19 (4):330–34. doi:10.1007/BF02505371.

 Google Scholar

Ferron, C., and C. Fleming. 2004. Co-Treatment of limonitic laterites and sulphur-bearing materials as an alternative to the HPAL process. International laterite nickel symposium, Charlotte, North Carolina, USA, 2004, Mar 14-18.Warrendale, Pennsylvania, USA: The Minerals, Metals and Materials Society, 245–61.

 Google Scholar

Quinn, J., J. Turner, and D. van der Muelen. 2009. The Combined Pressure Acid Leach (CPAL) process, batch and piloting testwork. In Recent Advances in mineral processing plant design, ed. D. Malhotra, P. R. Taylor, E. Spiller and M. LeVier, 140–54, Colorado, USA: Society for Mining, Metallurgy and Exploration (SME).

 Google Scholar

McDonald, R., M. Rodriguez, J. Li, D. Robinson, M. Jackson, and T. Hosken. 2012. The co-processing of nickel sulphide and laterite materials using low oxygen pressures Pressure Hydrometallurgy 2012, Proc. of the 42nd Annual Hydrometallurgy Meeting, Niagara Falls, Canada, September 30 - October 3. Westmount, Canada: Canadian Institute of Mining, Metallurgy and Petroleum, 211–25.

 Google Scholar

McDonald, R. G., and J. Li. 2020. The high temperature co-processing of nickel sulfide and nickel laterite sources. Minerals 10 (4):351. doi:10.3390/min10040351.

 Web of Science ®Google Scholar

Hubli, R., T. Mukherjee, and C. Gupta. 1983. A ferrous chloride-oxygen leach process for nickel-copper sulphide concentrations. Hydrometallurgy 10 (3):359–66. doi:10.1016/0304-386X(83)90065-8.

 Web of Science ®Google Scholar

Mukherjee, T., R. Hubli, and C. Gupta. 1985. A cupric chloride-oxygen leach process for a nickel-copper sulphide concentrate. Hydrometallurgy 15 (1):25–32. doi:10.1016/0304-386X(85)90064-7.

 Web of Science ®Google Scholar

Smyres, G. A., K. P. Lei, and T. G. Carnahan. 1985. Hydrochloric acid-oxygen leaching and metal recovery from a copper-nickel bulk sulfide concentrate. Report of investigations, 8999. US Department of the Interior, Bureau of Mines, 17.

 Google Scholar

Hubli, R., T. Mukherjee, S. Venkatachalam, R. Bautista, and C. Gupta. 1995. Kinetics of millerite dissolution in cupric chloride solutions. Hydrometallurgy 38 (2):149–59. doi:10.1016/0304-386X(94)00047-7.

 Web of Science ®Google Scholar

Habashi, F. 1973. Treatment of a low-grade nickel copper sulfide concentrate by nitric acid. transactions of the society of mining engineers of AIME (American Institute of Mining). Metallurgical and Petroleum Engineers 254 (3):228–30.

 Google Scholar