International Conference on Mining, Mineral Processing and Metallurgical Engineering (ICMMME'2013) April 15-16, 2013 Johannesburg (South Africa) An investigation of aggressive leaching of Uitkomst complex (South Africa) Ore and tailings M.S. Madiba, and A. F. Mulaba- Bafubiandi Abstract— Nickel in South Africa is mainly produced as I. INTRODUCTION by-product in the production of PGMS from the Uitkomst HE roughly boat shaped Uikomst complex in South Complex deposits. Pentlandite as the main nickel-bearing T Africa with a trough at the base is related to the Bushveld mineral is disseminated within a sulphide matrix of pyrrhotite, Complex in both composition and age except that metal pyrite and chalcopyrite while accessories are chromite and sulfides include pyrrhotite, pentlandite, chalcopyrite and platinum in solid solution with the sulphides. The sulphides pyrite predominate more than the platinum group elements. are hosted mainly in ultramafic rocks. Head grade in the South Africa nickel output was expected to probably decline, from earlier 0.7 to more recently 0.3% or even lower; standard froth flotation yields a concentrate of 7 – 9% nickel increase from 2008 to 2015 (Yager, et al., 2008). Particularly, at a recovery of up to 70%. by 2011 it was projected to nearly double because of A higher nickel recovery from such low – grade feed stock increased capacity at local nickel mines and the PGM mines is desired. To achieve this aggressive leaching as an (Yager, et al., 2007). Out of all South African mines alternative to flotation was employed on run of mine as well producing nickel, there is only one which is a primary and as on tailings materials. The usage of strong hydrochloric and only nickel concentrator while the rest produce nickel as co- sulphuric acid and also aqueous ammonia solutions at various product or by product. As a result of the latter statement this liquid-to-solid ratios was carried out at room temperature. research was narrowed to investigate only materials from the The tests were conducted on crushed run of mine ore from Uitkomst complex particularly the main mineralized zone MMZ zone in the Uitkomst complex of South Africa, with a (MMZ). head grade of 0.25% nickel and fine flotation tailings retrieved The concentrator process comprises of crushing, milling from dump with a head grade of 0.15% nickel obtained from a and flotation. The flow sheet consists of two stage crushing nickel mine. The materials were ground to 65% < 75 µm in a and one stage milling. Average run of mill is 0.58% grade laboratory rod mill before being leached. while concentrate grade is 9%. The concentrate is toll smelted Leaching of tailings in strong acids at room temperature and refined while the tailings are presently discarded. resulted in high nickel dissolutions, for example, close to Challenges to remove nickel from the feed due to 100% after digestion for 3hours with 1-molar hydrochloric or complexity of mineralogy of South African uitkomst complex sulphuric acid, but only at high liquid/solids ratios of L/S and chromite content coupled with consideration by mining ~100; 80% dissolution can be achieved after only 30 minutes companies to exploit low grade ore and tailings, have under these circumstances. motivated the need to investigate in this study alternative Leaching ore with acids was slower and less complete: process for the recovery of nickel from sulphide ore and hydrochloric was superior to sulphuric acid (~30% vs. 15% tailings. So far, current operation of South African mine nickel dissolution) and surprisingly lower liquid/solids ratios consists of floating directly after milling which gives were beneficial. Longer leaching times would yield higher recoveries of about 60 to 75%. The following is a proposed dissolutions. concentration processes which was under investigation to Leaching with aqueous ammonia solutions resulted in only attempt to improve recoveries (1) replacing flotation step by poor nickel dissolutions for both tailings and ore (~1%). leaching process using sulphuric acid, hydrochloric acid and ammonium hydroxide, independently. Keywords— hydrochloric acid; sulphuric acid; aqueous ammonia solutions; pentlandite; grade and recovery. Leaching process – The two materials which were run-of- mine ore (ROM), sulphide-nickel ore, and its tailings were examined. What is interesting is, most of operations utilize sulphuric acid leaching on oxidised ores rather than sulphides M.S. Madiba, and A. F. Mulaba- Bafubiandi are with Mineral Processing as it is generally known and proven that oxidised materials and Technology Research Group, Metallurgy Department, Faculty of leach reasonably easily compared to sulphides. Engineering and the Built Environment, University of Johannesburg, PO Box 5216, Wits 2050, South Africa, [email protected]. 154 International Conference on Mining, Mineral Processing and Metallurgical Engineering (ICMMME'2013) April 15-16, 2013 Johannesburg (South Africa) II. EXPERIMENTAL III. RESULTS A. Communition process A. Characterization of two samples: Nickel ore and flotation tailings were received, crushed and Representative portions of the sample were analyzed by milled before being pulverized. The milled sample was qualitative, backscattered SEM to determine the major gangue pulverized using a Siebtechnik pulveriser to make materials and ore minerals and their relative abundances. This analysis finer for production of effective and reliable results. The revealed that the samples consisted of various proportions of pulverized 1 kg’s samples of nickel sulfide ore were further silicate minerals which were found to be particularly abundant split into 250 g which were further divided into 10 on both run of mine ore and tailings; pyrite was also detected representative portions using small spinning riffle of 250 ml as one of the primary mineral contained in both samples and cups to give 25 g each. The 25 g samples were collected and consisted generally of pentlandite and pyrrhotite within the labeled for characterization tests and leaching testwork. sample (approximately 0.15% in flotation tailings and 0.25% in run of mine samples); the presence of iron oxide was B. Particle size analyzer confirmed with traces of magnesium, aluminum and chloride Microtrac, S3500, particle size analyzer with Tri- laser in the flotation tailings sample. Pentlandite and pyrrhotite technology was utilized on two materials to determine the minerals often occurred as discrete, liberated grains (often <50 contained percentage of particles of less than 75 μm for µm in float tailings and less than 75 µm in run of mine leaching process as it has been established that the finer the samples) and to a lesser extent as intergrowths with particle size the better the leaching behavior, resulting in transparent gangue. EDAX examination of the silicates improved recovery (Ntengwe, 2010; David, 2007). minerals confirmed that these contained predominantly iron, The pulverised 25 g’s samples were leached by; calcium, magnesium and aluminum; being present in smaller • Hydrochloric acid leaching at different leaching duration amounts were sodium, manganese, titanium, potassium, • Sulphuric acid leaching at different leaching duration chromium and chloride traces. The principal nickel bearing • Ammonium hydroxide leaching at different leaching sulphide minerals contained mainly pyrhotite and pentlandite, duration. and small traces of chromium. C. Hydrochloric acid leaching B. Particle size analyser: Two materials were leached with HCl independently for 30, The particle size used was found to be approximately at 45, 60, 90, 120, and 180 minutes to determine the effect of 70% passing 75 μm for both two materials when manual wet leaching time on nickel recovery at three lixiviant screening was employed and particle size analyzer used, concentrations of 1, 3 and 5M HCl acid. Solid/liquid ratio identified the amount of particles to be at approximately 80% used was 1: 20. 300 ml was added to 15 g of solid sample. For passing 75 μm which implies that efficient leaching might be each lixiviant concentration, the results of Ni Recovery versus expected since it is established that the finer the material the leaching time at constant solid ratio of 1:20 were plotted. more efficient is the leaching process. Then three graph lines of each lixiviant concentration were compared to determine the lixiviant concentration which gives C. Results of leaching of the tailings at different leaching improved Ni recovery. duration S/L ratio of 1: 40 and 1: 100 were investigated, using the Results of hydrochloric acid leaching – The following same procedure as for 1:20 solid ratio above. Then S/L ratios results were obtained when hydrochloric acid leach was of each lixiviant concentrations were also compared to employed at different leaching duration; 100 examine the one that renders better results. 100 1M HCl 90 3M HCl 90 80 D. Sulphuric acid leaching 80 70 70 Ni Rec. Ni Rec. 60 Methods used were the same as hydrochloric leaching 60 except that in this case sulphuric acid was used instead of 50 50 40 40 hydrochloric acid as lixiviant. 30 30 0 1 2 3 4 5 0 1 2 3 4 5 Time(hr) Time(hr) E. Ammonium hydroxide leaching 100 5M HCl 1:20 90 1:40 Same leaching method as sulphuric acid and hydrochloric 80 1:100 acid leaching was applied except that ammonium hydroxide 70 was used as lixiviant. Ni Rec. 60 50 After all mentioned leaching tests conducted, the results 40 30 that rendered better performance for each lixiviant were 0 1 2 3 4 5 identified and eventually compared. In all cases, AAS was Time(hr) used to analyse the amount of nickel dissolved. Fig. 1: 1M HCl, 3M HCl and 5M HCl leaching of the tailings at different S/L ratio 155 International Conference on Mining, Mineral Processing and Metallurgical Engineering (ICMMME'2013) April 15-16, 2013 Johannesburg (South Africa) 100 100 The results presented in the figure above also confirm that as 3M H SO 1M H2SO4 2 4 the leaching time increases the recovery also increases.