III / NICKEL AND COBALT ORES: FLOTATION 3491 NICKEL AND COBALT ORES: FLOTATION G. V. Rao, Regional Research Laboratory, Council of Sotation. Talc and other naturally hydrophobic mag- Scientific and Industrial Research, Bhubaneswar, India nesia-bearing minerals have a tendency to Soat with R Copyright ^ 2000 Academic Press sul des, resulting in a concentrate exceptionally high in magnesia. The presence of magnesia causes viscos- ity problems in the slag during smelting. Magnesia Introduction also promotes conditions favourable to hetero-coagu- R R Most of the world’s nickel is extracted from the lation of minerals, especially ne sul des with coarse gangue minerals, thus leading to nickel loss. mineral pentlandite, (Ni, Fe)9S8, which frequently oc- curs in ores containing predominantly pyrrhotite and R various non-sul des, some of which contain magne- Flotation Practice in Nickel Sul\de sium (Table 1). The nickel content in such sulRde ores is generally low (0.2}3%) and varies from place Deposits to place in the same deposit. The low nickel content Nickel sulRde minerals such as pentlandite can, in R of the present-day nickel sul de ores renders them general, be separated from their gangue by Sotation unsuitable for either direct smelting or hydrometal- using a thiol group of collectors like xanthates and R lurgical extraction, thus requiring bene ciation. alkyl dithiophosphates in the presence of variety of R The usual method of nickel extraction from sul de activators, depressants and dispersants. Since nickel ores is through the production of nickel matte after sulRdes contain other sulRdes such as pyrrhotite, pent- enriching the nickel content of the ore. This is com- landite and chalcopyrite, the enrichment of nickel is S monly carried out by magnetic separation, otation, generally carried out by two methods: or a combination of both after the ore is comminuted to below 200 m in size. The enrichment depends 1. production of bulk concentrate containing all sul- R upon the degree of rejection of the other sul de and Rdes together as smelter feed; R non-sul de gangue. The maximum grade of nickel 2. production of bulk chalcopyrite}pentlandite con- S achieved by otation is around 28% Ni. centrate by preferentially depressing the pyrrhotite followed by selective Sotation of chalcopyrite and Problems Associated with Sul\de Mineral Flotation pentlandite. The main problem encountered is selectivity of pyr- rhotite and also in some cases chalcopyrite during Although bulk Sotation of all sulRdes is relatively Sotation. A clean and satisfactory separation of simple the presence of pyrrhotite, which contains pentlandite from pyrrhotite by Sotation is difRcult minor amounts of nickel but substantial amounts of in practice since pyrrhotite typically contains inter- sulfur (Table 2), causes excessive sulfur dioxide emis- grown inclusions of pentlandite as well as nickel in sions during smelting of the concentrate. solid solution. In fact pyrrhotite often contains Since most of the sulfur contained in the Sotation 0.5}1% Ni that cannot be separated by physical concentrate is emitted from pyrrhotite, the rejection methods. The common occurrence of both mono- of pyrrhotite is important, especially in Canada, ow- clinic (magnetic) and the hexagonal (nonmagnetic) ing to stringent limits imposed on SO2 emissions from forms of pyrrhotite in association with pentlandite smelters in that country. The pyrrhotite from the also poses problems. Another type of alteration Canadian ores in the Sudbury region occurs in two which adversely effects Sotation recoveries is crystallographic forms having distinct characteristics. that tochilinite has Sotation properties similar to that The monoclinic pyrrhotite, being magnetic, can be of pyrrhotite. As a consequence it either reports to partly rejected by magnetic separation, while the the Sotation tailings, thereby decreasing the nickel hexagonal pyrrhotite is separated by Sotation. recovery, or, if it is effectively Soated, a signiR- Pyrrhotite is known to Soat poorly in alkaline cant amount of pyrrhotite accompanies it, diluting media; therefore the general practice is to selectively the nickel grade in the concentrate. Soat pentlandite from pyrrhotite, by maintaining There is a distinct difference in silicate min- a highly alkaline pH with cyanide as depressant and eralogy between types of host rock, which have their using thiols like xanthates or dithiophosphates as own problems with respect to rejection of gangue by collectors. Although it is possible to reject signiRcant 3492 III / NICKEL AND COBALT ORES: FLOTATION Table 1 Principal nickel and nickeliferous minerals in nickel tives, although their cost accounts for as much as sulfide deposits 60% of the reagent cost incurred in the plant. Formula Nickel content (%) Developments in Flotation Nickel minerals \ Primary Flotation of Nickel Sul de Minerals Pentlandite (Ni,Fe)9S8 25}41 Millerite NiS 65 Collectorless Wotation It is known that chalcopyrite Heazelwoodite Ni3S2 73 and pyrrhotite can be Soated without collectors } Geradorffite (Ni,Co,Fe)AsS 15 35 at electrode potentials of #400 mV and #50 mV Nickeline NiAs 44 S Awaruite Ni-Fe 25}75 respectively, and pyrite does not oat even at #700 mV. The underlying mechanism causing hy- Secondary drophobicity, although obscure, is attributed to sur- Violarite Ni FeS 33}40 2 4 face oxidation, formation of elemental sulfur and Bravoite (Ni,Fe)S2 17}24 Haapalaite 4(Fe,Ni)S.3(Mg,Fe)OH2 partial dissolution of mineral surface, leaving a ) Annabergite (Ni,Co)3(AsO4)2 8H2O sulfur-rich layer. S Nickeliferous minerals Collectorless otation studies on three lean nickel Primary sulRde ores, from Outokumpu Finnmines Oy (Enon- Pyrrhotite Fe(1\x)S Up to 1.5 koski (0.31% Ni and 0.14% Cu), Vammala (0.9% Mackinawite (Fe,Ni)9S8 Up to 9 Ni and 0.45% Cu) and Hitura (0.32% Ni and 0.11% Arsenopyrite FeAsS Up to 0.5 Cu)), ground in steel and ceramic mills at a pH range Secondary of 3}12 using polypropylene ether as frother revealed Pyrite FeS2 Up to 12 interesting details. Marcasite FeS2 Up to 6 The pentlandite, chalcopyrite and pyrrhotite from Smythite Fe3.25S4 Up to 5 S Tochilinite 6Fe S.5(Mg,Fe)(OH) Up to 5 Enonkoski ore ground in a steel mill oated easily 0.9 2 } Magnetite Fe3O4 Up to 1 without a collector at low pH values of 3 5 (Figure 1). The Soatability of these three minerals (Reprinted with permission from The Minerals, Metals and Mater- was improved when the same ore was ground in ials Society (TMS).) a ceramic mill (Figure 2). This suggests that collector- less Sotation in acidic pH can be adopted as a low- cost pre-concentration phase to obtain a bulk sulRde amounts of pyrrhotite in this way, the concomitant concentrate. pentlandite losses into Sotation tailings are highly unsatisfactory, in addition to the problem of water contamination due to cyanide. Effect of sulfur dioxide It is possible to effec- A typical example of a nickel sulRde plant operat- tively separate Cu}Ni ores from pyrrhotite gangue in S ing with highly oatable talcose gangue is at Trojan the absence of a collector, but with SO2 and an mine in Zimbabwe. The Zimbabwean sulRde deposits effective complexing agent such as diethylenetri- contain very low grades of nickel (0.6%) and copper amine (DETA) and a frother. The depressant action } S R (0.04 0.4%). The easily oatable non-sul de min- of SO2, which is greatly enhanced in the presence of erals dilute the grade of the concentrate and also DETA, does not affect the recovery of chal- result in nickel losses into the tailings. This problem copyrite. Figure 3 shows that 93% of chalcopyrite can be overcome by using depressants like car- can be recovered in 12 min in the presence of SO2, boxymethylcellulose (CMC) and guargum deriva- but the co-recovery of 12% pentlandite and 23% Table 2 Distribution of Ni and S in sulfide ores Ore source Nickel distribution (%) Sulfur distribution (%) Pentlandite Pyrrhotite Pentlandite Chalcopyrite Pyrrhotite Sudbury 87.0 13.0 12.5 9.8 77.7 Thompson 94.8 5.20 20.1 1.5 78.4 Shebandowan 91.8 8.20 16.0 7.6 76.4 (Reprinted from Agar GE (1991) with permission from Elsevier Science.) III / NICKEL AND COBALT ORES: FLOTATION 3493 Figure 1 Effect of pH on collectorless flotation recoveries on Enonkoski noritic ore after grinding in steel mill. (Reprinted from Heiskanen et al. (1991) with permission from Elsevier Science.) Figure 3 Collectorless flotation kinetics after treatment with sul- \1 \1 ! + fur dioxide (1.1 kg t SO2;35gt Dow froth 250, pH 9.5). pyrrhotite makes the grade unacceptable. The over- (Reprinted with permission from the Canadian Institute of Mining, all recovery of pentlandite and pyrrhotite could be Metallurgy and Petroleum.) restricted to 4.1% and 2.5% respectively by introduc- ing DETA in combination with SO2 (Figure 4). The differential separation of pentlandite and ency was greatly improved (Figure 6); however, the pyrrhotite from the resulting Sotation tailings with depressing effect is more effective when the sodium isobutyl xanthate collector and Dow froth ore is oxidized. ! 250 can also be achieved in the presence of SO2 and The activation products formed on pyrrhotite may R DETA. By using 1.4 kg SO2 per tonne of ore in be sul des of Ni, Cu and Ag, which are usually combination with 300 g DETA per tonne a concen- insoluble under reducing conditions. When the min- trate with an overall pentlandite recovery of about eral is oxidized, the activation products convert to 89% could be produced, while restricting pyrrhotite oxides, increasing the solubility in the presence of recovery to less than 12.5% (Figure 5). DETA and thus causing its depression. Problems due to process water The difRculty of Effect of pulp potential Pentlandite can be Soated selective depression of pyrrhotite at Canadian plants selectively from pyrrhotite by maintaining the pulp was found to be due to the process water containing heavy metal ions such as Ni2#,Cu2#,Ag2#, which were causing the inadvertent activation.