Pyrrhotite precipitation

A number of the observed minerals (formulae given in Table 16.4) do not form in the simulation. Wurtzite is metastable with respect to sphalerite, so it cannot be expected to appear in the calculation results. Similarly, the formation of pyrite in the simulation probably precludes the possibility of pyrrhotite precipitating. In the laboratory, and presumably in nature, pyrite forms slowly, allowing less stable iron sulfides to precipitate. Elemental sulfur at the site probably results from incomplete oxidation of H2S(aq), a process not accounted... 

As mentioned already, small amounts of electrum occur in epithermal base-metal vein-type deposits. Electrum is not observed in the epithermal base-metal vein-type deposits in which pyrrhotite occurs (e.g., Toyoha-Soya, Oizumi, and Hosokukura Pb-Zn deposits). However, electrum is found in epithermal base-metal vein-type deposits in which hematite is commonly observed (e.g., Osarizawa and Ani Cu-Pb-Zn deposits). This indicates that electrum precipitates in relatively high /s2 and /oj condition. 

From the mode of occurrence of opaque minerals it is considered that pyrrhotite and sphalerite were precipitated at an early-stage, gold, pyrite, marcasite, stibnite and cinnabar were precipitated at a late-stage, and arsenopyrite was precipitated throughout the mineralization period. 

Some of the discharged sulfide particles settle onto the chimney s exterior, where they are buried by the outward growth of anhydrite. Sulfide precipitation within the chimneys, causes copper, zinc, and iron sulfides to deposit and partially replace the anhydrite. Chimneys can build to several meters in height and their orifices range in diameter from 1 to 30 cm. Both the smoke and the chimneys are composed of polymetallic sulfide minerals, chiefly pyrrhotite (FeS), pyrite (FeS2), chalcopyrite (CuFeS2), and sphalerite or wurtzite (ZnS). 

The selective hydrolysis of metal ions to produce various forms of hydrated oxides is the most widely used form of precipitation. In particular, the removal of iron from hydrometallurgical process streams is a continuing problem. Iron enters the circuit as a constituent of a valuable mineral, such as chalcopyrite (CuFe2), or an impurity mineral, such as the ubiquitous pyrite or pyrrhotite. So far, effective removal of the iron has been achieved by the precipitation of iron(III) as jarosite (MFe3(S04)2(OH)6),401 goethite (FeOOH)402 or hematite (Fe203).403... 

Bostick and Fendorf (2003), 918 performed detailed laboratory studies and concluded that arsenic could form surface precipitates on FeS (including troilite, pyrrhotite, and mackinawite) under anoxic conditions as shown in the following reaction ... 

The sulphides of iron include Fe3S4 (spinel structure), Fe7Sg (pyrrhotite), FeS (troilite), and FeS2 (pyrites and marcasite). Ferrous sulphide rarely has the Fe S ratio precisely equal to unity, though stoichiometric FeS can be prepared. A microcrystalline form prepared by precipitation has been examined by X-ray powder photography and also by electron diffraction. This form (mackinawite) has... 

As pointed out by Seal et al. (2000), many studies of ancient hydrothermal systems have utilized equilibrium sulfate-sulfide sulfur isotope fractionation models, but these should be applied with great caution. As shown in Figure 9, seafloor hydrothermal vent fluid 5" Sh2S values do not conform to simple equilibrium fractionation models. Shanks et al. (1981) first showed experimentally that sulfate in seawater-basalt systems is quantitatively reduced at temperatures above 250°C when ferrous minerals like the fayalitic olivine are present. When magnetite is the only ferrous iron-bearing mineral in the system, sulfate-reduction proceeds to sulfate-sulfide equilibrium, but natural basalts contain ferrous iron-bearing olivine, pyroxene, titanomagnetite, and iron-monosulfide solid-solution (mss) (approximately pyrrhotite). It is the anhydrite precipitation step... 

The solutions from precious metal precipitation (now free of precious metals) were used in a program of copper concentrate enrichment. Copper concentrates from SGS flotation programs on NorthMet ore, corresponding to the PLATSOL feed source concentrate (either Cu-Ni concentrate or pyrrhotite concentrate) were obtained and used for the test (Table XI). 

The PLATSOL M treatment of a nickel rich concentrate and a pyrrhotite 3 cleaner concentrate was demonstrated. The PLATSOL autoclave process was combined with precious metal precipitation, copper concentrate enrichment, copper removal, iron/aluminum removal, mixed hydroxide precipitation of nickel and cobalt and finally magnesium removal. The copper concentrate enrichment process proved to be a novel addition to the NorthMet flowsheet. The copper in solution from the autoclave processing was recovered by metathesis on the copper concentrate in the enrichment process. The solvent extraction and electrowinning of copper process applied to precious metal free PLATSOL solutions is no longer required by using this metathesis process route. This development provides maximum operational flexibility for treatment of the NorthMet ore. 

---