Control of autoclave scaling during acid pressure leaching of nickeliferous laterite ore
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I.
INTRODUCTION
THE AMAX
sulfuric acid leaching process involves countercurrent digestion of nickeliferous laterite ore at high pressure and at atmospheric conditions.t After solid/ liquid separation, the nickel and cobalt values are precipitated by hydrogen sulfide ~5 and refined into nickel and cobalt metal. 16Research at AMAX began in the early 1970's at AMAX's Extractive Research and Development Laboratory in Golden, Colorado, using as a model Cuba's Moa Bay laterite acid pressure leaching operation. 2 Substantial improvements were made over the Moa Bay process, particularly in the areas of metal recovery, energy consumption, and feed versatility.3'4'5 A block diagram of the AMAX acid leach process is shown in Figure I. The ore is first screened into two size fractions. The coarse fraction, being higher in magnesium, is dried and calcined to improve its reactivity.17 It is then used for pH adjustment of the pregnant solution before the resulting leach residue is added to the high-pressure leach. The fine fraction is pumped directly to high-pressure leach. Optionally, the ore may be mined as separate "L" (limonite) and "S" (serpentine) fractions and handled in separate ore preparation circuits. The high-pressure leach consists of a multistage pressure vessel equipped with turbine agitators. Sulfuric acid is added after the slurry is preheated to operating temperature. The heating of the slurry is carried out in several stages using steam recovered from the multistage flashing, as well as high-pressure makeup steam. The flashed slurry is subjected to countercurrent decantation (CCD). Neutralized barren solution is used to displace the pregnant liquor from the tailings. Residual acid in the pregnant solution is reduced to an acceptable level for precipitation of metal sulP. B. QUENEAU, formerly with AMAX Extractive Research and Development, Inc., Golden, CO, is now Technical Assistant to the President, Hazen Research, Inc., Golden, CO 80403. R.E. DOANE, Process Engineer, M.W. COOPERRIDER, Mineralogist, and M.H. BERGGREN, Research Metallurgist, are with AMAX Extractive Research and Development, Inc., Golden, CO 80403. P. REY, formerly with COFREMMI, Paris, France, is now retired, Paris, France. Manuscript submitted October 7, 1983. METALLURGICALTRANSACTIONS B
tides by atmospheric leaching of roasted high-Mg ore. The clear liquor goes to sulfide precipitation, and the solids residue is pumped to the high-pressure leach to complete the dissolution of its metal values. 6 One of the operating problems that must be addressed in design of the pressure leaching system is scaling of reactor surfaces.7 The formation of alunite and hematite deposits on the autoclave walls has been well documented by Russian authors describing the Moa Bay operation. 7's Less recognized is the potential for formation of MgSO4" U 2 0 and silica scale. These latter compounds can become troublesome when higher grade nickeliferous laterites containing greater amounts of magnesium silicate are treated. One of the advantages of AMAX's process is
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