The lime-concentrate-pellet roast process for treating copper sulfide concentrates by R. W. Bartlett and H. H. Haung HE LIME- concentrate-pellet-roast proc­ drite (CaS04 ) within a lime-concentrate T ess (LCPR) developed in the Process pellet during roasting. Very little S02 ever Metallurgy Group at Stanford University is a leaves the pellet. Because anhydrite is in­ simple, low capital cost method of converting soluble in acid, sulfur is retained in the solid copper sulfide flotation concentrates to cath­ during a subsequent copper leaching. The ode copper while meeting the strictest leach residue includes anhydrite, Fe203, standards for S02 emissions into the atmos­ silicates and other insoluble components of phere. It is a combined pyro-hydrometallur­ the concentrate. This tailing may be cyanide gical prooess that fixes the sulfur as anhy- leached to recover gold and silver. COPPER CONCENTRATES WATER AIR ROBERT BARTLETT (OPTIONAL) Robert Bartlett is Associate Professor, Process Metallurgy Group, Applied Earth ~---SPENT Sciences Department, ELECTROLYTE Stanford University and is currently WASH chairman of the WATER TMS-Education and EMD-Publications BLEED Committees and a CYANIDE ~ _____-I member of the AU,Ag COPPER Executive Committee CATHODES COPPER STRIPPING of EMD. He received TAILING ELECTROL YSI S his B.S. (1953) and OR CEMENTATION Ph.D (1961) from TAILING the University of Utah. His current COPPER LIME OR interests are in the LIMESTONE area of metallurgical kinetics, transport processes, metallurgical Fig. 1-Flowsheet for the lime-concentrate-pellet-roast process modeling and process with direct electrowinning solution mining. TAILING 28-JOURNAL OF METALS, DECEMBER 1973 Fig. 2-Cross-section of a -112 + 3/8-in. diameter pellet roasted! 30 minutes The process is aimed at situations where a cathodes will probably result if a solvent ready market of sulfur or sulfuric acid is not extraction circuit is inserted between leach­ available at prices that justify the additional ing and electrowinning. Metal recovery in cost of processes that produce sulfur or sul­ the LCPR process is compatible with a pa­ furic acid. The LCPR process should also be rallel oxidized ore or dump leach/solvent considered in situations where acid can be extraction/ electrowin operation. used internally for dump leaching but where PELLET ROASTING the increased returns may be insufficient to The key step in the process is the roast. justify the additional cost of an acid produc­ Both pelletization, or equivalent agglomera­ ing process for copper concentrates. tion, and the use of lime as a reagent are The cost advantages of the LCPR process necessary to obtain satisfactory results. result from the direct, in situ fixation of sul­ The advantages of roasting chalcopyrite fur in the solids, low reagent cost and very and other copper sulfide minerals while si­ simple flow sheet. The flowsheet consists of multaneously converting the S02 to an inert green pellet preparation, pellet roasting and solid sulfate stimulated the present investi­ a modified sulfuric acid leach/electrowin gation. A brief history of the steps that led circuit (see Fig. 1). Gas treating facilities to the present process is of interest. Haver are not required and limestone is the only and Wong1,2 investigated a number of cal­ reagent. The roasting step is semi-autoge­ cium, magnesium and sodium compounds as nous with very low fuel requirements. It is possible sulfate forming reagents in con­ performed at low temperatures, does not junction with simultaneous roasting of a involve liqUid phases and can be conducted chalcopyrite concentrate. They found that on a traveling grate machine. Satisfactor~ re­ hydrated lime was an effective reagent while sults are obtained at temperatures from the other reagents tested did not fix enough 400°C to 600°C so that adequate tempera­ of the sulfur to meet the 90% emission stan­ ture control of the roasting reactions can be dard. Their results were based on rabbling a achieved in an industrial process. Capital stoichiometric mixture of concentrates and and operating costs should be .comparable reagent. The oxidation reactions are exother­ with iron ore pelletizing except that less fuel mic and the temperature of a rabbled pow­ is required but residence times are longer. der charge cannot be prevented from Limestones containing at least 5% MgC03 increasing markedly. Consequently, the can be tolerated by using a bleed to remove formation of copper ferrite (CuO· F~03) MgS04 in the hydrometallurgical circuit. at higher temperatures could not be pre­ Sulfuric acid lost in the bleed is compen­ vented and since copper ferrite does not sated internally by electrolysis of copper readily dissolve in sulfuric acid a bOiling sulfate produced in the roasting step. Al­ HCL leach is used in their process. This though the process permits direct electro­ complicates the process and increases the winning from the leach liquor, higher purity cost. DECEMBER 1973, JOURNAL OF METALS-29 The initial efforts at Stanford involved fluidized bed studies using a stoichiometric Major Reactions During Lime-Concentrate-Pellet mixture of lime and copper concentrates in Roasting which chalcopyrite was the major mineral. DEHYDRATION (WEIGHT LOSS) Temperature escalation and poor retention /l of sulfur resulted. However, in cases where CuFeS2 + 2Ca(OH)2" CuFeS2 + 2CaO + 2H.o t partial sintering of the charge occurred sul­ LOW TEMPERATURE OXIDATION (WEIGHT GAIN) CuFeS2 + 13/ 4 02" CuO + 1/2 Fe20a + 2S02 fur retention increased markedly. Then, CuFeS2 + 15/4 0 ..... CuSO, + 1/2 Fe203 + S02 separate kinetic studies on the oxidation of CuFeS2 + 7/2 0 ..... 1/2 (CuO . CuSO,) + 1/2 Fe,03 + 3/2 SO. chalcopyrite and sulfation of lime were HIGH TEMPERATURE OXIDATION (WEIGHT GAIN) made. At equal temperatures the rate of CuFeS2 + 13/4 02 .... 1/ 2 {CuO . FezOa} + 1/ 2 CuO + 2S02 lime sulfation is slower than the rate of chal­ SULFUR FIXATION (WEIGHT GAIN) copyrite oxidation but in the same order-of­ 2CaO + 2S02 + 0 ... 2CaSO. magnitude range. These results also showed that in a mixture of chalcopyrite and lime, core of the pellet from its reacted shell. Sul­ S02 is formed by mineral oxidation and this ration of lime occurs in the shell but pri­ gas subsequently reacts with lime to form marily in a thin diffuse zone near the ~ore/ HSIN-HSIUNG HAUNG anhydrite. Furthermore, the average S02 shell interface. The unreacted core of the residence time within the mixed charge must pellet is continually shrinking during roast­ Hsin-Hsiung Haung be a few minutes for substantial conversion is a graduate ing and the distance over which S02 must metallurgy student to anhydrite. The l'esidence time of gases diffuse to escape from the pellet increases at Stanford passing through a fluidized bed, even of in­ with roasting time. Hence, the residence University in the dustrial size, is only a few seconds and most time and probability of sulfation of S02 Applied Earth of the S02 produced will be swept out of the increases and the rate of S02 loss into the Sciences Department_ bed before it can react. However, when the gaseous efHuent decreases with roasting time. Born in Taiwan, bed became partially sintered, gas flow was he received a B.S. The net effect is that much more than 190% (1969) in Metallurgical blocked and S02 had to diffuse out of the of the sulfur can be retained in pellets with Engineering from sintered region to escape the bed. The longer a diameter of only 3fs in. if the temperature Cheng-Kung residence time caused a higher conversion of is limited. University (Tainan, the S02 to anhydrite_ It became clear that a Taiwan), and an M.S. Sulfur retention and temperature control pellet roast was necessary to achieve a suc­ are the two major advantages of using pel­ (1973) in Process cessful process and pellet studies were Metallurgy from lets. These advantages are crucial to the Stanford University. begun. success of the LCPR process. A major tem­ Presently he is Pellets have been made by pressing and perature escalation can be prevented be­ pursuing the Doctoral by balling with essentially identical results cause oxidation of copper sulfides is confined degree at Stanford obtained. The cross-section of a partial re­ University. to the core/shell interface within the pellet acted balled pallet is shown in Fig. 2. There and kinetically limited by oxygen diffusion is a sharp interface between the unreacted through the pores of the reacted shell of the core and the reacted shell. Electron beam pellet. Passing additional air through a bed of microprobe traverses across the interface pellets will not increase the roasting rate but show excellent correlation between calcium will increase the rate of heat removal from and sulfur in the shell and between copper the bed. Without pelletizing, additional air and sulfur in the core. will cause additional oxidation and heating. Oxidation of the copper sulfide minerals It is worth noting that lime is a natural occurs only at the interface separating the binder for agglomerating the green pellets Fig. 3-Weight gain and S02 emission during roasting Fig. 4-Roaster residence times for complete of 1.6 cm diam X 0.6 cm pressed disk pellets. reaction of balled pellets in air 2 (j) 0.7 ~ .... w co wI .JU 0.6 ~ .JZ w :> 05 ;: ~I 0 .7 "­ 00:: 0.6 w wW o.q W .J .... 0.5 I .Jw 0.4 I- <[~ 0.3 ~ OJ::! 0.3 0 02 ON '" 0 , 1 ~ 0 .230 405060 100 200 400 600 1000 0. ROASTING RESIDENCE TIME - MINUTES 30---JOURNAL OF METALS, DECEMBER 1973 and that the roasted pellet shell has ade­ quate crushing and impact strength. Flota­ Fig. 5-Effect of roasting temperature on copper extraction tion concentrates may need to be reground and sulfur retention in completely roasted pellets to provide particles small enough to possess 86 good balling characteristics.