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The lime-concentrate-pellet roast process for treating 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 Group at Stanford University is a leaves the pellet. Because anhydrite is in• simple, low capital cost method of converting soluble in acid, is retained in the solid copper sulfide flotation concentrates to cath• during a subsequent copper . 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 solution .

TAILING

28-JOURNAL OF , 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 is not extraction circuit is inserted between leach• available at prices that justify the additional ing and electrowinning. 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 or dump leach/solvent considered in situations where acid can be extraction/ electrowin operation. used internally for 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 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 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 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 . 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 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. The present 100 study is based on -200 mesh concentrates. 98 88 A series of kinetic and process optimiza• a:: a:: :J tion studies on pellet roasting were made lL.. 96 90 Wo using an apparatus in which both weight .-J 0 &w :J W 94 92 Or- change and S02 in the effiuent gas were if) z 00 continuously measured. Finally, the effiuent r- IO 30 lL..<"> Note the early release of S02' r- a... oc? the pellet blocks any ingress of oxygen. 0 Hence, in the LCPR process CaO rather 400 500 600 700 800 than Ca(OH)2, reacts with S02 and oxygen ROASTING TEMPERATURE °C to form CaS04' The major roasting reactions are listed in Table 1. Significant amounts of CUS04 and CuO · CUS04 are formed at low summarized in Figure 5. Nominal furnace temperature but anhydrite is the major sul• temperatures are given. There is a tempera• fate at all roasting temperatures. ture increase in the pellet between 50 and The roasting kinetics are nearly indepen• 100 D C that occurs during the first 30 min. dent of temperature because the small in• of roasting. Residue assays were corrected to crease in oxygen diffusivity with temperature account for a total copper balance. is offset by a decrease in porosity of the re• The remaining parameters were optimized acted shell. The experimental rates are at a higher, less-optimum temperature nearly identical with theoretical rates cal• ( 650 D C) where differences in performance culated on the basis that oxygen diffusion in could be seen. Water vapor from 0 to 6% had the pores of the reacted shell is the rate no effect. Sulfur retention was reduced by limiting process. The kinetic results will be only 3% as oxygen was reduced from 0.20 to reported separately.3 Residence times for 0.05 atm. complete reaction as a function of pellet dia• The calcined limestone was equal to re• meter are shown in Fig. 4. agent grade lime which was superior to cal• The LCPR process optimization study on cined calcite crystals. Reagent grade lime completely roasted single pellets included was used in most of the experiments. Cal• the following parameters: temperature, oxy• cined limestones vary enormously in their gen, water vapor, lime sources, and limel microstructural properties and their reac• sulfur equivalent stoichiometry. These pa• tivity with .4 Consequently, rameters were optimized with respect to each limestone deposit considered for use in sulfur retention and copper extraction into the LCPR process should be tested to verify sulfuric acid with a residual concentration of its suitability. about 30 gil H2S04, Lime-concentrate-pellet-roasting at lower Temperature is the most important pa• temperatures yields significant quantities of rameter and the optimum temperature for CUS04 and CuO· CUS04 as well as CaS04. both sulfur retention and copper extraction Consequently, substoichiometric amounts of is about 500°C where 99% of the sulfur can lime can be used with only a small increase be retained in the solid during roasting with in S02 emissions, an increase well within a stoichiometric lime charge and 99% of the permissable limits. Comparative results at copper can be extracted into a room tem• various lime amounts and temperatures are perature sulfuric acid leach. These excep• shown in Figure 6. A 100% lime stoichi• tional results make the process industrially ometry is the amount of lime required to attractive. The affects of temperature are convert all of the sulfur in the concentrates

DECEMBER 1973, JOURNAL OF METALS-31 to CaS04' The ability to use substoichio• reqUired production rate requires a grate metric amounts of lime is a very important area of 6,000 ft2 for preheating, roasting and consideration for the hydrometallurgical part cooling. Nominal roaster blowing rates are of the process. 135,000 cfm of air (25°C) and 770,000 dm While most of the experiments have been of recycle flue gas (575°C), distributed conducted with a predominantly chalcopy• nonuniformly through a series of windboxes rite concentrate, successful pellet roasting along the grate. Because of the decreasing has also been conducted with a / overall reaction rate with time, the draft pyrite concentrate. velocities through the bed are greatest at the initial windbox and decrease progressively ROASTING TEMPERATURE CONTROL at each windbox down the traveling grate USING A TRAVELING GRATE machine. Chemical reaction and heat release Initial process engineering calculations in• curves for - % + 3fs in. pellets are shown dicate that a traveling grate machine is well in Fig. 7. Because of the equipment sizes in• suited to the LCPR roasting requirements. volved it may be desirable to divide the The traveling grate would consist of an ini• plant into two strands with all of the above tial heating section followed by a long sec• figures halved for each strand. tion throughout which the draft rate would Capital costs for a complete roaster plant be controlled to maintain a pellet bed tem• including feed preparation, lime burning perature within the range between 400 and and all auxiliary facilities have been esti• BOO°C. mated at $19,000,000. Operating costs exclu• A preliminary design calculation of a tra• sive of amortization have been estimted at veling grate pellet roasting unit to produce 2.4¢ per lb of copper. 50,000 tpy of copper has been made. It was assumed that the equipment can be operated ROASTED PELLET LEACHING AND SOLUTION IMPURITY CONTROL 85% of the time. Consequently the required copper production rate is 7 tph. With the Leaching can be conducted after grinding copper concentrate used for roost of our ex• in agitators followed by a solid/liquid sep• periments, containing 23.0% Cu, 25.5% Fe, aration. Vat leaching of the pellets with 28.9% S, and with an 88% lime stoichiometry countercurrent solution flow following the the roasted pellets will contain 12.9% copper technology of vat leaching oxidized copper and the pellet discharge rate must be 55 tph. also appears attractive. Countercurrent The bulk density of the fired pellets is ap• leaching of pellets in the laboratory under proximately 125 lb./ cu ft. simulated industrial conditions yielded a A pellet size range of -% + 3fs in. has pregnant liquor (electrolyte feed) containing been selected. This size was chosen because only 3 to 4 gil iron, which is satisfactory for it is large enough to yield excellent sulfur reasonably efficient electrolysis. It is esti• retention and the residence time required for mated that 1.3 KWH per pound of copper complete reaction is not too long. This is the would be required. A flowsheet is shown in most commonly used size range in iron ore Fig. 8. pelletization and balling technology is well Limestones usually contain some magne• developed. It is assumed that all + % in. sium carbonate which is calcined to magne• balled pellets are removed by a trommel or sium oxide and readily sulfated during other screening device before roasting. roasting. Limestones usually divide into low When void space between the pellets (35%) magnesium and high magnesium varieties. is considered the effective bed density is 80 Low magnesium limestones generally con• lb/cu ft. tain 2-3% mole pct magnesium while high The heat released from the exothermic re• magnesium limestones contain 12-17% mole actions is directly proportional to the extent pct magnesium.5 The MgS04 is dissolved of reaction. Half of the chemical reaction during leaching and will accumulate in a and heat release occur during the first 15 closed cycle leach/electrowin circuit unless min. of reaction because of the spherical adequate steps are taken to remove it. Al• geometry of the pellet and because the though dissolved MgS04 does not appear to diffusion controlled reaction rate decreases adversely affect copper electrowinning from with further penetration of the reaction in• sulfate electrolytes, precipitation of excess terface into the pellet. MgS04 from a saturated solution cannot Heat transfer considerations dominate the readily be confined to the leaching step and design because of the necessity to maintain some precipitation may occur in the electro• the temperature in the range of 400 to winning cells. 600°C Bed depth is limited to 6 inches and The concentration of MgS04 in the elec• flue gas must be recycled to maintain the in• trolyte can be kept below a set level by let gas temperature near the minimum pellet bleeding a minor portion of the spent elec• temperature to prevent undercooling of pel• trolyte as shown in Fig. 1. Copper must be lets at the inlet to the bed. For the major stripped from the bled solution by electroly• test concentrate used in our experiments the sis or cementation. The end liquor is a

32-JOURNAL OF METALS, DECEMBER 1973 strong acid solution containing MgS04. bleeding spent electrolyte. The reduced Water pollution standards will undoubtedly amount of lime used in the roast also lowers require neutralization of this liquor with the necessary bleed rate compared with the lime or limestone before discarding it. Neu• bleed rate needed for a stoichiometric lime tralization with lime or limestone would pre• roast. With the correct reduction of lime in cipitate the magnesium and sulfate ions as the roast replenishment of acid from external magnesium hydroxide, Mg( OH)2, and gyp• sources is not required. In effect, use of some sum, CaS04' 2H20, respectively. The amount of the lime must be deferred from the roast of spent electrolyte removed depends on and used in the neutralization of the bleed the desired level of MgS04 in the electrolyte and stripped electrolyte solution. Fortu• and the magnesium content of the lime used nately, this can b e accomplished without in roasting. Significant emission of S02 because of the There are enormous tonnages of low mag• partial sulfation of copper in a lime roast sium limestones, containing less than 5 mole with substantially less than the stoichio• pct MgCOa, throughout the Basin and metric amount of lime; recall Fig. 6. Range Province of the western United The unit operating costs for leaching States, where most of the copper mines are should be lower than present operations on located. A simplified vat leach-direct elec• oxidized copper ores because the tonnage of trowin How sheet for 50,000 tpy copper is solids leached to obtain a unit of copper is shown in Fig. 8. This circuit was designed only about 5% of that encountered in leach• for a limestone containing 5 mole pct ing oxidized copper ores and purchased acid MgCOa and includes a bleed of about 8% is not required. Electrowinning is conven• of the spent electrolyte. Pellets would be tional and costs similar to present electro• made with 88% of the stoichiometric amount winning costs are expected. However, the of lime, Ca(Mg)O. The concentration of higher grade feed permits higher solution Mg+ + will be kept below 20 gil where its strengths than are often possible with low affect on electrolyte viscosity is minor.6 grade ores. Operating costs for the LCPR The amount of lime used in the roast must process in the direct electrowinning mode be decreased from the stoichiometric amount are expected to be below 8¢ per pound of to compensate for 1) incomplete fixation of copper for plants of 50,000 tpy copper size. sulfur during roasting, and 2) loss of acid by This includes lime burning, pellet prepara-

100~-----.-----.-----r------' Fig. 6-(Left) Lime content of pellets and sulfur retention

I- 98 w Fig. 7-Chemical conversion and heat release during ...J ...J roasting of -1/2 + 318 balled pellets W on a traveling grate a.. 96 z o ~ 94

86 ~------L-----L-----~--~~ 450 500 550 600 650 NOMINAL ROASTING TEMPERATURE-OC

DECEMBER 1973, JOURNAL OF METALS-33 tion, roasting, leaching and electrowinning. ometric amount of lime can be used, which This cost does not include general and ad• permits acid generation to compensate for ministrative expenses or amortization. partial bleeding of the electrolyte to control excess MgS04 and minor impurities. D SUMMARY ACKNOWLEDGMENT The LCPR process involves low tempera• The assistance of George M. Meisel, Metallur• ture roasting of a pellet composed of a mix• gical Engineer, Kaiser Engineers Division ' of Kaiser ture of flotation concentrates and lime. The Industries, with cost estimating is sincerely appre• oxidized copper is recovered as cathode ciated. copper in a conventional sulfuric acid leach• electrowin circuit. A solvent extraction/elec• REFERENCES trowinning circuit can be substituted for di• 1 Haver, F . P., and Wong, M. M., "Lime Roast of Chalco• rect electrowinning to improve cathode pu• pyrite Concentrates," oral paper, Pacific Southwest Minerals Conference, Las Vegas, April 1972 rity. This extremely simple process is 2 Haver, F. P., and Wong, M. M., " Making Copper Without Pollution," Mining Eng., 24, No.6, 52 (1972) successful because all of the critical charac• • Haung, H. H., and Bartlett, R. W ., "Kinetics of the teristics are optimum for pellets roasted be• Lime-Concentrate-Pellet-Roast Process for Copper Sulfide Concentrates," to be published tween 400 and 600°C. These characteristics • Borgwarot, R. H., and Harvey, R. D., "Properties of Carhonate Rocks Related to so.. Re.ctivity," Environ, Sci, are 1) 95+ % retention of sulfur as sulfates, and Tech., 6, No.4, 350 (1972) 5 Bathurst, R. G. C., Carbonate Sediments and Their primarily anhydrite, in the roasted pellets, Diagenesis, Elsevier Publishing, Amsterdam, 235-237 (1971) 2) 98+% extraction of copper by sulfuric 6 Andersen, T. N., Wright, C. N., and Richards, K, J., "Important Electrochemical Aspects of Electrowinning Cop• acid leaching, and 3) enough sulfation of per from Acid Leach Solutions." International Symposium on , ed. by D . J. I. Evans and R. S, Shoe• copper during the roast so that a substoichi- maker, AIME, New York, 171-202 (1972)

Fig. 8-Vat leach- .------, direct electrowln flowsheet for 50,000 ROASTED PELLETS (388,000 tpy) tpy copper using the test concentrate I WATER BATCH- COUNTERCURRENT LEACHING t 0 o :x: <[ « U) 0 <[ ...J o Z ...J ~ =>

L each Liquor DlStARD 52 giL Cu .... 120 pH = 4.3 gpm 3-49/l Fe Spent Electrol yte E c.. 0> 30 gil Cu 0 N 30 gil H2 SO4 r<) 3-4 glt Fe l

ELECTROWINNING PLANT .... 120 gpm (CONVENTIONAL DES IGN) Bleed

ELECTROWINNING 44,000 tpy COPPER STR IP copper cathodes 6,000 tpy impure l copper MARKET SMELTER or REFINERY

34-JOURNAL OF METALS, DECEMBER 1973