Sulfur Dioxide Pressure Leaching New Pollution-Free Method to Process Copper Ore R. A. Meyers,* J. W. Hamersma, and M. L. Kraft TRW Systems Group, Redondo Beach, Calif. 90278
Copper smelters in the United States emit 3.0 X 106 tons of sulfur dioxide into the atmosphere yearly, causing a major air pollution problem in some western states. The current industrial technology which gives rise to sulfur oxide air pollution involves pyrometallurgical treatment of copper sulfide ores. We have discovered a pressure-hydrometallurgical reaction which utilizes sulfur dioxide and hydrochloric acid to recover copper from copper ore while converting both the sulfur dioxide and the ore’s sulfide content to elemental sulfur. This embryonic discovery could provide a basis, after full engineering testing and development, for pollution-free refining of copper.
Copper smelters in the United States presently emit more than 75% of the total sulfur oxides generated by the nonferrous smelting industry. An average capacity copper smelter discharges sulfur dioxide into the atmosphere a t higher rates than some of the largest United States power plants. United States copper sulfide ores consist mainly of chalcopyrite (CuFeS2) with smaller amounts of chalcocite (CuzS), bornite (CusFeS4). and lesser copper minerals. Pyrite (FeS2) is frequently found in the ore matrix. These ores are currently processed by pyrometallurgical techniques involving air oxidation. Approximately one half of the US.smelters roast the ores prior to reverberatory furnace smelting, resulting in sulfur dioxide emissions. After the metal-bearing charge is placed in the reverberatory furnace, it is melted to form copper matte and slug. The matte is then transferred to the converters where air is blown through the matte to eliminate the remaining sulfur as sulfur dioxide, while silica flux is added to remove the iron as a silicate slag. Because of the multiplicity of flue gas exit locations and the variable sulur dioxide content of the emission streams, stack gas cleaning technology has met with only moderate success. In addition, many of the stack gas cleaning processes which have been tested for the control of sulfur oxides from copper smelters produce one or more products such as sulfuric acid, ammonium sulfate, or calcium sulfate which would be sources of secondary pollution if produced in the large amounts required for industry-wide pollution control. Consequently, a good deal of activity has been centered on the development of nonpolluting hydrometallurgical techniques which could be substituted for the older pyrometallurgical technology as new copper production capacity comes on stream. We believe that a hydrometallurgical process which could rapidly dissolve the ore copper content while converting the sulfide content directly to elemental sulfur would be most desirable, as elemental sulfur storage would create no secondary pollution problem. The key step in the development of this type of hydrometallurgical process is in finding an oxidizing medium which will rapidly dissolve the ore but will not oxidize sulfur to sulfate. 70
Environmental Science & Technology
A number of oxidizing agents have been evaluated for hydrometallurgical processing of copper sulfide ores. These include sulfuric acid, oxygen, ferric chloride, and ferric sulfate ( 1 ) . Processes based on these agents have been under investigation for more than 40 years. Of these, only the ferric salt leaching systems produce elemental sulfur as nearly the only sulfur product. Although processes based on ferric salt leaching were first described in the 1930s ( Z ) , none have as yet reached commercial production. A drawback of this latter system lies in the electrowinning of the copper metal from ferric salt solutions, wherein the ferric ion impedes the plating step. An unconventional approach for the pressure hydrometallurgical refining of copper ore is presented below. The process is based on a novel reaction, reported for the first time herein, in which solutions of hydrochloric acid and sulfur dioxide convert copper sulfide ores to elemental sulfur and soluble copper and iron chlorides as shown in Equation 1: 4CuFeS,
+
3S0,
+
4CuC1
12HC1 =
+
4FeC1,
+
11s
+
6H,O
(1)
This reaction mode has advantages over other leaching methods as both copper and iron ions are produced in the reduced form, thus less reductant is required for, and ferric ion is not present to interfere with, subsequent copper metal production. In addition, sulfur dioxide removed by flue gas scrubbing from the exit gases of existing pyrometallurgical smelters can be reduced to elemental sulfur by this reaction, thus providing the dual benefit of reducing sulfur dioxide to a storable form while producing copper. A sample of flotation concentrate from a large open pit mine in the state of Arizona was selected for demonstration of the concept. This concentrate is nearly pure chalcopyrite. The results (Table I) show that 99-100% of the copper and 96-9770 of the iron content of the concentrate is dissolved by action of aqueous sulfur dioxide and hydrochloric acid in a maximum residence time of 30 min a t 180°C and 180 psig. Initial studies a t lower temperatures
Table I. Extraction of Metal Sulfides. Metal s u l f i d e
Chalcopyrite (Cu FeS$ concentrate Chalcopyrite (Cu FeS& concentrate Chalcopyrite (CuFeS& concentrate Iron pyrite (FeSJd
Residence time, min
Metal extracted,b Copper
% by wt Iron
30
99.71 0.05 96.6 rt 0.1
60
98.81 0.05 97.3 f 0.1
120
99.8+ 0.05 96.8 1 0.1
60
...
16.31 0.1
a Extraction conditions: 9.77 m M metal sulfide (based on copper for
chalcopyrite, on iron for pyrite) extracted with 225 ml of water, 0.86M in sulfur dioxide a n d 3.6M in hydrochloric a c i d a t 180°C a n d 180 psig, in a Fischer a n d Porter glass pressure reaction vessel with Teflon-coated m a netic stirring bar system b Fercent metal extracted =(1:000metal extracted from residue with nitric acid)/(total metal in s t a r t i n g concentration) X 100. Metals determ i n e d by atomic absorption spectrophotometry. c Greeniee County Arizona: molar ratio Cui ooFeo.gsSl.so; 85.2% -200 mesh. X-ray powde; fraction pattern shows gieater t h a n 95% of t h e metal content in t h e form o f chalcopyrite with a small a m o u n t of pyrite. d E. H. Sareent Co.. 95% -200 mesh.
7
SULFUR, LEACHER
.
HCI, SO2
CUCI, FcCI2,
GAS SEPARATOR
SULFUR, HCl, CuCI, FeCI2
TAILINGS
I
FILTRATION
SULFUR
ELECTROWINNING
-
COPPER
TAILINGS
OXlDiZER
'-" HYDROLYSIS
OXYGEN OR AIR HCi
Figure 1.
-
HC' CuCI, FeC12
Process flow diagram
OXIDE
J
(i) Copper concentrate is leached with an aqueous solution of hydrochloric acid and sulfur dioxide; (ii) unreacted sulfur dioxide is vented from the slurry mixture and recycled; (iii) elemental sulfur and copper ore tailings are filtered from the slurry mixture; (iv) makeup sulfur dioxide is obtained by oxidization of a part of the product sulfur; (v) elemental copper product and ferric chloride are obtained by electrolysis of an acid solution of cuprous and ferrous chloride: and (vi) hydrochloric acid is regenerated by hydrolysis of ferric chloride giving iron oxide as a product
showed very little reaction below 150°C (100 psig). It is apparent that the iron content of the concentrate dissolves at a slower rate than the copper content. This is probably due to the small iron pyrite component of the concentrate which reacts more slowly under these conditions, as shown in Table I. Elemental sulfur and ferrous and cuprous ions were isolated from each experimental extraction in an amount, which corresponds within a relative 570, to that predicted from the reaction-shown in Equation 1. Excess sulfur over stoichiometric (Equation 1) was found for the 2-hr extraction while no excess was noted for the shorter duration runs. Catalyzed disproportionation of sulfur dioxide to sulfur and sulfur trioxide ( 3 S 0 2 = S 2S03) is possibly responsible for the excess sulfur. Reaction blanks were run under the same conditions as shown in Table I (18O0C, 180 psig), where the copper ore was omitted but trace amounts of cuprous ion were added, and in which sulfur was produced in amounts corresponding to the small ex-
+
cesses noted for the 2-hr extraction. Copper (>99% purity) was plated directly from the above solutions by reduction of the cuprous ion content, after flashing off residual sulfur dioxide. We believe that a pollution-free process could be developed utilizing the discovery presented herein (in combination with suitable state-of-the-art sulfur dioxide and hydrochloric acid regeneration steps) which could extract copper from copper ore with production of elemental sulfur as the sole sulfur-containing product. A block process flow diagram of one preliminary process design utilizing the technical approach described above, is shown in Figure 1. References (1) Subramanian, K . N., Jennings, P. H., Can. M e t . Quart., 11 (2), 387 (1972). (2) Sullivan, J. D., Trans. AIME, 106,515(1933).
Received f o r review M a y 2, 1974. Accepted S e p t e m b e r 27, 1974
Volume 9, Number
1 , January 1975
71
