Copper precipitate agglomerization process

Impure copper precipitates, obtained by cementation of copper from an aqueous solution, are converted into dense, unfriable, high-grade copper pellets suitable for separate smelting and refining as well as for being fed directly to anode or refining furnaces. Sufficient water is added to the copper precipitates to form a coherent, moldable mass of moist precipitates, which is then formed into pellets. The pellets are dried, and then heated to a sintering temperature of at least 750.degree. C. in a reducing atmosphere. The pellets are held at the sintering temperature for a time sufficient to form dense, unfriable, high-grade copper pellets, and the high-grade pellets are then cooled to near ambient temperature before being exposed to an oxidizing atmosphere such as air.

BACKGROUND OF THE INVENTION 
Field 
This invention relates to techniques for producing an agglomerated product 
from impure copper precipitates, and in particular, an agglomerated 
product suitable for separate smelting and refining. 
State of the Art 
In cementation of copper values from an aqueous solution, the copper 
precipitates are deposited in the form of a sludge, and the mother liquor 
is separated from the precipitates by draining and/or filtering. The moist 
precipitates are conventionally blended with flotation concentrates and 
fed to a reverberatory furnace. 
Agglomerating and briquetting of various smelter feeds has been recognized 
as a viable technique for reducing material loss and for providing 
convenient handling and rapid smelting. However, several problems have 
been encountered in the agglomerization and smelting of copper 
precipitates. Agglomerates, i.e., pellets, nodules, or briquettes, made 
from moist copper precipitates oxidize very rapidly when exposed to air. 
The oxidation reaction elevates the temperature of stored agglomerates to 
a red heat, and the "burning" agglomerates constitute a fire hazard. In 
addition, the copper agglomerates contain large amounts of oxygen and 
sulfur and are not suitable for conventional smelting and refining 
processes. Simple smelting of the agglomerated precipitate copper results 
in the generation of slag containing a high content of copper oxide. A 
more complex smelting process must be used wherein reducing agents are 
added to the charge to control the amount of copper oxide in the slag. The 
complex smelting processes are far more costly than conventional smelting 
techniques, and have never been of commercial significance. 
The present invention provides a process of economically producing 
agglomerated copper precipitates which can be smelted and refined by 
conventional smelting and refining techniques. In fact, the agglomerates 
produced by this invention are suitable for being fed directly to anode or 
refining furnaces. 
SUMMARY OF THE INVENTION 
In accordance with the invention, impure copper precipitates, containing 
from about 65 to over 95 percent by weight copper, from about 1 to 15 
percent by weight oxygen (principally as copper oxide, i.e. Cu.sub.2 O), 
and from about 0.1 to 2 percent by weight sulfur are thoroughly mixed with 
sufficient water to provide a coherent, moldable mass of moist 
precipitates having sufficient green strength for shape retention, and the 
moist precipitates are formed into pellets. The term "pellets," as used 
throughout the specification and claims, is meant to encompass any 
agglomerated, shaped mass including conventional spherically shaped 
pellets as well as cylindrically shaped pellets, nodules, briquettes, etc. 
The resulting green pellets are dried, preferably in a neutral or a 
reducing atmosphere, and the dried pellets are heated in a reducing 
atmosphere to a sintering temperature of at least 750.degree. C. The 
pellets are held at the sintering temperature for a time sufficient to (1) 
reduce copper oxides in the pellets to metallic copper, (2) to volatilize 
sulfur and lead impurities therefrom, and (3) to sinter the metallic 
copper in the pellets. The hot, sintered pellets are cooled in a neutral 
or reducing atmosphere to a temperature below about 150.degree. C. 
After being cooled, the pellets are essentially inert and can be exposed to 
air for indefinite periods of time without any adverse effects. The 
pellets are dense, uncrushable and extremely resistant to attrition in 
handling. They are high-grade pellets which contain at least about 75% 
copper, and as much as about 95% copper depending upon the copper content 
of the feed precipitates. The pellets contain no more than about 1% oxygen 
by weight as copper oxides and no more than about 0.1% sulfur by weight. 
The pellets behave like copper shot when melted in a furnace, i.e., they 
are readily digested. 
Preferably, the drying of the green pellets and the subsequent heating of 
the dried pellets are accomplished in an elongate kiln. The green pellets 
are introduced into the kiln at one end and conveyed through the kiln to 
the other end. The pellets are contacted with a stream of hot reducing 
gases which are passed through the kiln in the opposite direction to that 
of the pellets. The temperature of the hot reducing gases is such that the 
pellets are initially heated and dried and then progressively heated to 
the sintering temperature as they move through the kiln. The kiln is of 
sufficient length for the pellets to be maintained at the sintering 
temperature for about 5 to 30 or more minutes, i.e. a time sufficient for 
sintering of the pellets. Reduction of the copper oxides contained in the 
green pellets and the volatilization of sulfur and lead from same occur 
rapidly at the temperatures employed, and it has been found that these 
reactions are essentially completed within the time required for sintering 
the pellets. 
The reducing atmosphere which is maintained about the pellets preferably 
comprises a gas selected from the group consisting of carbon monoxide, 
hydrogen, and mixtures thereof. Advantageously, the reducing atmosphere is 
generated by the controlled combustion of fuel such as oil, natural gas, 
propane, or coal. The heat of combustion of the fuel, together with the 
exothermic heat derived from the reduction of copper oxide in the pellets 
supply essentially all the process heat required. 
The process provides for flexibility in the processing of impure copper 
precipitates. Fresh, dewatered precipitate copper has a typical oxygen 
content of from about 2 to 11 percent by weight. Upon exposure to air, the 
precipitates are readily oxidized to an oxygen content of from about 6 to 
15 percent by weight. This high oxygen content together with sulfur and 
lead impurities contained in the precipitates are responsible for many of 
the difficulties previously encountered in smelting and refining 
precipitate copper. Copper oxides contained in the impure precipitate are 
readily reduced to metallic copper during the heat treatment and sintering 
of the green pellets. In addition, sulfur and lead impurities are also 
removed from the precipitate copper during the sintering of the pellets.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
As illustrated, impure copper precipitates, obtained from cementation of 
copper from an aqueous solution with metallic iron, are mixed with water 
in a conventional disc pelletizer to form shape-retaining pellets. Water 
is added to the precipitates in an amount necessary to form a coherent, 
moldable mass which can be formed into the shape-retaining pellets having 
sufficient strength and coherency to withstand subsequent handling during 
the drying thereof. Preferably, enough water is added to the precipitates 
so that the moist mass contains from about 10-25 percent water by weight. 
The moist, uncured pellets, i.e., green pellets, are dried, and the dried 
pellets are subsequently subjected to a heat treatment in the presence of 
a reducing atmosphere. The drying of the green pellets can be accomplished 
in any conventional drying apparatus, such as traveling grate, horizontal 
belt, disc, tray, and hollow screw driers as well as vertical kilns and 
horizontal rotary kilns. 
The dried pellets are then heated in a reducing atmosphere to a sintering 
temperature of at least 750.degree. C for a time sufficient to (1) reduce 
copper oxides in the pellets to metallic copper, (2) volatilize sulfur and 
lead impurities from the pellets, and (3) sinter the metallic copper in 
the pellets, thereby forming dense, unfriable, copper pellets. Pellets 
containing from about 75% to 95%, or greater by weight are produced from 
corresponding precipitates containing from about 65% copper to about 75% 
copper by weight, respectively. The pellets contain no more than about 1 
percent by weight oxygen in the form of copper oxides. 
The drying of the pellets and the subsequent heat treatment of the dried 
pellets can be advantageously accomplished in an elongate kiln through 
which the pellets pass. The kiln can be of the rotary, horizontal type or, 
as shown in the drawing, of the vertical, shaft type kiln. In either 
apparatus, the green pellets are introduced at one end of the kiln and 
pass through the kiln in counterflow relation to hot process gases which 
are either introduced into or generated at the other end of the kiln. 
As shown in the drawing, the green pellets are introduced at the upper end 
of the kiln and pass therethrough under the force of gravity. The green 
pellets initially pass through a drying zone wherein they are contacted 
with hot process gases coming from the reduction-sintering zone. As the 
pellets progress through the drying zone, they are progressively dried and 
heated by the counterflow of process gases. 
The dried pellets continue to move through the kiln into the 
reduction-sintering zone wherein they are progressively heated to a 
temperature of at least 700.degree. C., preferably between about 
750.degree. and about 950.degree. C. The pellets are maintained at this 
temperature for a time sufficient to reduce the copper oxides contained 
therein to metallic copper and to sinter the pellets. Sulfur and lead 
impurities are volatilized and removed from the pellets during the heat 
treatment. 
The hot process gases flowing countercurrent of the pellets in the 
reduction-sintering zone must have reducing characteristics. The reducing 
gases preferably contain CO and/or H.sub.2 in mixture with inert gases 
such as CO.sub.2, H.sub.2 O, and nitrogen. Hot, reducing gases can be 
supplied by burning a fuel with a sub-stoichiometric air supply. The 
temperature of the hot, reducing gases should be no greater than about 
950.degree. C to avoid melting the pellets in the vicinity of the gas 
inlet. The temperature of the gases can be controlled by injecting cooling 
water into the gases prior to contact of the gases with the pellets in the 
kiln. The hot gases flow through the kiln countercurrent to the movement 
of pellets. 
The oxygen content of dewatered, precipitate copper obtained by cementation 
of copper from aqueous solutions ranges from about 2% to 11% or more. Upon 
standing and exposure to air, the precipitates are readily oxidized, and 
the oxygen content of the precipitates rapidly increases to about 6% to 
15% by weight. It is this high oxygen content which is responsible for 
many of the difficulties previously encountered in processes for smelting 
and refining precipitates. 
The reducing gases rapidly react with the copper oxides contained in the 
pellets as the pellets pass through the reduction-sintering zone of the 
kiln. Depending on the reducing gas being used, the reduction of copper 
oxides proceeds according to the following reactions: 
EQU Cu.sub.2 O + H.sub.2 .fwdarw. 2Cu + H.sub.2 O 
EQU cuO + H.sub.2 .fwdarw. Cu + H.sub.2 O 
EQU cu.sub.2 O + CO .fwdarw. 2Cu + CO.sub.2 
EQU cuO + CO .fwdarw. Cu + CO.sub.2 
unlike similar reactions involving ferrous oxide, the above reactions are 
stoichiometric, i.e., the reactions go to completion. 
Solid and liquid carbonacious materials can also be used as a reductant. 
Such materials can be suspended in the hot gases which are introduced into 
the reduction-sintering zone of the kiln, or they can be introduced into 
the reduction-sintering zone as an admixture with the dried pellets. 
Carbonaceous materials, such as hydrocarbon fuels and carbon itself, do 
not reduce copper oxides, but, instead, reduce water to H.sub.2 and CO and 
reduce CO.sub.2 to CO in a reductant regeneration reaction. When a solid 
carbonaceous reductant is used, it is preferable to mix it with the moist 
precipitates during the formation of the green pellets. However, the solid 
reductant can also be mixed with the pelleted precipitates at any time 
following the formation of the pellets and prior to the reduction and 
sintering of the pellets. 
The copper oxides in the pellets are rapidly reduced at the temperatures 
employed in the reduction-sintering zone of the kiln, and it has been 
found that the reduction is not limited, at normal operating conditions, 
by diffusion of the reducing gases into the pellets or of the products out 
of the pellets. From a practical standpoint, if an adequate quantity of 
reductant is present during the reduction-sintering stage, i.e. at least 
stoichiometric amounts of reductant, the copper oxide reduction is 
completed well within the time required to sinter the pellets. 
The reduction of the copper oxides in the pellets is exothermic and 
supplies a major portion of the process heat for the drying and sintering 
of the pellets. Additional process heat is supplied by the heat content of 
the hot, reducing gases being introduced into the kiln. Advantageously, 
the hot, reducing gases are produced by the combustion of fuel with a 
substoichiometric air supply. Such combustion produces hot gases 
containing substantial amounts of CO, H.sub.2 and carbon. Depending on the 
oxide content of the pellets and the amount of reductants introduced into 
the reduction-sintering zone, the gases being introduced to such zone may 
need to be diluted with an inert or neutral gas or other inert cooling 
medium to avoid overheating the pellets in the reduction-sintering zone. 
If dilution is necessary this can be attained by injecting water, steam, 
or an inert gas such as nitrogen or carbon dioxide into the 
reduction-sintering zone of the kiln. 
In addition to the reduction of copper oxides to metallic copper, the heat 
treatment of the present process results in effective removal of 
substantial amounts of sulfur and lead impurities from the pellets. The 
amount of sulfur and lead contained in typical copper precipitates ranges 
up to about 1% for sulfur and up to about 0.4% for lead. Up to 90% or more 
of the sulfur and up to 65% or more of the lead is removed from the 
pellets during the heat treatment of this invention. The off-gases from 
the kiln are treated in conventional scrubber apparatus to remove sulfur, 
lead, and other impurities which accumulated therein during the heat 
treatment of the pellets, and the gases are then released to the 
atmosphere. 
The sintered pellets leaving the reduction-sintering zone must be cooled to 
near ambient temperatures before they are exposed to air or other gases 
containing oxygen or an oxidizing agent. This cooling can be accomplished 
in a variety of ways, but in all cases, a gas seal must be provided to 
prevent air infiltration to the reduction-sintering zone, or to the hot, 
sintered pellets coming therefrom prior to their cooling to near ambient 
temperature. 
As shown in the drawing, it is convenient to incorporate a cooling zone as 
an integral portion of the vertical shaft kiln so that the sintered 
pellets pass directly from the reduction-sintering zone into the cooling 
zone. As shown, the cooling zone can be formed by providing a cooling 
water jacket around the appropriate section of the vertical shaft kiln. 
The pellets pass through the cooling zone, and the cooled pellets are 
removed from the vertical shaft kiln through an air lock feeder. The 
cooled pellets are then transferred to appropriate storage means prior to 
their being transported to a smelter or refinery. Instead of the indirect 
cooling which is illustrated, the pellets could be cooled directly using 
water sprays or inert gas cooling. 
The invention will be further described with reference to the following 
examples; however, the examples are intended to illustrate the invention 
and are not to be construed to limit the scope of the invention. 
EXAMPLE 1 
Precipitate copper obtained from cementation of copper from aqueous leach 
solutions was mixed with sufficient water to provide a coherent, moldable 
mass of moist precipitates. The moist material was formed into wet balls 
on a conventional pelletizing disc. The resulting pellets (green wet 
balls) were dried in a low temperature drying oven. 
A portion of the dried precipitate pellets weighing 177 grams was placed in 
a stainless steel boat. The boat was placed in an electric resistance 
furnace maintained at 800.degree. C by an electronic controller. 
Immediately upon placing the pellets in the furnace, natural gas (94% 
methane) was introduced into the furnace. The flow of natural gas was 
maintained to produce a short (2 in.) flame at the end of a 7 mm diameter 
ceramic vent tube located on the furnace. After 30 minutes, the natural 
gas was shut off and pure nitrogen introduced into the furnace. The 
pellets were drawn into a water-cooled section of the furnace tube shell 
where they were rapidly cooled to room temperature. The pellets weighed 
138 grams. The pellets were hard and could only be broken by striking with 
a hammer. 
A series of tests similar to the above was conducted using various furnace 
temperatures and heating times. The composition of the resulting product 
pellets is shown in Table 1 as compared to the composition of the dried 
feed pellets prior to the heating and sintering thereof. 
______________________________________ 
Time Temp., 
% Cu %S %O Min. .degree. C 
______________________________________ 
Dried feed pellets 
80.7 .65 12.14 
-- -- 
Product From Test 1 
95.0 .17 1.16 5 800 
2 95.8 .05 .806 10 800 
3 95.8 .06 .757 30 800 
4 95.4 .07 .814 5 850 
5 96.2 .04 .643 10 850 
6 94.9 .06 .631 30 850 
7 95.5 .06 .715 5 900 
8 94.6 .08 .640 10 900 
9 94.8 .09 .631 30 900 
______________________________________ 
EXAMPLE 2 
Precipitate copper was pelletized on a disc pelletizer to produce wet 
balls, having a size of about 1/4 to 7/16 inch. The balls were screened to 
remove the -8 mesh fraction and then charged to an indirect fired rotary 
kiln. The kiln was heated to 860.degree. to 900.degree. C by natural gas 
burners and the discharge end was cooled by water sprays. The kiln was 
rotated at a speed of between 1.5 to 3 rpm. Natural gas was injected into 
the inside of the kiln shell at a rate sufficient to produce a natural gas 
flame 1 inch long at the kiln discharge. Approximately 68 lbs. of the wet 
precipitate balls were charged to the kiln over a time period of 1 hour 
and 10 minutes. The kiln product was dense and essentially unbreakable. 
The average residence time of the balls in the kiln was 15 minutes. The 
composition of the sintered product is shown in Table 2 as compared to the 
composition of the material which was fed to the rotary kiln. 
TABLE 2 
______________________________________ 
% 
Cu Fe S O 
______________________________________ 
Feed 85.2 1.7 .46 7.67 
Product 95.6 1.6 .014 1.05 
______________________________________ 
EXAMPLE 3 
Precipitate pellets, as prepared by the method described in Example 1, were 
charged to a vertical tube furnace 2 inch ID by 24 inch high that had been 
preheated to 700.degree. C. The bed occupied approximately 8 inches of the 
furnace. Upon charging, the furnace temperature controller was increased 
to 850.degree. C and hydrogen gas was simultaneously introduced into the 
bottom of the furnace at about 2.5 liters per minute. The off-gases from 
the furnace were vented through a ceramic tube 7 mm in diameter. The gas 
was continuously tested for flammability. The off-gas ignited after 20 
minutes. This corresponded to completion of the reduction reaction. The 
hydrogen flow was stopped, the furnace power shut off, and the charge 
rapidly cooled with an inert gas flow. The pellets were removed from the 
furnace and quenched in water. The resulting pellets were dense and 
essentially unbreakable. They were in all respects similar to the product 
of Example 2.