CA1192411A - Copper refining plant - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A copper refining plant is provided with a refining furnace for the refining of converter copper. According to the invention, a suction blower is operated to place the furnace under an underpressure of 5 to 20 mm H2O and the gases, substantially free from outside air, are drawn through a raw gas duct where they are sprayed with a scrubbing solution, e.g. aqueous sodium hydroxide, are then passed through a differential pressure scrubber and are drawn by the suction blower thereto to the stack. The differential pressure scrubber and the blower are controlled to maintain a subatmospheric pressure but such that foreign air is not introduced into the gas.

Description

Cross References to Related Applications and Paten~s Thi~ applica~ion is related to ~he following United States Patent Nos.
4,31G,727 of 23 February 1982, 4,145,193 of 20 March 1979, 4,218,241 of 19 August 1980, 4,007,025 of 8 February 1977, 4,052,042 of 4 October 1977, 3,773,472 of 20 November 1973, -~
3,726,065 of 10 April 1973, 3,844,745 of 29. October 1974, 3,976,454 of 24 August 1976, 3,791,108 of 12 February 1974, 4,123,238 of 31 October 1978, 4,1S2,123 of 1 May 1979, 3,799,520 of ~6 March 1974, 4,055,331 of 25 October 1977, 3,631,656 of 4 January 1972, 3,820,307 of 28 June 1974, 3,906,078 of 16 September 1975, 4,093,434 of 6 June 1978, 3,854,908 of 17 December 1974, 3,844,744 of 29 October 1974, Field of the Invention Our present invention relates to the refining of copper andl more particularly, to a copper refining plant or apparatus of the type in which a copper refining furnace is provided with a gas cleaning apparatus.
Background of the Invention The provision of gas cleanin~ systems in metallurgical plants has become incrP~ ngly important in recent years because of the requirements ~,., h that such plants comply with environmental pollution regulations and in order to protect the environment, plant personnel and persons living in the vicinity of or downwind of such plants.
Hence steel making plants have been provided wlth gas cleaning apparatuses which, inter alia, include Venturi or annular gap scrubbers Csee the aforementioned copending applications and patents) O Anmll ~r gap scrubbers also are useful because the movable body defining the annular gap can be and generally is utilized to control or establish a pressure dif-ferential.
1~ ~n the refining of copper it is known to provide a refining furnace having a substantially horizontal gas outlet passage, a gas cleaning unit and a clean gas passage running to a chimney or stack from which the gas can be discharged into the atmosphere.
The furnace, which is vastly different in design and func~ion from the ferrous metallurgy systems using the aforementioned gas cleaners 9 can be used to refine copper from a converter, i.e. so-called converter copper, to a higher quality product.
In the operation of a plant of the aforedescribed type, the copper ore or slurry is usually first processed in a cupola-type or shaft furnace or in a smelting furnace in which copper matte is formed.
The copper matte is then processed in the converter and air at a pressure of about 1 bar, is blown into the copper matte in the converter via nozzles as part of the refining process. Iron and sulfur in the copper matte are oxidized and the oxidized copper forms with slagging materials, which are added in the cold state, a low viscosity slag which remains above the converter copper until it is decanted at the end of the blowing process.
The oxides of sulfur, in the form of sulfur dioxide and sulfur trioxide, are carried away with converter waste gases.
The converter waste gases are generally sub~ected to a gas clean-ing process to recover the sulfur. The converter copper, which is tapped ~7 from a converter generally contains about 98~o by weight copper and can berefined further in the refining furnaf.e.
To the extent that the converter copper is not already in a liquid state, it must be melted beore it i9 introduced into the refining furnace or while it ~s in the refining furnace and, to assist in the ~melting process, copper scrap can be added.
The refining furnace is fired with gas and/or oil and/or coal dust.
The refining process has an initial oxidizing state followed b~ a r~durin~ state carried out at a temperature of about 1300C.
A pyrometallurgically refined copper is the end product.
To obtaln copper of higher conductivity and indeed a r~
electrical conductivity, impurity metals such as silver and gold must be removed. For this purpose, the pyrometallurgically refined copper is first cast into anode plates and the high purity copper metal is electrolytically recovered by copper electrowinning, i.e. the deposition of the high purity copper at the cathode of the electrowinning process utilizing an electro-light.
The copper refining furnaces are thus frequently referred to as anode furnaces since they provide the metal which is usually subjected to the electrowinning process.
During refining in the latter furnace, a slag i6 formed which contains most of ~he ch~m~cAl impurities of the raw copper.
In the gas cle~n~ng systems hitherto used in copper refining plants for such refining furnaces, the gas ~leAn~ng device is generally provided in, as part of or in association with, a waste heat boiler which reduces the temperature of the raw gases exiting from the refinlng furnace.
Dusts (particulates) deposit in the waste heat boiler from the gas stream under the effect of gravity and sedimentation phenomena result-ing from change in direction of ~low and/or change in flow cross section.
The collected dust is recovered in hoppers from which it can be withdrawn.

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This dust contains a significant proportion7 generally up to about 50% by weight, copper.
To avoid loss of this copper, the dust is either recycled to the copper treatment stages and ~sed as a copp&r-containing raw material, or is recycled to the refining furnace. In either case, difficulties have been encountered in h~n~ling the dust.
For example, if it is recycled to the refining furnace it tends to be entrained with the exiting raw gas and thus participates largely ln a useless recirculation through the furnace.
It has also been found to be a problem that sulfur dioxide is not significantly removed from the raw gas by the prior art systems.
~ hile the sulur dioxide concentration is relatively small in the raw gas because auxiliary air is drawn along with it, it nevertheless poses a problem upon release into the atmosphere. The low concentration of sulfur dioxide, however, makes the removal of sulfur dioxide from the raw gas e~Ll~ ~ly expensive.
Thus, in copper refining systems, there have been significant problems which have only been exacerbated by the requirements of laws or the like preventing even the least release of sulfur dioxide into the atmosphere.
As far as we are aware, moreover, the development of metallurgical plant gas cleaning systems, e.g. of the type described in the aforementioned patents, have not been used to attack the problems described with respect to copper refinlng furnaces and have not been able to be used for such purposes because of the differences ln the characteristics of copper refining furnaces and the metallurgical units described in the afore mentioned patents.
Ob~ects of the Invention It is the principal object of the present invention to provide an improved copper refining plant whereby disadvantages of earlier copper .;~ , ,, refining plants and particularly pro~lems arising from the gas cleaning associated with copper refining furnaces, are obviated.
Another ob~ect of the invention is to provide an improved copper refining apparatus which permits ~he sulfur dioxide to be removed at rela-tively low cost from the raw gas of a copper refining furnace.
It is also an ob~ect of the invention to provide a method of operating a gas cleaning apparatus for a copper refining plant so as to enable the low cost removal of the small quantities of sulfur dioxide in the exhaust gases.

Summary of the Invention These ob~ects and others which will become apparent hereinafter are attained, in accordance with the present invention, in a system of the type described, i.e. a system which ccmprises a copper refinlng furnace, a raw gas duct extending hori~ontally from thi~ furnace, a gas cleaning device, a clean gas duct leading from the gas cle~n~ng device, and a stack from which the clean gas is discharged. According to the invention, the gas ~le~n~ng device comprises a gas c]~An~ng washing to~er or scrubber provided with a multiplicity of scrubbi~g no~zles for introducing a sulfur dioxide agent into the gas stream and at least one differential pressure scrubbing device. According to an important aspect of the invention, the lower portion of the washing tower is provided with a sump collecting the sludge, the pure gas duct upstream of the stack or chimney i5 provided with a suction blower which draws the pure gas from the scrubber and introduces it into the stack, while the refining furnace is provided as an underpressure furnace, i.e. a furnace operated at subatmospheric pressure in which, with the suction blower, a subatmospheric pressure of 5 to 30, preferably about 10 mm water column is maintained.
According to another important aspect of the invention, a control circuit is provided which has at least one sensor in the underpressure furnace and/or in the raw gas product, responsive to the pressure, and which i ~ ~ A~

controls the blower and/or the differential pressure scrubber to ensure an operation of the system such that foreign air is not drawn into the latter.
The sludge from the sump of the scrubbing tower can be trans-formed into copper withou~ difficulty~ It can be added to the copper ore, for example, or recycled directly to the refining furnace without any danger that dust will be entrained in the exiting gas. Depending upon the manner in which the refining urnace is fired or heated, the sludge may contain various amounts of soot which is burned in the refining furnace and thus contributes to the heating therein.
We have found, surprisingly, that the aforedescribed scrubbing system, at least in part because of the operation at subatmospheric pressure to the blower without inducing the entry of foreign air into the raw gas, produces a raw gas whose sulfur dioxide concentration is high enough that the differential pressure scrubber, i.e the annular gap scrubber, and the tower, will be able to remove the sulfur dioxide efficlently.
The clean gas which is obtained is practically free from sulfur dioxide, The sulfur trioxide in the raw gas is simultaneously scrubbed out.
The system of the invention also results ln an effective removal of particulates from the raw gas, i.e. a collection of dust therefrom in the form of a sludge which is readily handled and can be simply treated to obtain copper.
The operation with raw gas evacuation free from foreign alr means that the suction is applied such that practically only reaction products are drawn through the system from the refining furnace to the blower, these reaction products being the reaction produc~s of the aforedescribed oxidiz-ing and reducing steps in the treatment of converter copper.
This of course means that the furnace chamber is closed at least sufficiently to permit the subatmospheric pressure to be generated and maintained, and to prevent the entry of outside air into the system at least until the gas emerges from the scrubber.

The control of the operations also is comparatively s:Lmple requiring only low cost and conventional control circuitry.
The only measurements required are those of s~atic pressure and, once the subatmospheric pressure for a particular apparatus can be ascertained a~ which the reactlon products from the furnace can be withdrawn without entrainment of outside air, this value can be established as the control or set point value which is compared with the actual value of the suction or pressure. The difference determined by the comparator is applied as a control signal to the suction blower and/or the differential pressure unit.
In some cases it is also advantageous to control both of the two controllable units, namely, the differential pressure unit and the suction uni~.
The differential pressure unit is preferably an annular gap scrubber or washer with a flow passage having a Venturi-type construction and/or a diffuser and a body disposed in the passage and of tapering con-figuratlon. This body is axially shiftable to vary the width of the annular gap and hence the differential pressure across the latter.
It has been found to be advantageous, moreover, to provide addi-tional nozzles in the horizontal stretch of the raw gas duct for introducingthe sulfur dioxide scrubbing agent into the gas stream.
The raw gas passage, the scrubbing column and/or the c:lean gas passage can be formed as a waste heat boiler according to the invention.
The scrubbing agent is preferably a recirculated sodium hydroxide solution.
The copper containing waste is separated as a sediment from the liquid phase which contains the sodium sulfate and sodium sulfite in solu-tion and as precipitates.
To this end, the sludge outlet of the column is provided with a thl~kPnçr and a clarifier, the sludge passing from the thickener into the !.

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clarifier and as a sediment from the latter is introduced dlrectly into the refining furnace or previously dried, e.g, by introducing it into a flash drying furnace.
Water and scrubbing solutions decanted from the settling and thickening tanks or basins are delivered by pumps or the like to the no~zles, Fresh sodium hydroxide can be added to this scrubbing water and to the extent that the scrubbing water contains sodium sulfite and sodium sulfate in solution, the sulfate and sulfite can be precipitated with or without oxidation and with the addition of calcium hydroxide to precipitate the sodium salt and thereby regenerate sodium hydroxide.
According to an important feature of the invention, the decanted water is fed to the column and to the scrubbing nozzles in the raw gas duct.
The thickening uni~ can preferably be a plate-type or 1~ r thickener in which dewatering is carried out in the narrow gaps be~ween the plates. An intermediate vessel is connected to the thl~kener and serves as a dehardening stage for the treatment of the water.
8rief Description of the Drawing The above and other ob;ects, features and advantages of the presen~ invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:
Figure 1 is a diagrammatic side elevational view of a copper refining plant according to the invention;
Figure 2 is a view similar to Figure 1, drawn to a larger scale and showing further details of a portion of the plant of Figure l; and Figure 3 is a similar view showing yet another embodiment of the copper refining plant according to the invention.
Specific Descriptlon The copper re~ining plant shown in Figures 1 and 2 comprises a copper refining furnace 1 which, for the reasons previously described, may be substantially sealed from the ambient atmosphere and can be a conven~lonal refinin~ furnace for the upgrading of converter copper to anode copper.
This furnace may be in the form of a rotary kiln furnace and can be fired by conventional means. It has an outlet la at one end connected to a down-wardly extending transition member 2a of a raw gas duct 2. '~he raw gas duct 2 extends generally horizontally to a gas cleaning unit 3 from which a clean gas duct 4 extends to terminate in a stac~ or chimney 5.
In addi~ion to receiving converter copper, the furnace 1 generally also receives copper scrap.
As has been previously noted, the reactions in the furnace 1 are those which commonly occur in such copper refining furnaces, generally an oxidation followed by a reduction at an elevated temperature of, say, 1300C, the evacuated reaction products being gases which can contain sulfur dioxide, sulfur trioxide as well as dust rich in copper.
This raw gas traverses the duct 2y the washing device 3 and the clean gas duct 4 before entering the chimney 5.
As is especially apparent from Figure 2, the scrubbing unit 3 comprises a sulfur dioxide scrubbing column 6 provided with a plurality of scrubbing nozzles 7 for contacting the gas with a scrubbing solution, pre-ferably an aqueous solution of sodium hydroxide. The scrubbing column alsoincludes a differential pressure scrubber represented at 8.
The differential pressure scrubber comprises a Venturi-like flow passage 9 and an axially slidable tapered body 10 which is displaced by a servomotor 40. More speciflcally, the gas arriving in the column 6 from the duct 2 ls directed upwardly by a baffle 41 and passes into a cylindrical duct 42 provided with some of the spray nozzles 7.
At the upper end of this duct, a converging section 43 terminates in a Venturi constriction 44 from which a diffuser 45 extends.
The diffuser 45, in turn, lies within a frustoconical duct section 46 which diverges upwardly but terminates below the top 47.

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The top 47 is provided with a seal 48 receiving a sha~ 49 connected to t~e frustoconical body 10, the lower end of the shaft being guided in a bearing 50 held by a spider 51, Thus the gas is induced to sharply turn upwardly (arrow 52), the æharp direction change resulting in some separation of solids from the gas stream by inertia.
To the extent that the gas stream has not previously been satu-rated by the scrubbing liquid~ it is saturated in lts upward flow through the duct 42 and its velocity increased through the annular gap of the scrubber which has been designated at 53. The gap width is reduced by lowering the body 10 and is increased by raising this body.
The gap 53 is relatively long and hence intimate turbulent mixture of the water droplet and the gas occurs within this gap; as the gases flow out of this gap and upwardly, they are directed downwardly into the annular space 54 around the section 4 because the droplets within the section 46 are collected at 55 while additional liquid is connected at 56 at the bottom of the baffle 41 for delivery to the sump 11~
The clean gas duct 4 opens laterally into the column to carry away the scrubbed gas.
The sump 11 at the lower part of the washing tower 6 is provided with a sludge outlet 12 and can be formed at an upper portion with a scrub-bing liquid outlet 60 from which a pump 61 can decant the scrubbing liquid and force it into one or more of the scrubbing nozzles.
Within the clean gas passage 4 upstream of the chimney or stack 5, we provide a suction blower 13, i.eO a high volume axial intake tangential outflow fan which blows the gas up into the stack. The refining furnace is formed as an underpressure furnace, i.e. a furnace operating at subatmospheric pressure so that the blower 13 is dimensioned to maintain a pressure of 5 to 20 mm H20, preferably 10 mm H20, below atmospheric pressure.
A control circuit represented generally at 14 is provided in accordance with the invention -- lo --~1 This control circuit comprlses a sensor lS or 15' or measuring the pressure in the raw gas pass~ge 2 or in the subatmospheric pressure furnace 1l a comparator 14a receiving these pressure values and lines 14b and 14c running to operators 14d and l~e controlling the pump motor and the gap width respectively. A set point value for the desired degree of suction is applied by a set point generator 1~lf.
The control circuit 14 has been shown more generally in Figure 3.
The control circuit principles can be those which are disclosed in Servo-m~ch~n~m Practice, McGraw-~ill Book Company, New Yor~, 1960, and the pressure sensors and operators can be thus described in this publication as well.
The control circuit 14 permits the displacement or rate of opera-tion of the suction blower 13 and/or the differential pressure developed across the differential pressure scrubber 8 to be adjusted so that the selected subatmospheric pressure is maintained to preven~ the intake of outside air during the drawing of the raw gas through the scrubber.
As a result, practically only the reaction products of oxidation and reduction treatments o~ the converter copper in the furnace are drawn off.
The sludge which collects in the scrubber sump 11 is recycled, in the embodiment shown, directly to the refining furnace 1 for which purpose a recirculating line 17 and a slurry pump 17a are used.
Excess sludge can be collected in the settling tank 17b which can also collect sludge which separates at the blower 13 or in the chimney or stack 5.
It has also been found to be advantageous to provide additional scrubbing nozzles 16 directly in the raw gas passage 2 for feeding the scrubbing agent, i,e. the aqueous solution of sodium hydroxide into the gas stream. Preferably a plurality of such no7zles are provided in axially spaced relat1Onship.

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The raw gas passage 2, part of the washing column 6 and the clean gas passage 4 are formed as a waste heat boiler, i.e. arP provided with boilers formed with tubes as shown to heat water (and produce s~eam) or superheat steam as is conventional with waste heat boilers.
As has previously been described, moreover, the scrubbing agent is preferably regenerated and recycled.
According to our invention, the scrubbing agent ls an aqueous solution of water and sodium hydroxide and, as can be seen from Figure 3, the sludge outlet 12 can be connected by a pump 21 to a basin.
Mixing tank 63 is provided with a stlrrer 64 and into which addi-tives can be fed for combination with the sludge.
From this tank 63, sludge is introduced into a 1 Am~l 1 fl or plate th~rkpner 17 from which it is transferred via line 65 into the clarifying or settling basin 19.
From the settling basin, the sludge may be returned to the refin-ing furnace via the line 17 and the pump 17a (not shown) or this sludge can be discharged and dried, e.g. as represented by the pile 24', and trans-ferred, e.g. by a bucket loader 66 to a location in the metallurgical plant at which it is handled like the copper ore.
The liquid decanted from the settling basin 19 passes into the compartment 66 from which it is fed by a pump 67 to a buffer or intermediate vessel 22 which can be combined with a water dehardening unit 23 adding a chemical agent which reduces the hardness of the water, and in accordance with the response of a float controlled valve 68.
The liquid phase from the intermediate vessel 22 is transferred by a pump 68' and a pump 69 to the nozzles 7 and 16 previously described.
The solids 2~1 consist of about 50% metal oxide, mainly copper oxide; the balance being free carbon so that it can serve as a fuel.
The intermediate vessel 22 serves a control function in that it stirs the decanted and recycled scrubbing solution and also provides for the ~. .

addition of the dehardening agent. The float valve 68, in addition permits make-up water to be added.
The dehardening agent tends to limit the formation of calcium carbonate and magnesium salt in the nozzles 7 and 16 which otherwise result from the high pH of the recycled water at the relatively high temperature to which the nozzles are subjected.
Lye can be added from the tan~ 71 ~o the recycled water while additives designed to generate lye can be added from the tank 72.
During the oxidation period, a 30% lye solution may be added to absorb the sulfur dioxide peaks which may occur at ~he beginning of the oxidation process.
In Figure 3 we have shown a modification of the blower 113 drawing the reaction products from the furnace 101~ a damper valve 114e being here provided in place of the motor control unit 14e; in this embodiment, more-over, the configuration of the annular gap scrubber 108 has been modified somewhat and it can be seen that the body 110 is here located only in the diffuser portion 145. The control circuit 114 has been shown in a somewhat different form as well and the sensor 115 is shown to be located at the bend of the outlet pipe 102. Otherwise the elements in the 100's series in Figure 3 function in the manner described with respect to the corresponding elements without the lOO~s digit of the respective reference numerals.

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a copper refining plant having:
a refining furnace for refining converter copper and generating sulfur dioxide in a raw gas stream;
a raw gas duct extending from said furnace;
a gas cleaning unit connected to said raw gas duct;
a clean gas duct connected to said gas cleaning unit; and a stack for discharging clean gas, the improvement which comprises:
a suction blower member in said clean gas duct for inducing an underpressure of substantially 5 to 20 mm H2O in said furnace, in said raw gas duct and in said unit;
a scrubbing column provided with nozzles for introducing the sulfur dioxide scrubbing solution into gas withdrawn from said furnace in said unit, and a differential pressure scrubber in said column having a control member for varying the pressure differential across said differential pressure scrubber; and control means connected to at least one of said members and responsive to the underpressure generated by said blower member for control-ling said underpressure to extract gas from said furnace substantially with-out entrainment of outside air through said scrubber therewith.

2. The improvment defined in Claim 1 wherein said column has a sump collecting a copper-containing sludge, further comprising means for recycling said sludge to said furnace.

3. The improvement defined in Claim 1, further comprising scrubbing nozzles for spraying said scrubbing solution into said raw gas duct.

4. The improvement defined in Claim 1 wherein at least part of at least one of said ducts and said column is formed as a waste heat boiler.

5. The improvement defined in Claim 1, further comprising means forming a solution of sodium hydroxide for delivery to said nozzles at said scrubbing solution.

6. The improvement defined in Claim 1, further comprising:
a thickener for collecting sludge from said column;
a clarifying basin receiving said sludge from said thickener;
and means for recycling decanted liquid from said clarifying basin to said nozzles.

7. The improvement defined in Claim 6 wherein the last mentioned means includes means for feeding decanted liquid from said basin to spray nozzles in said raw gas duct.

8. The improvement defined in Claim 6 wherein the last mentioned means includes an intermediate vessel.

9. A method of operating a copper refining plant having:
a refining furnace for refining converter copper and generating sulfur dioxide in a raw gas stream;
a raw gas duct extending from said furnace;
a gas cleaning unit connected to said raw gas duct;

a clean gas duct connected to said gas cleaning unit; and a stack for discharging clean gas;
said method comprising the steps of:
evacuating said unit, said duct and said furnace to a sub-atmospheric pressure of 5 to 20 mm H2O whereby the gases extracted from said furnace and passed through said unit are substantially free from outside air;

spraying in said unit into said gases a sulfur dioxide scrubbing solution to form a mixture of said solution and said gases;
passing said mixture through an annular gap scrubber; and controlling the gap width and the suction applied to said ducts, said unit and said furnace to maintain said gases free from outside air.

10, The method defined in Claim 9, further comprising the steps of:
collecting a sludge at said unit;
thickening said sludge;
settling solids from said thickened sludge and decanting a liquid therefrom; and recycling the decanted liquid to the scrubbing of said gases.