US2847294A - Method of purifying and desulfurizing zinc sulfide ores and concentrates - Google Patents

Description

Aug. 12, 1958 c. c. LONG ETAL METHOD OF PURIFYING AND DESULFURIZING ZINC 7 SULFIDE ORES AND CONCENTRATES Filed Jan.

2 Sheets-Sheet 1 0 536m 525.3% T CE: 22; M12586 A R ow mm @v on 536m W mocfiafiwfi T 2w: 2.2; 4 0625%; .w x om INVENTORS o CARLETON c. 1.0m, & HERAND K. NAJARIAN Jyrw%7- d/uow ATTORNEYS Aug. 12, 1958 Filed Jan. '7, 1952 METHOD OF PI'JRI SULFIDE CRES AND CONCENTRATES C. LONG ET AL PRECIPITATOR $122 is 9 64 65 FYING AND DESULFURIZING ZINC 2 Sheets-Sheet 2 3 ELECTRICAL .fll ,-ss

9 9| ELECTRICAL 87 WASTE HEAT PRECIPITATOR F: BOILER 89 INVENTORS CARLETON C. LONG &

HERAND K. NAJARIAN ATTORNEYS 1 product metal.

United States Patent METHOD OF PURIFYING AND DESUL'FURIZING ZINC SULFIDE ORES AND CONCENTRATES Application January 7, 1952, Serial No. 265,318

3 Claims. (Cl. 75-9) This invention relates to a method and apparatus for purifying zinc sulphide ores and commercial zinc sulphide concentrates, whereby there is attained a high degree of purification, and subsequently roasting the purified concentrates in a highly economical manner. Briefly, this invention contemplates initial purification of the concentrates by removal therefrom of the major portion of such constituents as lead, cadmium, tin, and the like, but with only partial oxidation of the sulphide constituents. The so-purified concentrates, partially depleted of fuel value (i. e., sulphide sulphur), are then in a subsequent and separate operation desulphurized and oxidized by roasting in a fluidized bed. Both the initial purification and the subsequent desulphurization are carried out essentially autogenously. The combination of purification and subsequent final desulphurization as separate steps carried on in substantially separate apparatus but with both being achieved autogenously marks a distinct forward step in Zinc metallurgy and in the economy of zinc smelting.

In the art of zinc metallurgy, one of the most challenging problems is the production of high grade low-lead zinc from a zinc sulphide concentrate containing substantial amounts of lead. Zinc concentrates containing less than 0.1% lead are much less commonly available than those concentrates containing 0.1% to 1.0% lead. Even more abundant are zinc concentrates containing more than 1% lead; and in fact, lead contents as high as 3% are not unusual.

In the present state of the art, production of high grade zinc metal or zinc oxide requires either preselection of concentrate feed, wet purification, or rectification of the Rectification of product metal involves fractional distillation of high-lead zinc; e. g., prime western grade This method, while technically satisfactory, inherently involves added cost of the extra metallurgical step. Wet purification by the well-known electrolytic Zinc process is generally more costly than the processes of Zincpyrometallurgy and finds its most extensive use only where the Zinc ores contain significant values in byproduct metals such as gold and silver. Direct production ofhigh grade zinc metal by pyrometallurgical processes is achieved only when the lead content of the zinc concentrate feed is limited to what can be removed in the roasting and sintering operations. For example, in the well-known electrothermic method for zinc production, it is generally found necessary to limit the lead con when to do so results in lower recovery of zinc, as happens in the present state of the art of beneficiation. Highleadzinc concentrates are used to produce prime western zinc for which there is a large and steady demand. There is also a large and increasing demand for high grade zinc. In view of the ready outlet for high-lead zinc concentrates, there appears little possibility of substantially improving the supply of low-lead zinc concentrates at the mines. The producers of zinc metal must, therefore, develop more efficient and lower cost purification procesess.

An object of this invention is to provide a method and apparatus whereby a relatively pure high grade zinc calcine may be made at reasonable cost from a high-lead relatively impure zinc sulphide concentrate.

Another object of this invention is to provide a method and apparatus for beneficiating a high-lead relatively impure zinc sulphide concentrate whereby two products are produced; one a low-lead relatively pure zinc calcine suitable for the production of high grade zinc, the other a lead-enriched calcine suitable for the production of prime western zinc.

Another object of this invention is to provide a method and apparatus for beneficiating a high-lead relatively impure zinc sulphide concentrate whereby two products or fractions are produced; one a relatively pure zinc calcine suitable for the production of high grade zinc Without further refining, the other fraction having a relatively high concentration of impurities such as lead, cadmium, and so forth, the proportion of the second fraction being relatively small compared with the first fraction.

A further object of this invention is to provide a method and apparatus whereby the above-described objects may be achieved by essentially autogenous means without necessity of combustion of substantial amounts of added fuel.

A still further object of'this invention is to produce as a by-product of this purification and desulphurization process a high-strength sulphur dioxide gas suitable after purification and dilution with air for manufacture of sulphuric acid by the contact process.

Another object of this invention is to provide a method and apparatus for purifying and roasting zinc sulphide concentrates at an over-all cost substantially lower than that achieved by prior art methods.

The term zinc calcine as used in this specification is that commonly employed in the art to designate an essentially dead roasted material; i. e., the product obtained by substantially complete oxidation of a zinc sulphide concentrate.

It is known that by subjecting the commercial zinc sulphide concentrates containing relatively large amounts of impurities to an oxidizing roast in any of Well-known processes such as roasting in multiple hearth roasters, suspension-type roasters, rotating drum masters, and the like, appreciable amounts of volatile impurities such as lead, cadmium, and the like, remain in the final roasted product, making the product unsuitable for production of high grade zinc metal or zinc oxide pigments without further purification.

By the method of the invention disclosed herein, lowlead calcine suitable for the production of high grade zinc metal or zinc oxide can be produced from high-lead zinc sulphide concentrates; i. e., zinc concentrates containing more than 0.5% lead. Briefly, the invention consists of obtaining purification and subsequent desulphurization in two distinct steps, whereby in the first step the lead-containing zinc sulphide material is subjected to a high temperature and a high sulphide sulphur environment wherein the major portion of the impurities is eliminated, and then, in the second step, passing the thuspurified material through a fluid bed reactor for substantially complete removal of the sulphur by oxidation. Under these conditions, the lead-containing constituents readily sublime and are carried oft-with the exit gases from the first step or purification operation, while other minor metals whose sulphides, oxides, or intermediate reaction products are volatile, such as cadmium and tin, are vaporized insubstantial amounts and'are removed along with the lead,

In the usual method of carrying out the purification step, a portion of the original sulphide content of the concentrate feed is oxidized to provide the necessary heat. The purified partially oxidized intermediate productwhich, for nomenclatural convenience will hereinafter be designated as PDC, i. e., partially desulphurized concentrate-ordinarily does not contain enough heat value to permit completely autogenous roasting by the methods conventionally employed in the art. That is to say, PDC cannot be dead roasted in a multiple hearth furnace, a suspension roaster, a rotary kiln, or the like, without the use of substantial quantities of auxiliary fuel, such as natural gas. The invention described herein provides a method and apparatus bywhich the roasting of PDC may be completed entirely autogenously.

In one of the preferred methods of the invention, as the first step in purification and desulphurization or roasting procedure, sulphidic zinc concentrates are fed to the top hearth of a conventional type multiple hearth roaster. The concentrates successively pass downwardly through a multiplicity of hearths while being mechanically agitated by means of rabbles and being spread over several hearths and transported toward the bottom discharge opening while hot gases are circulated upwardly over the several hearths. I

This method of operating such an assembly, whereby practically complete elimination of impurities such as lead and very high percentage elimination of cadmium and the like is obtained, differs from the conventional roasting of zinc concentrates in multiple hearth roasters as follows:

The top open hearth of the multiple hearth roaster is used mainly for drying of the concentrates. Depending upon the capacity desired, the first, second, or third hearths following immediately below the top hearth are used to gradually heat the material to a temperature permitting ignition of sulphidic materials in the charge whereby when the material drops on to the next following hearth below the sulphidic minerals begin to ignite. This hearth and the several hearths following are used as reaction hearths in which sufficient preheated oxygen-lean gases of high sulphur dioxide content are allowed to circulate upwardly over each one of the several reaction hearths partially oxidizing the sulphidic minerals to maintain a temperature and atmosphere in each of several hearths which will permit volatilization of the volatile compounds of metals such as lead, cadmium, and the like. As the products of combustion, due to strict control of air supply, are maintained at or near maximum possible concentration with respect to sulphur dioxide content, a preponderantly reducing atmosphere surrounds the material being agitated and transported downwardly over the reaction hearths. Preferred temperature range for volatilization of impurities in the zinc concentrates and for incidental partial oxidation thereof as they pass over the reaction hearths, is between 850 to 950 C. The lowest two or three hearths in the multiple hearth furnace immediately below the lowest reaction hearth are used for the dual purpose of preheating the small amount of'air required to temperatures suflicient to oxidize the minerals in the lowest of the reaction hearths and, also, to cool the purified material before its discharge from the roaster. The preheated air may comprise hot gas from another furnace, such as a subsequent desulphurization reactor or other desirable source, and may be so regulated as to be capable of burning sufficient sulphidic minerals in the reaction hearths to maintain the desired temperature. Additional air or heated gas is supplied to each one of the reaction hearths necessary to oxidize an increment of sulphidicminerals to maintain optimum operating conditions.

When purifying commercial zinc concentrates having 30% to 32% sulphur, oxidation of somewhat less than one-half of the sulphur content of the concentrates is usually found sufiicient to furnish the heat necessary to complete the purification and partial desulphurization operation as outlined hereinbefore during normal operations without the necessity of supplying extraneous heat. Assaysyof product discharged from the multiple hearth roaster show from 18% to 20% sulphur remaining in the partially desulphurized product. Zinc ores and zinc concentrates containing sulphur lower or higher than 30% to 32% can be purified by this method.

7 centrates, however, a further improvement is employed.

' centrates fed to the furnace.

By thoroughly drying and preheating if necessary the concentrate before it is fed onto the top hearth of the multiple hearth furnace, the furnace can be run with a hot top. By this is meant that the temperature of the upper hearths of the furnace is kept sufiiciently high to prevent condensation and recycling of lead sulphide, lead oxide, and other impurities on and with the incoming ore. Without wishing to confine the invention to a par-' ticular theory, it is thought that for very high lead content feeds the furnace gases in the upper part of the roaster may become supersaturated, with respect to constituents volatilized from the charge, if the temperature is allowed to drop too much. Presumably the supersaturation is relieved by condensation from the gas of the excess sublimed constituents. Some of the so formed particulate matter may settle out on the incoming ore where it has the effect of increasing the percentage of impurities in the charge.

Whether the above hypothesis be correct or not, the practical effect and the remedy as described have been discovered.

The volatile impurities in the zinc concentrates that are driven off in the reaction hearths together with very fine particles of zinc concentrates entrained in the gases from the multiple hearth roaster are carried out of the furnace, and are recovered in well-known apparatuses such as cyclones, electrical precipitators, and the like. A typical analysis of the fume and dust collected from the gases discharged from the multiple hearth furnaces which are used for purification and partial desulphuriza tion shows lead assay of 18% to 20%, zinc assay of 38% to 42%. The amount of solid fume material discharged with the furnace gases from such an operation is in the neighborhood of 2.5% of the weight of con- When a multiple hearth roaster is being used in a conventional manner for dead roasting of commercial zinc concentrates, the amount of dust and fume driven out of the furnace with the furnace gases usually runs from 12% to 15% of the weight of concentrates fed to the furnace with a typical average assay of 4.5% to 5% lead and about 50% zinc. It is evident that in the practice of the present invention, due to low volume of gases being circulated in the furnace and consequent low velocity and lower exist temperature, much less zinc concentrate is driven out of the furnace as dust, and the dust collected in the cyclones or electric precipitators is much higher in lead content, making it more economical to further process the same for separation of lead, cadmium, zinc, etc.

As the second step in the purification and roasting of commercial zinc concentrates in accordance with this method, the partially roasted and purified product from the multiple hearth furnace is passed through another furnace where complete desulphurization is obtained.

To obtain autogenous roasting of concentrates, substan-' tial savings in fuel and economical operation, it is preferred to carry on the second step in a fluid bed furnace.

5 arrangcdso thatasubstantial depth of the PDC is maintained upon the furnace hearth and so that the air'required to complete the oxidation is so injected as to obtain the agitationof the PDC being oxidized. A mixture of gases such as S; and air or S0 and oxygen may also be the vehicle of agitation.

It is well-known in the art of zinc metallurgy that a certain amount of lead is removed from zinc sulphide concentrates by .usual roasting under strongly oxidizing conditions in multiple hearth furnaces under carefully controlled conditions. In the pyrometallurgical zinc process, for example, lead content of the product metal is influencedfin major degree'by'the amount of lead eliminated during roasting. With good practice, it ispossible to .eliminate about 90% of the lead during the usual roasting in the multiple hearth furnace. For example, a concentratecontaining 0.5 lead' can be made to produce a calcine containing about 0.05% lead. From a concentrate' containing 1%flead, a calcine containing 0.1% to 0.15% lead can'befproduced. v

In the subsequent operation 'of sintering, additionalbut less important'amounts of lead are removed, mak ing it possible to produce fromthe smelting furnaces a high .grade metalcontaining as low as 0.025% lead when the feed to the roaster contains as much as 0.5% lead.

Whereas prior art processes attempt to remove lead. by roasting; i. e.,' oxidation, this invention makes'it possible to removelead and similar impurities more efiectively by minimizing oxidation and maintaining as high a concentration of sulphides as possible. 1 In this processthere is practically no free oxygen in the exit gases from the first stage or purification reactor, quite in contrast to prior art processes in which the exit gases commonly contain 6% to 71/2% sulphur dioxide and 7 to 10% oxygen. In the process of this invention, the sulphur dioxide contentof the exit gases approximates 14% to 16%, depending on the composition of the air or other fluidizing gases entering the reactor.

The process is preferably operated continuously with high-lead zinc concentrate feed entering the purification reactor at a predetermined rate and deleaded PDC being withdrawn from the reactor at a related rate. Heat is normally supplied by admitting air to the reactor and partially burning the concentrates.

Whena multiple hearth roaster is operated according to this method 'for initial purification and partial desulphurizationof commercial zinc concentrates, much larger tonnages canbe treated in a given size of furnace than when the same "furnace is used for roasting inconventional manner.

In a typical run of the purification step, zinc sulphide concentrates are'fe'd' at the rate of 132 tons'per day to a Nichols-Herreshoif furnace-20'"6 inside diameter, with twelve super-imposed'hearths and having anormal oxidizing or dead roasting capacity of '60 to 70 tons'of concentrates per day.- The zinc concentrate feed contains 1.05% lead and31.4'% sulphur. As the ore progresses downwardly through the furnace, its temperature is increased so that on seven of the twelve hearths the temperature is above'900 C., and on four of the hearths the temperature is above 950 C. At the same time, a small amount of air is admitted in the lower part of the roaster in order to partiallybu'rn the concentrates to secure the necessary heat. The lead content progressively decreases as the ore passes downward'through the roaster. The ore (PDC) discharged from the roaster contains 0.027% lead and '-18.1%.sulphur.- 1 a In another run, the same species of zincconcentrates are fed at the rate of 1 25 'tons'per day to the aforementioned multiple'hearth furnace; and the dischargedprodnot contains 0.012% lead and 17.1% sulphur. 'Other runs, described more fully hereinafter, give similar results. a C I During some of these runs, :it isfound convenientto burn "fuel gas on the fourth, tenth,:an'd the twelfth hearths of the furnace in order better to control the.t em'-' perature distribution. With some concentrates fuel may be used as a convenience, but its use is not a necessity. In another run for example, "122 tons per day of concentrate containing 0.55% lead are fed tov the multiple hearth furnace; and the discharged material contains 0.020% lead. No fuel gas is burned during this run.

The elimination of cadmium by the method of this invention is also much greater than that of prior art processes. In the run first-mentioned hereinbefore, the cadmium content of the feed material is 0.10% while the cadmium content of the material discharged from the furnace is only 0.027%. i

In another run, zinc sulphide concentrate is fed to the aforesaid multiple hearth furnace at the rate of approximately 100 tons per day. The feed assays approximately 57% zinc, 0.55% lead, and 32% sulphur. Duringoperations, temperature of the second hearth from the top is maintained at approximately 800 C., the third hearth .at 880 C., fourth to ninth hearths inclusive at 900 to 935 C., tenth hearth at 800 C., eleventh hearth at 765 C., and the twelfth hearth at 525 C. The following results are obtained. For a 24-hour period when the amount of concentrate fed to the roaster is 93.9 tons, the product assays 0.007% lead and 19.5% sulphur. For a 24-hour period when the amount of concentrate fed to the roaster is 98.6- tons, the product assays 0.009% leadand 19.0% sulphur.

As mentioned in the first described run, when the furnace is fed with a zincvsulphide concentrate assaying 55.0% zinc, 1.05% lead, and 31.4% sulphurat the rate of 132 tons per day, the product assays 0.027% lead nd 18.1% sulphur. When the furnace is fed with .zinc concentrates assaying 54.0% zinc, 2.08% lead, and 31.5% sulphur at the rate of about tons per day, the product assayed 0.054% lead and 16.5% sulphur. While specificruns have been described, it is not wished to limit this invention to the conditions described in the runs. In other runs, the invention is practiced successfully using different temperatures than the ones recited hereinbefore. It will be readily understood that the concentrate production of ,various mines will influence to a considerable extent the temperature and atmospheric conditions required, the more refractory ores or concentrates requiring temperature and reduction conditions of the higher order. In some of the runs, temperatures in excess of 950 .C. are used on two to five or more of thehearths with good results. By means of preheated gasessuch .as derived from combustion of auxiliary fuel or'as derived from the subsequent "desulphurizing step to be described, great leeway is provided in exercising control both over temperatures and over sulphur content of the concentrates during passage through the furnace.

' Itisnecessary 'to remove the sulphur from the PDC in orderto prepare a high grade calcine suitable for ,agglomeration for'feeding'to reduction furnaces. To this end, an autogenous desulphurizing step is carried out in a gas agitated bed operated under oxidizing conditions. The preferred practice of this invention, therefore, calls for the passing of the purified product (PDC) discharged from the multiplehearth furnace into a boiling bed reactor. Through tuyer'es in the Walls or floor of this reactor, air'or other oxidizing gas is introduced in quantities sufficientboth toagitate and to oxidize the particles of zinciferous material. 'By means of this process, it is ,possible to desulphurize autogenously purified .PDC. containing as little as 1.0%. sulphide sulphur.

The purified and desulphurized zinc calcine produced by the practice disclosed herein issuitable after. agglomeration for making of .high purity zincmetal-directfrom reduction furnaces without the necessity'ofsubsequent redistilling or refining and for manufacture of commercial lead-free zinc-oxide pigments.

The invention will be described further with reference to the drawingswherein:

Fig. 1 is a diagrammatic view of one form of apparatus in which the process of the invention is performed; and

'Fig. 2 is a digrammatic view of another form of apparatus'suitable for carrying out the process of the invention.

Referring to the drawings, particularly to Fig. 1, represents a multiple hearth furnace and 11 a fluidized bed reactor. Zinc sulphide zinc concentrate feed, preferably dry, enters the multiple hearth furnace 10 through seal 12 from suitable conveyor 13 through a bin 14. Part of exit gases from the fluidized bed reactor 11 may enter the multiple hearth furnace directly or, after being divested of fume products in dust collecting apparatus 15 may enter the multiple hearth furnace 10 through duct 16 and be admitted to various hearths in desired quantities and pass upwardly through the furnace in counter-current flow relationship to the downwardly moving zinc concentrates. The gases leave the multiple hearth furnace 10 through duct 17, pass through waste heat boiler 18, through fan 19, through a dust and fume separating device 20, which'suitably may be a Cottrell type electrostatic precipitator, and through duct 21 to further process- 1ng apparatus such as a contact sulphuric acid plant which is not shown. The shaft 22 of the multiple hearth furnace 10 and the rabble arms 23 attached thereto are rotatedat a suitable speed such as l R. P. M. by drive umt 24. The rotating rabbles cause the concentrates to feed across the hearths and down through the roaster in a manner well-known to those skilled in the art. The ore which has been treated in the multiple hearth furnace leaves through conveyor 25 and star valve 26 which discharges into surge bin 27. From this bin pipe 28 delivers the deleaded concentrate to feeder 29 which supplies deleaded sulphur-containing ore to fluidized bed reactor 11. Feeder 29 may suitably be a screw conveyor driven by a variable speed drive 30. The material delivered by feeder 29 to reactor 11 is maintained in a fluidized state in the bed 31 by means of gas such as air blown in through tuyeres 32 located preferably in the floor of the reactor 11. Air is supplied to tuyeres 32 from a suitable supply such as a blower 33. An amount of air is supplied which will give a linear upward gas velocity in reactor of between one and three feet per second. Gases leaving reactor 11 pass through duct 34 to enter dust separation device 15, suitably a refractory-lined cyclone. Part of the gas leaving the cyclone 15 through exit duct 35 is diverted to the lower portion of the multiple hearth furnace through duct 16 as described hereinbefore, while the remainder of the gas passes through duct 36 to waste heat boiler 37 thence through fan 38, Cottrell 39 and on through pipe 40 to a further processing apparatus such as a sulphuric acid plant which is not shown.

Overflow of oxidized calcine from the fluidized bed 31 passes through an overflow port 41 and seal 42 through conveyor 43 to a bin 44 and thence by way of conveyor 45 to the next processing step. Dust separated in cyclone 15 and in waste heat boiler 37 may join through conveyors 46 and 47 the overflow product delivered into bin 44. Dust and fume recovered by Cottrell 39 pass by conveyor 48 to a further processing step. Alternatively, the Cottrell dust from conveyor 48 may be diverted to the bin 44 or may be returned, if desired, to raw concentrate feed entering furnace 10 through conveyor 13.

Dust recovered in waste heat boiler 18 passes through conveyor 49 to the next processing step or suitably may be returned to multiple hearth furnace 10 through conveyor 13. Leady fume collected by Cottrell passes by conveyor 50 to the next processing step.

In another variation of the invention, referring to Fig. 1, exit gases from the reactor 11 are passed directly mto multiple hearth furnace10. It is also possible to discharge PDC from the bottom hearth of furnace 10 directly into reactor 11.

In some circumstances, it is not necessary to place a dust catcher 15 ahead of waste heat boiler 37. Also,

8 it is not always necessary to pass multiple hearth furnace exit gases through waste heat boiler 18, for example when operating with a cold top furnace. For purposes of illustration, however, there has been shown a more general arrangement of our invention.

In the operation of this process, it is convenient to vary the proportion of fluidized reactor exit gases diverted through duct 16 to multiple hearth furnace 10 by varying the speed of either or both fans 19 and 38. Enough gas is diverted through furnace 10 to develop and maintain temperatures suitable for eflicient deleading. Experiments have shown that it is desirable to maintain temperatures in excess of 900 C. on at least two of the hearths of furnace 10 and preferably to maintain temperatures in excess of 950 C. on at least five hearths. The concentrate feed rate through the system is adjusted so that with an air velocity through reactor 11 of one to three feet per second there will be a suitable stoichiometric excess of oxygen in the exit gases passing through duct 34. In general, it is desirable to maintain an oxygen concentration of 1% to 7% in these gases.

In another of the preferred methods, as the first step in purification of sulphidic zinc'concentrates, the concentrates are fed continuously through a first fluid bed reactor and thereafter through a second fluid bed reactor.

Fig. 2 illustrates this arrangement for carrying out the invention. In purifying reactor 51 a bed of zinc concentrates is maintained in an agitated condition by injection of air through tuyeres 53 located in the floor of the reactor. Air is supplied from any convenient source such as blower 54. Zinc concentrates, preferably thoroughly dry, are delivered from a stockpile or other facilities, not shown, by conveyor 55 to feed bin 56. A conveyor 57, which suitably may include a weighing device, delivers concentrate to reactor feeder 58 which may suit-. ably be a screw conveyor driven by variable speed drive 59.

Temperature of the reactor 51 may be controlled by varying the ratio between air input rate and concentrate feed rate. In general, it is found preferable to maintain a fixed air input rate and to vary the concentrate feed rate, for example by means of variable speed drive 59, to control the temperature within the reactor bed 52.

It is particularly convenient to control the bed temperature automatically by means of a conventional temperature recorder and controller which, through suitable linkage, adjusts the concentrate feed rate in response to variations indicated by a thermocouple in the boiling bed. A thermocouple may be located 6" to 18" above the bottom of the bed; but because of the remarkable uniformity of temperature in a properly operated fluidized bed, location of thermocouple is not at all critical. 7

It is the preferred practice to maintain as high a bed temperature as possible without incipient fusion of the particles in the bed. If too high a temperature is reached, the bed may sinter and lose its mobility characteristics. The highest workable temperature depends upon the species of concentrates being treated as some compositions fuse at lower temperatures than others. In general, it has been found possible to operate consistently with bed temperatures of the order of 1000 C. It is also interesting to note that in comparative tests in which the same species of zinc concentrates was treated in a multiple hearth furnace and in the mobile bed reactor,

the operating temperature attainable before sintering or I agglomeration occurred was significantly higher in the latter reactor.

Gaseous reaction products together withthe sublimed impurities and smaller particle size constituents of the concentrate feed leavethe top of the reactor 51 and flow through short duct 60 to a dust separating device 61 which may conveniently be a cyclone separator. Alternatively the cyclone can be located within the reactor or in an upward extension thereof. However, with presently available materials of construction, it is usually -'9 more convenient to locate the deduster exteriorly to, but contiguous with, the reactor.

Exit gases leave cyclone 61 through a duct 62 and pass successively through a waste heat boiler 63, a fan 64, a fume-separating device 65, which suitably may be a Cottrell electrostatic precipitator, and on through a duct '66 to further processing apparatus, not shown, such as a contact sulfuric acid ,plant. Dust settled in waste heat boiler 63 is withdrawn through pipe 67 and delivered to bin '68 or alternatively is returned to feed bin 56 by suitable conveyingmeans, not shown. Leady'fume and other impurities volatilized by the reactor are recovered in Cottrell 65, withdrawnby conveyor 69, and are passed to a.-further processing operation for recovery of the contained metal values.

Dust caught in the cyclone 61 passes through downspout 70 to bin 71 which may'deliver'through pipe 72 to a feed bin 73 or the dust may'be returned to reactor 51 as described hereinafter. The nature of the cyclone dust depends on the mode of operation of the reactor and the temperatures of the gas .in the rdedus'ter, as will be disclosed hereinafter. In general, the greater the linear gas flow rate upward through the reactor and the smaller the size of the concentrate particles, 'the more will be the amount of dust carried over with the exit gases. Conversely, lower air rates and larger particle sizes tend to lessen the amount of dust in the exit gases. There is a lower limit to the gas flow rate below which the boiling bed tends to lose its "mobility characteristics. In general, gas flow rates of l t'o'-3 feet per second maintain satisfactory mobility of the bed. Particle size and particle size range vary considerably from one species of concenrate to another. Even-fairly'large size particles, such aspellets formed by therolling-actionof concentratesin-storageor-in passing through a dryer or tumbling action conveyor, will fluidize satisfactorily if the size range is not too narrow.

If the exit gas from reactor 51 is allowed to chill before or when it enters the deduster 61, there is a tendency for some of the sublimed. and volatile compounds to come down with the dust. If it is desired to blend this dust with the purified concentrates which overflow from the reactor 51, then thegases are kept hot until they leave the .deduster. duce .a lead-enriched product as well as a lead-impoverishedmateriaLit is convenient to chill the gases before they enteror while they .pass through-the deduster. For thislatterpurpose the deduster may'take the form of a waste heat boiler followed by a cyclone. Under some circumstances it is desirable to recireulate a part of the cyclone dust from the bin 71 back through the reactor 51. For this purpose the dust is passed throughpipe 74 to join with the concentrate at feeder 58 through bin 56, or it can enter the reactor51 through a similar but separate feeder, not shown.

As indicated above, one of the methods of practicing this invention contemplates chilling the exit gases leaving reactor 51, by passing'said gases first through a waste heat boiler and thereafter'through a'cyclone in the reverse order o f'that shovvn'in Fig. 2. By operating in this manner it ispossible'to producea lead-enriched product suitable, after desulphurization, for the production of prime western zinc metal, leaded Zinc pigments, and so forth. For this mode of beneficiation, cyclone dust frombin 71 and boiler-dust from bin 68 are comingled and desulphurized in afluidized bed reactor not shown which, in effect, operatesin parallel with reactor75 and is essentially .a duplicate of'theiequipment assoc'iated'therewith.

-Aswconcentrate enters through feeder '58, PDC is withdrawn from .thereactor .51. Thismay be done in any convenient mannerone especially. convenient way being that there is little danger of short circuiting between feed and overflow. Whether the product is withdrawn from the surface-of the bed, from the bottom, or from an intermediate level, or Whether the feed be introduced above, at, or beneath the top of the bed,-seems to make little ditference; the overflow product is not contaminated with feed material.

From 'bin 73 PDC is withdrawn at regulated rate through conduit 79 by feeding device 80, suitably a multiple screw conveyor driven by variable speed drive 81. Feeder 8t) discharges PDC into reactor 75 where it forms the fluidized bed 82. Air for agitation and oxidation of boiling bed 82 is supplied by suitable means such as blower 83 through tuyeres 84 suitably disposed in the bottom or lower'walls of the reactor 75. An amount of air is supplied which will give an upward gas velocity in reactor 75 or" the-order'of l to 3 feet per second. Gases leave'reactor 75 through .shortduct 85 and enter dust separation device 86, which suitably may be a'refractorylined cyclone or acyclone lined with corrosion-resistant steel :of the 28% chromium variety. Gases pass from dust separation device 86 through duct 87 to waste heat boiler'fifi, thence through fan "89 and Cottrell 90 and on through pipe 91 to further processing operation such as a On the other hand, if it is desired to .pro-

sulfuric acid plant, not shown. Prune collected by Cot- =trell passes by convey0r 92 to asubsequent processing step. Dust recovered in waste heat boiler 88 passes by conveyor 92 to join the dust'dis'charged from collection device 86 in bin 93.

Overflow from boiling bed 82 0f reactor 75 passes through pipe95, star valve 96 and pipe 97 to bin 93. This overflow. product, together with the dust recovered from cyclone-86 and waste heat boiler 88, constitutes the final purified oxidized zinc calcine product of this process.

Further description of this invention is given in the following example. In this example which pertains to the first orpurificationstep, the reactor consists of a steel shell 256 high by 4"1l diameter lined with 4 /2" of insulatingbrick-and 4%" of firebrick up to 7'3 level and with 4 /2" of firebrick above this point. To conserve heat the external surface of the reactor shell is lagged with insulationsuflicient to raise'the steel temperature to 200- 250 C. Reactor gases pass through a cyclone, thence through aflueand variable speed fan to a contact sulfuric acid plant gas 'purification system. Dry zinc concentrates enter the reactor at about the five-foot level and are fed by a 4 diameter screw conveyor which in turn is supplied by a feeder of the constant weight type-equipped with a variable speed drive fforready feed weight adjustment. A star gate arrangement seal-s the entry to the screw feeder.

Air-is supplied -.to the reactor'through a number of tuyeres inserted through the refractory floor of the reactor or alternatively through spokes 'of various lengths inserted horizontally through'the wall of the reactor at floor top level.

Reactor 'bed overflow .ports are provided at various levels, but for the results-reported here the overflow ports at S-foot and 3 /2-foot elevations are used.

To start a reactor of this type it is usually suflicient-to preheat the brickwork to abright'cherry red temperature, turn on the air and start the concentrate feed. Some species of oncentrate ignite more readily than others. Sometimes it is convenient to light OE With a readily 'burnable high iron concentrate and then switch to the less readily combustible material. On occasion it has also been found helpful to preheat the air before it enters the reactor.

Temperature of the-boiling bed'is measured with thermocouples inserted through the reactor wall. In a properly Operating bed there are no more than minor temperature differences fromside to side and from top to bottom of the bed. Temperatureis controlled 'by rate of feed addition. With dry concentrates and unpreheated air it is entirelypractical to maintain a sulphide sulphur 11 concentration in the bed of 24% with a 32% sulphur feed and to operate at 1000-1100 C.

In Table l are presented data typifying the operation of the above-described reactor when purifying zinc sul- 12 Concentrate feed enters the highest reactor in the series and passes downward through appropriately disposed overflow conduits to the gas agitated bed below. While theoretically any number of stagesmay be placed in phide concentrates. In runs 17A through D the gases series flow as described,'practically it is considered that emerging from the boiling bed are chilled before and durthree such stages are the workable limit in the present ing their passage through the cyclone deduster. In runs state of the art, especially if bed temperatures of the 19, 20A, and 20B, an attempt is made to conserve the order of .1000 C. are employed. heat in the gases between the time they leave the bed and In another arrangement it is found desirable to comthe time they leave the cyclone. Heat conservation is bine in stagewise relationship a boiling bed purification accomplished by lagging the upper part of the reactor section and a boiling bed desulphurizing section. The with heat insulation material and by using a refractoryoperation of the boiling bed desulphurizer is the same lined cyclone. With bed temperature of 1050 C., the as that previously described for the purifier except that temperature of the gases leaving the cyclone is 950 C. an excess of air is employed in the desulphurizer whereas Table 1 Run No. 17A 17B 170 17D 19 20A 20B Bed Depth, feet 5 5 5 3.5 3. 5 3. 5 Feed Rate, Tons/Day 10.4 12.5 17.4 14.5 18.4 21.0 21.8 Airflow Rate, C. F. M. STP- 200. 220 222 225 225 22s 2 Bed Temperature, C 1, 050 1,075 1,000 1,075 1,050 1, 050 ,050 Percent of Product Appearing in Bed Overfl0w 64. 1 62.3 61. 2 64. 67.6 67. 1 59.2 Percent of Product Appearing in Cyclone Heavies 22.9 27.9 27.7 18.0 13.4 8.6 13.8 Percent of Product Appearing in Cyclone Lights 13.0 9. 8 11. 1 17.4 19.0 24.3 27.0 Lead Content of Feed, percent 1. 13 1.13 1. 45 0. 85 0.35 0. 36 0. 61 Lead Content of Bed Overflow, percent 0.04 0. 07 0. 028 0.016 0.016 0.024 Lead Content of Cyclone Heavies, percent. 1. 61 2. 1.36 0. 24 0.21 0. 60' Lead Content of Cyclone Lights, percent 4. 26 8. 02 7. 50 3. 75 2.07 1.41 2.00 Percent of Lead in Feed Appearing in Bed Overflow 3.8 1.9 1.9 1.9 2.9 2.9 2.2 Percent of Lead in Feed Appearing in Cyclone Heavies 51.0 36.7 41.5 26.5 10.1 4.8 13.2 Percent of Lead in ig ts 45.2 61.4 57.6 71.6 87.0 92.3 84-6 Other modes of practicing the invention are described a stoichiometric deficiency of air, with respect to the rate hereinafter. of input of metallic sulphides, is maintained in the puri- It has been found that, for a given set of conditions, fication step. Temperature control in the desulphurizer the higher the sulphide sulphur content of the bed of a can be achieved by varying the quantity of excess air or first or purifying reactor, the greater is the degree of by introducing into the bed a vaporizable coolant such volatilization of impurities. In the simplified version of as water or drip liquors from nearby acid plant or by the invention described -above--and which is entirely m ans f Water Steam'generating tubes Suitably P adequate for most applicationsheat and temperature 40 Automatic temperature control can be achieved by reguof the bed are developed by burning a portion of the lating the addition of coolant by means well-known in sulphides in the bed. The amount of sulphide thus rethe art. quired to be burned can be substantially lessened by pre- In desulphurizing in a gas agitated bed we have found heating the incoming air and by preheating the incoming it not only possible but entirely practical to operate with zinc sulphide concentrates. For example, with air preonly a few percent stoichiometric excess of air and yet heated to 800 C. and concentrate feed preheated to obtain calcine running consistently less than 1% total 500 C., the attainable sulphide sulphur content of the sulphur content. The gas from such a reactor contains bed rises from about 24%, when operated under the conapproximately 12-14% S0 It is convenient to reguditions previously described, to about 29%. late the concentrate feed rate automatically by means Air preheating is conventionally accomplished by reof an oxygen analyzer such as the Beckman Oxygen cuperative or regenerative means such as shell and tube Analyzer and controller. This device continuously samheat exchanger or a pebble heater. We have found, howples and determines the oxygen content of the gases leavever, that a much simpler and cheaper way of preheating ing the gas agitated desulphurizing bed and in turn air consists of burning the fuel, such as natural gas, adjusts the concentrate feed rate to maintain the oxygen under pressure right in the air duct (brick-lined) ahead content of the exit gases at a predetermined value. Good of the tuyeres. The hotter the mixture of air and prodresults are obtained even when the oxygen content is ucts of combustion, the less sulphide sulphur needs to be held as low as 1% O in the exit gases. burned in the bed fluidized by these gases. When superimposing a purifying bed furnace on a In certain applications all or a part of the heat redesulphurizing bed furnace, it is necessary to adjust the quired in the purification step is supplied by heat transgas flow rates to give a linear flow rate in each of the fer surfaces suitably disposed in or around the boiling reactors in series of about 1 to 3 feet per second. bed. Such heat transfer surfaces may consist of refrac- Again using lead as an example, it has been observed tory or alloy tubes through the interior of which pass that the lead content of the solids which pass out of hot fluid, for example, flame and Products of combusthe purifying reactor with the gases tends to be assotion from gas burner positioned at lower end of tube. i d ith th aller particles. This circumstance Tubes or other heat transfer surfaces may also be heated makes i ibl t on entrate the lead by recycling the by within contained electrical resistance elements. Alcyckme, boiler, d fl dusts through the d l i ternatively, molten salts or molten metal or alloy at suitactor or reactors d assing the gases through an elecable temperature In11y be circulated through the tubestrostatic precipitator (Cottrell) for recovery of lead Further enhancement of p y elimination may be fume. For an extreme degree of lead concentration a attained y StageWiSe arrangement of boiling beds- I11 multiple stage Cottrell is used. Material collected in the such an arrangement, exit gases from the lowermost d first stage is recycled through the deleaders while ahigh P through tllytireS in a refractory septum P g lead product is recovered from the second and third the lowermost reaction chamber from the one immediatestages of the precipitator. 1y above it and fluidize the solids in the higher reactor. The gas ag tated bed desulphurizer and oxidizer 1s a to a high of 24 to 26% S.

highly desirable component of this invention if it is to be practiced in its most economical form. Other desulphurization methods suffer major disadvantages by comparison. Sulphide sulphur content of PDC from the purification step ordinarily may vary from a low of 10% To dead roast this material in a multiple hearth roaster requires the combustion of large amounts of auxiliary fuel such as natural gas. The sulphur dioxide content of gas from such a roaster is of the order of 6 to 7% S by volume. Desulphurization of PDC in a suspension roaster also requires the use of substantial amounts of auxiliary fuel. The exit gas from this roaster would contain about 7 to 9% S0 by volume.

By carrying out the desulphurization step in a gas agitated bed, it is possible to roast PDC entirely autogenously and to produce an exit gas of the order of 12% S0 by volume. Compression of the S0 into so much smaller volume yields important economies in the operation of the subsequent gas purification equipment and catalyst system of the acid plant. S0 the volume of gas required to make one ton of H SO is about 128,000 cubic feet (at 95% conversion eificiency in the acid plant); whereas, at 12% S0 the volume required for one ton of H 50 is only 64,000 cubic feet.

Compared with suspension roasting, a gas agitated bed desulphurizer as disclosed in this invention, is relatively non-sensitive with respect to particle size of the PDC to be roasted. With a suspension roaster, on the other hand, it is necessary to comminute the feed to a high degree to insure satisfactory operation even with the use of auxiliary fuel.

When operated as a finish desulphurizer, the fluidized bed reactor is not efficient as a purifier. By the methods disclosed in this invention, it is now possible to accomplish the substantially complete elimination of those impurities which under controlled conditions of temperature and atmosphere will sublime or become volatile, such as compounds of lead, tin, cadmium, and the like. This For example, at 6% invention provides means to accomplish both purification and desulphurization in an economical and commercially practicable manner. By doing the purification before the desulphurization We are able to utilize the gas agitated bed method With all its advantages; whereas, if, for example, non-deleaded concentrates were fed to the fluidized bed desulphurizer, only a lead-containing calcine Would be produced.

We claim:

1. The process for purifying and desulphurizing zinc sulphide ore containing volatilizable compounds of metallic impurities such as lead, tin and cadmium which comprises passing an oxidizing gas in'contact with the ore in a first oxidizing zone at a rate regulated to oxidize at least 10% and not more than of the sulphite content of the ore and to maintain the temperature in at least a substantial portion of said first zone in the range of 850" C. to 1050 C., and thereafter completing the oxidation of the ore by transferring the partially oxidized ore to a second oxidizing zone through which an oxidizing gas is blown to maintain a bed of the ore in a fluidized state and to oxidize autogenously the remaining sulphide content of the ore.

2. The process as defined in claim 1 wherein said first zone comprises a plurality of interconnected, vertically superposed, horizontal sub-zones and said ore and gas are passed therethrough in countercurrent relation.

3. The process as defined in claim 1 wherein, in said first zone, said gas is passed upwardly through a bed of said ore at a velocity to maintain the bed in a fluidized state.

References Cited in the file of this patent UNITED STATES PATENTS 735,903 Picher Aug. 11, 1903 1,992,049 Young Feb. 19, 1935 2,035,699 Fowler Mar. 31, 1936 2,120,475 Stimmel et al June 14, 1938 2,650,159 Tarr et a1 Aug. 25, 1953 2,689,176 Klepetko et al Sept. 14, 1954 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 2 847, 294 August 12, 1958 Carleton 0, Long et al.

It is herebi certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 14, line 15, for "sulphite" read sulphide a Signed and sealed this 21st day of October 1958.

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner of Patents

Claims (1)

1. THE PROCESS FOR PURIFYING AND DESULPHURIZING ZINE SULPHIDE ORE CONTAINING VOLATILIZABLE COMPOUNDS OF METALLIC IMPURITIES SUCH AS LEAD, TIN AND CADMIUM WHICH COMPRISES PASSING AN OXIDIZING GAS IN CONTACT WITH THE ORE IN A FIRST OXIDIZING ZONE AT A RATE REGULATED TO OXIDIZE AT LEAST 10% AND NOT MORE THAN 70% OF THE SULPHITE CONTENT OF THE ORE AND TO MAINTAIN THE TEMPERATURE IN AT LEAST A SUBSTANTIAL PORTION OF SAID FIRST ZONE IN THE RANGE OF 850*C. TO 1050*C., AND THEREAFTER COMPLETING THE OXIDATION OF THE ORE BY TRANSFERRING THE PARTIALLY OXIDIZED ORE TO A SECOND OXIDIZING ZONE THROUGH WHICH AN OXIDIZING GAS IS BLOWN TO MAINTAIN A BED OF THE ORE IN A FLUIDIZED STATE AND TO OXIDIZE AUTOGENOUSLY THE REMAINING SULPHIDE CONTENT OF THE ORE.

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