US3847595A

US3847595A - Lead smelting process

Info

Publication number: US3847595A
Application number: US00357536A
Authority: US
Inventors: S Liang; E Milner; G Toop; R Anderson
Original assignee: Teck Metals Ltd
Current assignee: Teck Metals Ltd
Priority date: 1970-06-29
Filing date: 1973-05-07
Publication date: 1974-11-12
Anticipated expiration: 1991-11-12

Abstract

A process for separately recovering lead values and sulphur values from lead sulphide ores and concentrates in which ore or concentrate in particulate form in admixture with an oxygen-rich gas is charged into a furnace containing a molten bath consisting of lead oxide-containing slag or of lead and a lead oxidecontaining slag cover. Oxidation of the lead sulphide is initiated in a combustion zone above the bath and is completed after oxidized lead sulphide particles have impinged onto and penetrated the surface of the bath. Sufficient oxygen-rich gas is supplied to provide the heat of oxidation to maintain a flame in the combustion zone at a temperature of at least 1,300*C. and to maintain said bath at a temperature of at least 1,100*C. Oxygen supplied to the furnace maintains a lead content of at least about 35 percent, as lead oxide, in the slag in order to maintain slag fluidity and to keep the sulphur content of the slag at a low level, thereby assuring production of low-sulphur lead values. Sulphur values are recovered as concentrated sulphur dioxode.

Description

United States Patent 11 1 Liang et al.

[ Nov. 12, 1974 LEAD SMELTING PROCESS {22] Filed: May 7, 1973 [211 App]. No.: 357,536

Related US. Application Data [63] Continuation-impart of Ser. No. 50,749, June 29,

I970, abandoned. v

[52] U.S. Cl 75/77, 75/78, 423/89 [51] Int. Cl C22b 13/00 [58] Field of Search 75/77, 78, 63; 423/89 [56 1 References Cited UNITED STATES PATENTS 3,300,301 l/l967 Malmstrom 75 77 3,326.67] 6/1967 Worner 1. 75 77 X 3,281,237 10/1966 Meissner et al 75/77 Primary Examiner-Herbert T, Carter [57] ABSTRACT A processfor separately recovering lead values and sulphur values from lead sulphide ores and concentrates in which ore or concentrate in particulate form in admixture with an oxygen-rich gas is charged into a furnace containing a molten bath consisting of lead oxide-containing slag or of lead and a lead oxidecontaining slag cover. Oxidation of the lead sulphide is initiated in a combustion zone above the bath and is completed after oxidized lead sulphide particles have impinged onto and penetrated the surface of the bath. Sufficient oxygen-rich gas is supplied to provide the heat of oxidation to maintain a flame in the combustion zone at a temperature of at least l-,300C. and to maintain said bath at a temperature of at least l,lOOC. Oxygen supplied to the furnace maintains a lead content of at least about 35 percent, as lead oxide, in the slag in order to maintain slag fluidity and to keep the sulphur content of the slag at a low level,

- thereby assuring production of low-sulphur lead val- 19 Claims, 1 Drawing Figure PATENTED NOV 12 1974 This application is a continuation-in-part of application Ser. No. 50,749 filed June 29, 1970, now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to an improved process for the smelting of lead sulphide ores and concentrates.

The earliest known process for smelting lead sulphide ore involved oxidizing the ore on an open grate. The principal reactions taking place in such a smelting operation are as follows:

PbS Pb S0 2PbS 30 2PbO 280 The metallic lead so formed in accordance with Equation I was collected for subsequent refining while the lead oxide slag was separately reduced, for example, using coke, according to the following equation:

2PbO c 2Pb co III form an intermediate material containing a major proportion of lead oxide according to Equation II. Such roasting reactions have, for instance, been carried out so as to provide a lead oxide sinter which is subsequently transferred to a blast furnace for reduction with coke according to Equation III.

The well known roaster-reaction process differs from the roast-reduction process in that the initial oxidation is only partial so as to provide an intermediate material containing lead oxide according to Equation ll, unconverted lead sulphide, and lead sulphate according to the equation:

PbS PbSO,

The resulting intermediate material may then be heated at an appropriate temperature essentially in the absence of .oxygen to cause reaction to occur between the unconverted lead sulphide and the oxidized products according to the following equations:

PbS PbSO 2Pb 2SO One might consider as a roast-reaction process one that involves charging the lead sulphide ore to a reverberatory furnace in which it is heated with a supply of air to form such an intermediate product, the air supply being then discontinued and the furnace temperature increased to cause the reactions of Equations V and VI to take place, with metallic lead and sulphur dioxide produced as end products. It will be appreciated that, unless the lead sulphide on the one hand, and the lead oxide and the lead sulphate on the other hand, are present in exactly stoichiometrically equivalent amounts in the intermediate material, there will be either an excess or a deficiency of available oxygen in the second stage of such a roast-reaction process. This is a difficult balance to maintain; Therefore, roast-reaction processes have been developed so as to be operated deliberately with either an excess or a deficiency of oxygen.

In the process of US. Pat. No. 2,416,628, lead sulphide concentrate is melted and partially oxidized in an electric furnace. Oxygen is provided by blending the concentrate with oxidic materials such as recycled fume and partially roasted concentrate containing lead oxide and lead sulphate. By avoiding the use of oxidizing gases, and by the formation of a low vapor pressure Pb-PbS solution, volatilization of lead with off-gases is substantially decreased in this stage. However, subsequent air blowing of the Pb-PbS solution in a separate furnace, even under a molten glass cover, was found to result in volatilization of as much as 40 percent of the lead requiring subsequent recovery in a gas purifying plant and recycle of the flue dust by mixing it with feed to the electric furnace.

Operation with close to stoichiometrically equivalent amounts of sulphidic and oxidic feeds is the object of US. Pat. No. 2,797,158. Alternate batches of sintered material, one with an oxygen deficiency and the next with an oxygen surplus, are obtained by roasting lead sulphide concentrate admixed with oxidized material in appropriate proportions. Several such batches are combined to form a charge which is heated to a temperature between 1,100C. and -l,250C. to produce metallic lead and volatile sulphur compounds as set forth in the reactions of Equations V and VI.

In the operation of the process of US. Pat. No. 2,797,158, as further illustrated in Transactions of the Metallurgical Society of AIME, 224, 1962, pages 939-944, a circulating load of zinc increases continuously in the fume that evolves from the heating of the sintered material. Although Canadian Pat. N 0. 574,935 provides for the removal of this zinc, which otherwise makes fluidity of the slag difficult to maintain, separate blast furnace or hydrometallurgical treatment of the fume is required.

All the operations hereinbefore described have the disadvantages-that material must be transferred physically between the separate stages and that full advantage cannot be taken of the exothermic nature of the reaction between lead sulphide and oxygen. Consequently,-fuel or electrical energy is required during the second stage to melt the intermediate reaction product i and to sup'ply the heat required for the endothermic reactions between lead sulphide, and lead oxide and sulphate. Moreover, separate discharging of gases from the two stages and the dilution of the sulphur dioxide with gaseous combustion residues complicates the recovery of said sulphur dioxide. Additionally, high grade concentrates containing as much as 80 percent lead sulphide which are presently available from flotation units present certain problems when treated on conventional sintering machines. To overcome these problcms, it has been necessary to dilute such concentrates with slag-forming materials and/or with recycled material and fly ash, or to use specially designedupdraft sintering machines In order to avoid these difficulties, several proposals have recently been made for the conversion of leadsulphide ores and concentrates in single stage operations. Most of these recent proposals have involved feeding the lead sulphide and an oxygen-containing gas such as air to a furnace to obtain the direct conversion to metallic lead in accordance with Equation 1.

To operate known direct smelting processes efficiently, it appears essential that the lead sulphide and the oxygen-containing gas be introduced into the furnace in intimate admixture with each other and in such relative proportions that as little lead oxide as possible is formed by the reaction of Equation II. Shortcomings such as batch operation, need for external heat, dilution of sulphur dioxide by-product and refractory failure are still encountered in these lead smelting processes.

SUMMARY OE THE INVENTION By the process of the present invention, several unexpected advantageous results may be obtained under the conditions of operation, including the use of an oxygenrich gas which, combined with the available oxygen content of solid oxidic material, such as PbO and PbSO that may be charged to the process, is sufficient to ensure an excess of oxygen over that stoichiometricallyrequired for conversion of the lead sulphide completely into metallic lead and conversion of other sulphides, e.g., zinc sulphide, if present, to oxides. With this excess of oxygen, a slag is formed containing an excess of lead oxide which, according to the reaction of Equation V, drives off, as sulphur dioxide, sulphur contained in lead sulphide entering the molten slag. As a result, the lead settlinginto the bullion is very low in sulphur. We have found, with a slag containing at least 35 percent lead as lead oxide and kept above l,lOC., such sulphur rejection is assured. Substantial recoveries of low-sulphur lead bullion were obtained with slags containing as much as 55 percent lead as lead oxide.

In accordance with the present invention, the lead sulphide ore or concentrate is charged in particulate form and in admixture with said oxygen-rich gas, preferably containing at least 75 percent oxygen, downwardly through one or more nozzle feeders into a furnace which contains a molten bath consisting of lead and a lead oxide-containing slag cover. Oxidation of the downwardly moving charge of lead sulphide is initiated inside the furnace prior to forceful impingement of the particulate material onto the molten bath and penetration of the surface therof, and oxidation continues after such impingement and in contact with the molten bath. The forceful impingement of particles on the molten bath serves to ensure completion of the desired reactions which are believed to proceed according toEquations I, [I and V. The furnace thus is operated to ensure that the formation of PbSO is thermodynamically unfavorable in the gas and slag phases in the combustion zone of the furnace,'thereby avoiding the reaction of Equation IV.

The process of the invention is intended to be operated continuously but may be operated batchwise in conjunction with a reduction step in the same furnace. The removal of molten lead and slag material from the smelting furnace may be intermittent if desired. The slag, for example, may be withdrawn from the furnace either intermittently by the provision of a suitable taphole or continuously by the provision of a slag overflow weir. The lead oxide-containing slag can be subsequently reduced to recover the leadtherefrom.

The lead obtained from the furnace is soft lead, low in arsenic and antimony. Operating experience shows that essentially all of the arsenic and antimony in the feed reports to the slag.

Special refractories are not required for the furnace. A good grade of chrome-magnesite brick provides a satisfactory lining. The furnace is compact because of the short distance between the feeder nozzle and the molten bath. The concentrate or ore feeder system provides sufficient mixing with the oxidizing gas to give ignition and a sustained flame. The furnace may be rectangular or oval in shape with one or more concentrate or ore feeders.

The furnace is operated with a bath temperature of 1,1001,300C. although the temperature in proximity to the stream of material impinging on the molten bath is normally above 1,500C. High temperature of the combustion flame prevents the formation of PbSO which is unstable at 0.1 atmosphere oxygen partial pressure at temperatures above 1,300C. The point of impingement and the position of the flame should be far enough from the furnace walls to minimize damage to the refractory lining on the walls. The nozzles through which the feed stream enters the furnace should be dimensioned to provide the required supply rate and should be disposed a sufficient distance above the surface of the molten bath to permit adequate oxidation of the lead sulphide prior to impingement, and to avoid nozzle plugging by accretions.

Although thermodynamic calculations indicate that it is technically possible to use oxidizing gas with as low as 60 percent oxygen in this process, heat .retention in the furnace is considerably imporved by the use of gas containing at least percent oxygen, so as to reduce the heat and dust losses from the furnace due to the throughput of inert gases,e.g., nitrogen. Such use of oxygen produces exhaust gases having a high concentration of sulphur dioxide. At this concentration, sulphur dioxide can be recovered more readily than at concentrations formed by oxidation with air.

In order to obtain a substantial lead fall in the furnace as low sulphur bullion, operation with a slag containing from about 35 to 55 percent lead in the form of lead oxide is preferred. Slag containing less lead permits too much sulphur to enter the bullion. Because of a higher proportion of zinc and iron,.Which is present in the ferric form, additional flux is required to maintain fluidity of slags that contain less than 35 percent lead. It is not necessary to have more than about 55 percent lead in the slag for production of a low sulphur bullion. However, it may be convenient, as hereinafter explained, to recover as lead oxide in a low sulphur slag, all, or nearly all, the lead that is not given off as fume.

The particle size normally found in commercial lead concentrates, for example flotation products, is suitable for the combustible feed to the furnace.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to the drawing, it will be seen that a rectangular furnace indicated generally therein at 10 has a sloping floor 12 extending from a lead well14 at one end of the furnace to a restricted slag overflow weir 16 at the other end. A rising passage 18 extends upwardly and outwardly from the lead well 14 through the end wall of the furnace 10 to provide a seal and to permit the overflow of molten lead from the furnace into a lead-receiving vessel 20. An exhaust flue 22 extends through the roof of the furnace to a dust recovery unit and a sulphur dioxide recovery system, not shown. A slag-receiving vessel 17 is shown for receiving the slag material from the slag overflow weir 16.

At one end of the furnace, two

feed nozzles

24 and 26 extend through the roof of the furnace 10. The nozzle 24 is supplied by a line 30 with an oxygen-rich gas into which particulate lead sulphide ore or concentrate is introduced by a screw conveyor 32 from a concentrate hopper 34.-The nozzle 26 is fed from hopper 38 by a screw conveyor 36 with particulate oxidic material which may include recycled fume, flux-forming material and sulphate leach residues from an electrolytic zinc plant. As the temperature of the slag and its lead content are higher than in the conventional processes, the slag, to be sufficiently fluid, requires only a small input of siliceous flux material, if any, thus providing the advantage of a lower slag volume. Flux material is fed, if required, from the feed hopper 38 by screw conveyor 36 into nozzle 26 to maintain the required slag fluidity.

In operation, the furnace 10 contains a molten bath 48 consisting of molten lead 40 covered by a molten slag layer 42 which contains lead oxide. The particulate lead sulphide ore or concentrate in intimate admixture with gaseous oxygen is introduced into the furnace through the nozzle 24 and the stream 43 issuing from this nozzle ignites in the space above thebath 48 to provide a high temperature flame in combustion zone 44 in which part of the oxidation of the lead sulphide occurs. In practice, temperatures as high as l,700C. or higher are obtained within the flame. Such temperatures were estimated -by thermodynamic calculations. Temperature readings could notbe obtained with thermocouples sheathed in stainless steel, which has a melting point of about 1,500C. Rapid destruction of these thermocouples indicated flame temperatures substantially above l,5( )0C. Presence of fume precluded the use of optical devices;

Before the oxidation of the lead sulphide particles is complete, the flame forcefully impinges-on the molten bath 48 disturbing the slaglayer 42 as shown at 46. This forceful impingement on the molten bath serves to allow reactions according to Equations I, II and V to reach equilibrium at an enhanced'rate. The injection feed nozzle 24 is disposed inwardly from the furnace walls to reduce damage by the high temperature flame to the refractory linings of these walls. The distance from the discharge end of the nozzle 24 to the surface of the molten bath is sufficient to prevent plugging of the nozzle by accretions and to permit sufficient oxidation of the lead sulphide concentrate prior to impingement in order to get a bath surface temperature of at least l,l00C. but short enough to ensure that the desired forceful impingement is obtained. The distance from the nozzle to the bath surface is dependent on the size of furnace, tonnage of concentrate treated and velocity of the downward stream. For a pilot plant of the type shown in the attached drawing, and treating ten tons of concentrates per day, 36 inches were effective. For larger furnaces, this distance should be increased to a range between 4 feet and 7 feet.

Lead formed during the oxidation enters the molten lead and the lead well 14 to be removed from the furnace 10 through the overflow passage 18 away from the area of impingement. Some of the lead oxide formed during the oxidation enters into the slag layer 42 and overflows at 16 also away from the area of impingement, into the slag-receiving vessel 17. The slag may be treated in a reduction furnace to recover its lead content. The sulphur dioxide formed, any unreacted oxygen, other gaseous products and fume rich in lead oxide leave the furnace via the exhaust flue 22 for subsequent treatment.

The temperature of the molten slag bath 42 during operation is in the range of from about 1,100C. to about 1,300C. except in the area of impingement 46 where it is impinged upon by particles from the hotter combustion zone, in which area the temperature is somewhat higher. The lead oxide-containing slag, which has an exceptionally low sulphur content and which leaves the furnace at opening 16 beyond the area of impingement, passes to a reduction furnace in which this slag can be reduced in a conventional manner such as smelting with the addition of a suitable reducing agent such as coke.Most of the fume entrained in the Y off-gases is recovered easily and returned to the furnace. Fume settles well by gravity if the flue 22 is sufficiently large in cross section to provide low gas veloc ity, thereby reducing the load on separating apparatus such as a bag-house or an electrostatic treater. Fume that is collected outside the furnace as flue dust may be returned to the furnace as it is collected. Agglomeration of dust particles and blending with lead sulphide feed are not required. Higher temperatures are obtained in the pre-impingement combustion zone 44 if the flue dust and other oxidic material, which reactendothermically, are not fed into the furnace through the nozzle 24. It is preferred that nozzle 26 be used, whereby the flue dust and'other oxidic materials are fed into the furnace ,in the stream designated by numeral 47 in proximity to oradjacent to stream 43. The portion of the fume that falls freely in the flue drops onto the bath surface below the flue', where lead sulphate in the fume decomposes by reaction, 'according to Equation V1, with lead sulphide also contained in the fume. Also, lead sulphate in the fume decomposes thermally when the fume falls on the part of the bath, near the combustion zone, that is above 1,300C. The gas stream from the furnace 10 passes from the dust separator to a conventional recovery system where its sulphur dioxide content is recovered as a product, for example, liquid 80,.

For controlled operation of the furnace, the oxygenrich gas with the particulate feed is admitted at a prede termined rate. This oxygen, which must be sufficient to maintain the furnace temperatures, is also the principal supply of oxygen that is available to form the lead oxide in the slag. When substantial recoveries of metallic lead bullion are to be obtained, the required amount of oxygen-rich gas may be estimated as a percentage of the calculated stoichiometric requirement for treatment of lead concentrate to convert the lead sulphide completely to lead and sulphur dioxide. This oxygen requirement depends on the composition of the lead concentrate feed, allowance being 'made'for oxygen used to convert Zinc sulphide and iron sulphide to oxides and for lead-containing, oxidic material that may be separately charged to the process. When substantial recoveries of lead bullion are obtained, inward leakage of air into the furnace, which provides hygienically desirable operation under slightly less than atmospheric pressure, is small and may be neglected for this calculation, which is illustrated by example in Table 1.

TABLE I Example Calculation of Oxygen Requirement for 100 Pounds of Lead Concentrate Containing 74% Pb, 4% Zn, 3% Fe.

Reaction stoichiometric O Gaseous oxygen used is x% of 17.4 pounds, an empirically determined value of x being chosen to provide sufficient lead oxide in the slag to ensure a low level of sulphur in the bullion and to keep the slag temperature above 1,100C. This value depends on operating conditions, mainly composition of solid feed and handling of evolved fume. if little or none of the fume, which contains available oxygen, mainly as lead oxide, is returned to the process, there will be insufficient lead oxide in the slag to ensure production of low sulphur bullion unless a relatively large x factor is used. Operation with a large X factor increases lead in the slag at the expense of lead in the bullion. Recycling of much or all of the culated stoichiometric requirement for treatment of the lead concentrate to convert the lead sulphide completely to metallic lead and sulphur is sufficient to maintain bath and combustion zone temperatures and to provide a slag that contains at least 35% lead as lead oxide and is substantially sulphide free. A preferred range between 102 'and percent of the calculated stoichiometric requirement ensures substantial recovery of lead as low-sulphur'bullion. Oxygen additions near percent, although directing more lead into the slag as lead oxide, are of considerable value for short time periods during which increased heat output is needed to maintain control of furnace temperature. Rapid consumption of free oxygen in the preimpingement reaction assures an oxygen partial pressure that is too low for the formation of stable PbSO A lowsulphur slag, from which lead is recovered by subsequent reduction, is formed. All the heat for the process is provided by the oxidation reaction, except during start-up operations when the furnace is heated initially by using natural gas or oil.

The following examples present results obtained in the operation of the process of the present invention in a 10 tons-per-day pilot plant. This operation produced exhaust gas containing more than 85 percent sulphur dioxide. it will be understood, however, that the scope of the invention is not restricted to these examples.

EXAMPLE 1 A lead concentrate was admixed with a gas containing 97 percent oxygen and charged into a furnace similar to that shown in the FIGURE. The concentrate contained 63.3% Pb and 18.3% S with sulphides of iron and zinc as the main impurities. The concentrate feed rate was 9 pounds per minute. The oxygen supply with the concentrate feed was 106 percent of the calculated requirement to convert all the lead sulphide to metallic lead. No flux materials were used and no flue dust was recycled to the furnace. The distribution of lead, by weight, in the concentrate, slag, metal and flue dust was as follows:

Concentrate Slag Lead Bullion Flue Dust Lead (1b.) 36,000 17,000 7,400 I l 1,200

' Product analyses, weight per cent, are tabulated.

Pb Fe Zn S As Sb CaO SiO Slag 51.5 19.9 8.6 0.2 0.18 0.23 0.84 2.9 Lead 0.07 0.0006 0.007 Flue Dust 77.4 3.7 3.9 5.0 0 25 0.12 0.3 0.4

fume reduces or eliminates oxygen lo'ss via fume, and effective operation is obtained with a lower x factor. Addition of solid oxidic lead containing material, not

' derived from the measured output ofconcentrate, per- The slag contained 0.1 percent sulphate sulphur and had an average temperature of 1,280C. The exhaust gases contained 84% S0 1.1% 0 9% N 3% H 0 and 1% C0 The flue dust contained 2.5% sulphate sulphur.

EXAMPLE 2 A lead concentrate was charged, with gas containing 97 percent oxygen, into the furnace. The concentrate contained 72.5% Pb and 17.2% S with sulphides of iron and zinc as the main impurities. The concentrate feed rate was 1 1 pounds per minute. The oxygen supply with Concentrate Slag Lead Bullion Fluc Dust Lcad (11).) 34,000 6,300 19,200 8,000

Product analyses, weight per cent, are tabulated.

flue dust contained 3.4 percent sulphate sulphur.

EXAMPLE 4 A lead concentrate was charged, with gas containing 97 percent oxygen, into the furnace. The concentrate contained 74.5% Pb and 16.2% S with sulphides of iron and zinc as the main impurities. The concentrate feed rate was 14.5 pounds per minute. The oxygen supply with the feed was 102.3 percent of the calculated requirement to convert all of the lead sulphide to metallic lead. Siliceous flux containing 81% SiO was fed into the furnace at 18 pounds per hour. ln this case, 95% of the flue dust was recycled to the furnace. The distribution of lead, by weight, in the concentrate, slag metal and flue dust was as follows:

Pb Fe Zn S As $1) CaO S102 Slag 38.7 13.1 15.8 0.4 0.020 0.029 4.9 12.0 Lead 028 0.0o01 0.0015 Flue Dust 77.7 1.1 1.9 7.6 0.006 0.004 0.35 0.2

The slag contained 0.05 percent sulphate sulphur and Concentrate Slag Lead Bullion Flue Dust 4 O had an average temperature of 1,150 C. The exhaust Lead ("1) 98,500 24000 73000 300 gases contained 88% S0 0.06% 0 2.3% N 3% H 0 and 3% C0 The flue dust contained 3.4 percent sulphate sulphur.

Product analyses, weight per cent, are tabulated.

EXAMPLE 3 A lead concentrate was charged, with gas containing 97 percent oxygen, into the furnace. The concentrate contained 74.1% Pb and 16.1% S with sulphides of iron and zinc as the main impurities. The concentrate feed rate was 10.2 pounds per minute. The oxygen supply with the feed was 100.2 percent of the calculated requirement to convert all of the lead sulphide to metallic lead. Siliceous flux containing 81% SiO was fed into the furnace at pounds per hour. In this case, 85 percent of the flue dust was recycled to the furnace. The distribution of lead, by weight, in the concentrate, slag, metal and flue dust wasas follows:

Concentrate Slag Lead Bullion Flue Dust Lead (10.) 54,000 13,000 36,400 2,100

Product analyses, weight per, cent, are tabulated.

The slag contained 0.1 percent sulphate sulphur and had an average temperature of 1,180C. The exhaust gases contained 87% S0 0.5% 0 6% N 3% H 0 and 3% C0 The flue dust contained 3.3 percent sulphate sulphur.

The foregoing analyses indicate that operation at a combustion zone temperature above that at which lead sulphate decomposes and the maintenance of a relatively high level of lead oxide in the slag provide effective decomposition of lead sulphide by the reactions of Equations I and V. The differences between the total sulphur and the sulphate sulphur analyses of the flue dusts in the foregoing examples indicate that about half the total sulphur is in the sulphide form. It appears that, when fume falls onto the bath surface, which is above 1,100C., elemental lead and sulphur dioxide are formed by the reaction according to Equation Vl. These factors ensure production of low-sulphur slag and a low-sulphur bullion. The relatively high level of Pb Fe Zn S As Sb I had an average temperature of 1,160C. The exhaust gases contained 88% S0 3% H 0 and 3% C0 The The amount of lead recovered in the flue dust, mainly as lead oxide, i.e., 20 to 30 percent of the lead in the preliminary sintering step. Return to the furnace of lead oxide fume, recoverd with relative ease from the exhaust gases, contributes significantly to the output of lead product without substantially increasing the lead oxide content of the slag. As indicated in Example 4, the lead content of the flue dust recycled to the furnace, estimated as 20,000 pounds of lead, is of the same order of magnitude as the 24,000 pounds of lead reporting to the slag. With recycling, an equilibrium is established. The reaction according to Equation V is promoted. Lead oxide from the fume reacts with lead sulphide, also in the fume, to yield lead and sulphur dioxide. Impurities such as zinc, arsenic and antimony, evolved in the initial reaction, do not build up in the fume.

It should be noted that, with a substantial amount of lead reporting as oxide to the slag in all the examples, the method of calculating the stoichiometric oxygen requirement allows for the presence of an excess of oxygen in the process as it is operated. With admission of small excesses of inlet oxygen through the feed nozzle,

' as calculated for Example 3, or even with small apparent deficiencies, occurrence of a high content of lead oxide in the slag may be unexpected. However, there was sufficient oxygen available to produce the high lead oxide slag because small amounts of unoxidized sulphide were retained in the slag and bullion, some iron occurred in the slag as Fe O rather than Fe O and a small but significant input of oxygen occurred through operation of the furnace at less than atmospheric pressure to fulfill hygienic requirements. Nitrogen analyses of the exit gases show some net input of oxygen by meansof air leakage into the furnace.

Example 4, with 102 percent of the calculated stoichiometric requirement of oxygen in the inlet gas, produced lead bullion with 0.25 percent compared with 0.56% S for Example 3, which had 100 percent of the calculated oxygen requirement in the inlet gas. In order to ensure a low sulphur content in the bullion, operation with at least 102 percent of the calculated oxygen requirement in the inlet gas is preferred. However, projections of test data show that, by permitting increased sulphur in the bullion, operation of a furnace is feasible with a gaseous oxygen supply that is as low as 98 percent of the calculated requirement to convertthe sulphide to lead metal. With incidental oxygen additions as noted above, in quantities that are not readily predictable, sufficient lead oxide will form in the slag to maintain fluidity and to promote decomposition of all but a small quantity of lead sulphide by the reaction of Equation V. A slag temperature that is above l,lC. will ensure low sulphate sulphur.

When operating a furnace large enough to treat 100 tons per day of lead concentrate we observed that a substantial portion of the oxygen required to form a Y slag containing at least 35 percent lead as lead oxide may be supplied by means other than the oxygen introduced with the particulate feed. This is illustrated in Example 5 in which no lead bullion was formed. All the lead values in the lead sulphide in the feed were converted to lead oxide-containing fume and low sulphur, lead oxide-containing slag. In this example, the quantity of oxygen-rich gas in admixturewith the particulate feed was sufficient to maintain the temperatures required in the process, but was less than the quantity stated in each of the preceding examples.

EXAMPLE 5 A lead concentrate was admixed with a gas containing 97 percent oxygen and charged into a furnace similar to that shown in the FIGURE. The concentrate contained 65.5% Pb, 18.0% S, 9.8% Fe and 3.5% Zn with sulphides of iron and zinc as the main impurities. The concentrate, 122,828 pounds, was fed at a rate of 98 pounds per minute. The oxygen supply in admixture with the particulate concentrate was 83 percent of the calculated stoichiometric requirement to convert all the lead sulphide to metallic lead. No flux materials were used and no flue dust was recycled to the furnace. In this test, lead bullion did not form and only 25 percent of the exhaust gas was sulphur dioxide. It was evident that a substantial amount of oxygen entered the furnace as air by means other than nozzle 24.

Concentrate Slag Lead Bullion Flue Dust Lead (1b.) 80,450 57,550 22,750

Product analyses, weight per cent, are tabulated.

had an average temperature of 1,150C. The exhaust gas, although lower in S0 than the exhaust gases of Examples 1 to 4, stillexceeded the concentration, about 12 percent, that may be used for the manufacture of sulphuric acid. N I

This example shows that the gaseous oxygen that entered the furnace in admixture with the particulate feed was sufficient to maintain the slag temperature above l,l00C. However, this amount of oxygen, 83 percent of the calculated stoichiometric requirement to convert all the lead sulphide to metallic lead, was not enough to convert all the lead to lead oxide. With this 100 tonsper-day furnace, inward leakage of air was substantially greater than with'the tighter l0 tons-per-day pilot furnace which, in Examples 1 to 4, produced exhaust gas containing more than S0 We have, therefore,

found that a portion of the oxygen requirement to maintain the desired lead oxide level in the slag can be provided from an extraneous source such as by inward leakage of air, while the oxygen entering the furnace in admixture with particulate feed maintains temperatures that are sufficiently high to ensure low-sulphate sulphur in the slag.

Oxygen supply must be adequate to obtain the lowsulphur slags disclosed in Examples 1 to 5 and the lowsulphur lead bullions disclosedin Examples 1 to 4, and to provide the elevated operating temperatures that ensure low sulphate sulphur in the slags. To meet these specific oxygen requirements, there must be, firstly, sufficient oxygen in the form of oxygen-rich gas containing at least 60% oxygen in admixture with the particulate feed to initiate and sustain oxidation of the lead sulphide in a combustion zone flame having a temperature above l,300C. and to provide sufficient heat of oxidation to maintain the molten bath at a temperature above l,lC. and, secondly, sufficient total oxygen to maintain a slag that is sufficiently high in lead as lead oxide to react with impinging lead sulphide and to reject, as sulphur dioxide, the sulphur from lead sulphide that enters the slag. Example 2 indicates that a level of about 35 percent lead in the slag is sufficient to meet the lead oxide requirement. Examples 1, 3 and 4 show that, with lead in the slag as high as 55 percent, there are still substantial recoveries of lead as bullion. Example 5, with 0.3 percent total sulphur including 0.l percent sulphate sulphur in the slag, shows that the process may be operated to recover in a low-sulphur slag, all, or nearly all, of the lead values that are not evolved withthe fume.

Operation of this process with at least 35 percent lead as oxide in the slag will produce slag and bullion that are low in sulphur. In order to withdraw from the furnace a substantial portion of the lead as metallic lead bullion, operation with a slag containing from about 35 percent to about 55 percent lead is preferred. For operation of a furnace that has a low inward leakage of air, as indicated by at least 80% S0 in the exhaust gas in Examples 1 to 4, it is desirable to control recovery of lead as bullion by introducing a predetermined quantity of oxygen-rich gas with the particulate feed. For operation of a furnace that has a greater inward leakage of air, sufficient oxygen must be supplied to form a slag that contains sufficient lead oxide to react with and reject sulphur, as sulphur dioxide, from the sulphides contained in the impinging particles. The temperature of the slag must be high enough to decompose sulphates entering the bath as settled fume containing both sulphide and sulphate sulphur, or as recycled fume and other oxidic materials charged to the furnace near the combustion zone. Maintenance of at least 35 percent lead as oxide in the slag and maintenance of the slag temperature above l,l00C. will prevent entry of sulphur into an underlying layer of lead bullion. Operation to oxidize all the concentrate lead values to fume and low-sulphur slag, as in Example 5, and subsequent reduction of the slag to recover metallic lead, offer ecological advantages over sintering of concentrate and reduction of the sinter.

Although the foregoing smelting process is preferably operated with sufficient oxygen to form a slag containing the 35 to 55 percent lead requirement which ensures substantial recovery of bullion low in sulphur, arsenic and antimony, or with sufficient oxygen to convert the lead sulphide completely to oxide and sulphur dioxide, it is evident that, as the degreeof oxidation increases, lead values within the bath will have decreasing proportions of lead as bullion.

What we claim as new and desire to protect by Letters Patent of the'United States is:

1. A process for separately recovering lead values and sulphur values from a lead sulphide ore or concentrate without prior sintering which comprises: charging lead sulphide ore or concentrate in particulate form and in admixture with an oxygen-rich gas containing at least about 60 percent oxygen downwardly into and through the combustion zone .of a furnace to impinge cess, whereby oxidation of said lead sulphide is initiated and sustained to produce lead oxide and sulphur dioxide in a flame in said combustion zone above the area of impingement onto said molten bath and, after such impingement, oxidation of residual lead sulphide is continued by reaction within the molten slag; supplying in the furnace sufficient oxygen gas in admixture with the ore or concentrate to provide heat of oxidation to maintain said flame at a temperature of at least l,300C. and said molten bath at a temperature above l,l00C., and sufficient total oxygen to maintain at least 35 percent lead as lead oxide in the slag, said heat of oxidation being provided by reaction with said oxygen-rich gas; and withdrawing from said furnace molten material containing lead values low in sulphur content as molten lead oxide-containing slag plus any molten lead that may be formed and gaseous material containing sulphur values and entrained lead oxide fume.

2. A process as claimed in claim 1 in which said process is conducted continuously.

3. A process as claimed in claim 1 in which from about 35 percent to about 55 percent lead as lead oxide is maintained in the slag.

4. A process as claimed in claim 1 in which lead values contained in the molten material withdrawn from the furnace include soft lead containing less than 0.001 percent arsenic and less than 0.01 percent antimony.

5. A process as claimed in claim 2 said molten bath consisting of lead and a substantially sulphide free lead oxide-containing slag cover, said oxygen-rich gas providing at least about 98 percent of the calculated stoichiometric requirement of oxygen for treating said lead sulphide ore .or concentrate to convert the lead sulphide completely tometallic lead and sulphur dioxide, and separately withdrawing from said furnace molten lead oxide-containing slag, molten lead, and a mixture of gaseous material and entrained fume.

6. A process as claimed in claim 2 in which said oxygen-rich gas issupplied in an amount sufficient to provide oxygen corresponding to from about 98 percent to about 120 percent of the calculated stoichiometric re quirement.

7. A process as claimed in claim 2 in which said oxygen-rich gas is supplied in amount sufficient to provide oxygen corresponding to from about 102 percent to about 110 percent of the calculated stoichiometric requirement.

8. A process as claimed in claim2 in which all the lead values contained in the molten material withdrawn from the furnace are in the form of lead oxide.

9. A process as claimed in claim 2 in which the lead oxide content of said material containing lead values is reduced to form substantially sulphur-free lead.-

10. A process as claimed in claim 2 in which solid oxidic material selected from the group consisting of reonto and penetrate the surface of a molten bath therein having lead values in the form of substantially sulphide free lead oxide-containing slag derived from said procycled fume, flux-forming material and sulphate leach residues is charged into the furnace in a stream that is in proximity to but not coincident with the stream of lead sulphide ore or concentrate.

11. A. process as claimed in claim 1 in which said oxygen-rich gas contains at least percent oxygen.

12. A process as claimed in claim 1 in which said molten bath is maintained at a temperature within the range of from about l,l00C. to about 1,300C. by the heat of oxidation in the combustion zone.

13. A process as claimed in claim 2 in which said stream of lead sulphide ore or concentrate in particulate form and in admixture with an oxygen-rich gas is charged downwardly into the furnace at a distance of at least about 36 inches above the slag cover,

14. A process as claimed in claim 1 in which said concentrate is a froth flotation product.

15. A process as claimed in claim 1 in which the temperature during oxidation of the ore or concentrate in the combustion zone prior to impingement upon the molten bath is above about 1,500C.

16. A process as claimed in claim 1 in which the temperature during oxidation of the ore or concentrate in the combustion zone prior to impingement upon the molten bath is about 1,700C.

17. A process as claimed in claim 1 in which the molten material containing lead values withdrawn from the furnace contains less than 1 percent sulphur and is withdrawn at an opening beyond the area of impingement.

18. A process as claimed in claim 1. in which said lead sulphide ore or concentrate is charged in admixture with the oxygen-rich gas at a velocity sufficient to provide forceful impingement of said charge onto said slag cover for penetration of the surface thereof and acceleration of the oxidation reactions.

19. A process as claimed in claim 5 in which said lead oxide-containing slag is treated with a reducing agent for recovery of lead therefrom.

Claims

1. A PROCESS FOR SEPARATELY RECOVERING LEAD VALUES AND SULPHUR VALUES FROM A LEAD SULPHIDE ORE OR CONCENTRATE WITHOUT PRIOR SINTERING WHICH COMPRISES: CHARGING LEAD SULPHIDE ORE OR CONCENTRATE IN PARTICULATE FROM AND IN ADMIXTURE WITH AN OXYGEN-FICH GAS CONTAINING AT LEAST ABOUT 60 PERCENT OXYGEN DOWNWARDLY INTO AND THROUGH THE COMBUSTION ZONE OF A FURNACE TO IMPINGE ONTO AND PENETRATE THE SURFACE OF A MOLTEN BATH THEREIN HAVING LEAD VALUES IN THE FORM OF SUBSTANTIALLY SULPHIDE FREE LEAD OXIDE-CONTAINING SLAG DERIVED FROM SAID PROCESS, WHEREBY OXIDIATION OF SAID LEAD SULPHIDE IS INITIATED AND SUSTAINED TO PRODUCE LEAD OXIDE AND SULPHUR DIOXIDE IN A FLAME IN SAID COMBUSTION ZONE ABOVE THE AREA OF IMPINGMENT ONTO SAID MOLTEN BATH AND, AFTER SUCH IMPINGEMENT, OXIDATION OF RESIDUAL LEAD SULPHIDE IS CONTAINED BY REACTION WITHIN THE MOLTEN SLAG; SUPPLYING IN THE FURNACE SUFFICIENT OXYGEN GAS IN ADMIXTURE WITH THE ORE OR CONCENTRATE TO PROVIDE HEAT OF OXIDIATION TO MAINTAIN SAID FLAME AT A TEMPERATURE OF AT LEAST 1,300*C. AND SAID MOLTEN BATH AT A TEMPERATURE ABOVE 1,100*C., AND SUFFICIENT TOTAL OXYGEN TO MAINTAIN AT LEAST 35

5. A process as claimed in claim 2 said molten bath consisting of lead and a substantially sulphide free lead oxide-containing slag cover, said oxygen-rich gas providing at least about 98 percent of the calculated stoichiometric requirement of oxygen for treating said lead sulphide ore or concentrate to convert the lead sulphide completely to metallic lead and sulphur dioxide, and separately withdrawing from said furnace molten lead oxide-containing slag, molten lead, and a mixture of gaseous material and entrained fume.

6. A process as claimed in claim 2 in which said oxygen-rich gas is supplied in an amount sufficient to provide oxygen corresponding to from about 98 percent to about 120 percent of the calculated stoichiometric requirement.

8. A process as claimed in claim 2 in which all the lead values contained in the molten material withdrawn from the furnace are in the form of lead oxide.

9. A process as claimed in claim 2 in which the lead oxide content of said material containing lead values is reduced to form substantially sulphur-free lead.

10. A process as claimed in claim 2 in which solid oxidic material selected from the group consisting of recycled fume, flux-forming material and sulphate leach residues is charged into the furnace in a stream that is in proximity to but not coincident with the stream of lead sulphide ore or concentrate.

11. A process as claimed in claim 1 in which said oxygen-rich gas contains at least 75 percent oxygen.

12. A process as claimed in claim 1 in which said molten bath is maintained at a temperature within the range of from about 1, 100*C. to abOut 1,300*C. by the heat of oxidation in the combustion zone.

13. A process as claimed in claim 2 in which said stream of lead sulphide ore or concentrate in particulate form and in admixture with an oxygen-rich gas is charged downwardly into the furnace at a distance of at least about 36 inches above the slag cover.

15. A process as claimed in claim 1 in which the temperature during oxidation of the ore or concentrate in the combustion zone prior to impingement upon the molten bath is above about 1,500*C.

16. A process as claimed in claim 1 in which the temperature during oxidation of the ore or concentrate in the combustion zone prior to impingement upon the molten bath is about 1,700*C.

18. A process as claimed in claim 1 in which said lead sulphide ore or concentrate is charged in admixture with the oxygen-rich gas at a velocity sufficient to provide forceful impingement of said charge onto said slag cover for penetration of the surface thereof and acceleration of the oxidation reactions.