US2598743A - Zinc smelting - Google Patents

Description

June 3, 1952 R. K. wARlNG ET Ax. 2,598,743

` ZINC SMELTING Filed Jan. 12, 1950 CDC) FIG,

ZoTHr-:R SLAG coNsTlTurfzNTs [loZ-(ZLIMm/Z'SILICM] oliva valus- :awl'l INVENTOR S BY Zak?, M, v TTMM/EYS Patented June 3, 1952 ZINC SMELTING Robert K. Waring, Luther D. Fetterolf, and Thomas L. Hurst, Palmerton, Pa., assgnors to The New Jersey Zinc Company, New York, N. Y., a corporation of NewJersey Application January 12, 1950, `Serial No. 138,136

(Cl. 'i5-14) Claims. l

This invention relates to thesmelting of oxidic zinciferous material and, more particularly, to the electric `arc furnace smelting of oxidic vzinciferous ore.

Because of its inherent simplicity and the absence of such requirements as vspecial charge preparation, the electric arc furnace smelting of zinc has attracted much .thought and speculation. Numerous proposals have been advanced heretofore for the electric furnace smelting of zinc, but to the best oi .our knowledge those proposals which have been reduced to practice have not been successful in producing massive zinc metal. These consistently discouraging results over a period of decades have not been caused by the diculty of smelting the zinc ore because such an ore can be smelted with facility at the temperatures which are readily obtained in an electric furnace. The diiculty encountered 'heretofore has resided in the condensationof the zinc vapor produced by electric furnace smelting. In 4spite of more optimistic predicitions, physical .embodiments of prior art proposals vhaveresnlted in the production of vast quantities of kblue powder to the substantial exclusion of massive metallic zinc.

In a general sense, the smeltinginethod of our invention, applied to the lsmelting ,of oxidic vzinciferous ore, resembles those of .the 'prior kart in that it produces three productszinc vapor, a molten slag, and a metallic iron product resulting from reduction of the -iron oxide which invariably accompanies the Zinc in its ores. As pointed out hereinafter, our method is also applicable to the smelting of substantially iron-free voxidic zinciferous materials and in such oase the products of the smelting operation do not include metallic iron. We have found, however, that when the smelting of oxidic'zinciferous material is carried out at a temperature Within the range of1250" to 1450 C. at the interface between armoltenslag bath and a floating layer of the `zinciferous charge, the zinc vapor produced by such smelting is substantially free from the dust-forming iml vcan successfully smelt zinciferous material Vin an electric arc furnace while substantially preventing the development of a temperature in excess of 1450" C. in any portion of the slag exposed to the furnace atmosphere, this 'temperaturexbeing measured as the .temperature of the slag being tapped lfrom the `furnace. The necessary slag uidity can be insured by themaintenance ofthe slag composition within certain well-'defined limits in accordance with ourinvention,

Oxidic zinciferous ores generally comprise zinc, cadmium, lead, copper, silver and iron, essentially in the form of oxides, each of which is readily Areducible by carbonaceous ,material at temperatures Within the range of about l250 C. yto 1450 CV., as well as oxides of calcium, magnesium and silicon which are not readily reducible underthese conditions. ln heating the entire mass of a smelting charge to a temperature Awithin such a range in an electric furnace, it is a normal characteristic of such an operation that a substantial portion of the charge is vheated to an appreciably higher temperature. When a portion of a charge derived from the reduction of oxidic zinciferous ore is heated `to a temperature substantially kabove 1450 C., there is a pronounced tendency for one or more of the gangue constituents-lime, magnesia and silica-as well as manganese oxide, if present, to be volatilized either directly or indirectly, or both. These ,constituents may be volatilized directly in the form Vof the oxides per se, or they may be volatilzed indirectly first by `reduction of the oxides to the metallic form, followed bv volatilization of the metals which are then reoxidized by `Carbon monoxide and carbon dioxide in the furnace atmosphere. Volatilization of these charge components in a relatively `hot portion of the charge is followed by their solidication in a cooler portion of the chamber, and the soiidied materials thereupon appearA in the furnace atmosphere in the form of dust- `like particles. These particles comprise the aforementioned dust-.forming impurities which contribute to the formation of blue powder and which indicate the existence .of furnace conditions Which lead to the production of Ylarge amounts of blue powder instead of massive metallic zinc. `By maintenance of a `molten slag temperature within the range of 1250" .to 1450" C., which is made possible by control of the slag uidity in accordance with our invention, `local overheatingof the slag or charge to any `tern- -perature substantially in excess of 15450 C. is

avoided and the production of .dust-forming impurities in the zinc vapor-bearing smelting gases is inhibited.

Accordingly, it will `be observed `.that we have developed a method Aof smelting an oxidic zinciferousiV material Vin the presence of carbonaceous reducing materialin' an electric arc furnace with Vthe resulting production of a molten slag and smelting gases bearing metallic zinc vapor substantially free from dust-forming impurities. In the smelting of oxidic zinciferous ore, a molten iron product will also be obtained. The method of our invention comprises charging the zinciferous material and reducing material into the furnace in a loose and dry condition, establishing in the charge composition such relative amounts -of the zinciferous and reducing materials as to effect reduction of at least a major portion of the zinc component of the charge and such reduction of any iron present in the zinciferous material (as in the case of a zinc ore) as topreclude the presence in the molten slag of more than 6% by weight of iron oxide (calculatedas Fe) further establishing in the charge composition such relative proportions of calcareous and siliceous material, with respect to the other slagforming constituents, that upon smelting of the zinciferous material the resulting molten slag will contain not more than 85 by weight of lime (CaOI) andy silica (SiO2) in a ratio between 0.9:1 and 1.2:1 and not-exceeding theV ratio, with respect to the total amount of the other constituents of the slag, represented by the line AB in Fig. 1 of the accompanying drawing, maintaining the slagin the molten condition at a temperature of at least 1250 C. by electric arc heating, and `'smelting the dry charge on the surface of the molten slag at a temperature not in excess of 1450 C. The line AB in Fig. 1 represents an experimentally determined critical relationship between Vthe lime-silica ratio and the total amount of other constituents present in the slag. Adherence to this relationship in the mannerl recited "assures therequisite fluidity of the slag for effective practice of our smelting method. However, optimum slag composition, corresponding t'o optimum slag fluidity, is obtained in accordance with 'our invention by so proportioning the aforementioned calcareous and siliceous material that the lime plus silica does not exceed 83% and the lime-silicaratio falls within the range of 1.05':1 and 1.15:1 and not exceeding, with respect to the total amount of other constituents of the slag, the ratio represented by the line CD in Fig. 1. In the interestl of a complete understanding of the invention, but not as any limitation thereof, it will be described herein particularly with respect to the smelting of an oxidic zinciferous ore. In the course of our smelting operation, the

zinc, cadmium, lead, copper and silver oxides present in the ore are readily reduced. Iron oxide in the ore is also largely reduced to metallic iron but it appears that some of the nascent iron tends to reduce the zinc oxide and thus become reoxidized to ferrous oxide. Although stable equilibrium conditions are not established in such a continuous smelting operation, it is nevertheless a fact that some metallic iron is formed simultaneously with the metallic Zinc and that, depending largely upon the amount of reducing material present, some ferrous oxide remains unreduced in the slag. When the metallic iron is produced in the medium of a suitably fluid slag and in the presence of carbonaceous reducing material, globules of carburized molten iron form in the slag and ultimately coalesce and tend to liquate therefrom in a body below the slag. The resulting body of molten iron normally contains such an amount of dissolved carbon as to render it molten at temperatures as low as about 1150o to 1200 C. Inasmuch as the smelting heat is supplied to the upper portion of the charge in an electric arc furnace, the temperature prevailing in the lower portion of the furnace below the slag will generally be somewhat lower than that of the slag itself. In order to make possible continuous operation of the furnace, the metallic iron must be maintained in a tappable molten state while the smelting operation proceeds at a temperature within the range of 1250 to 1450" C. The iron will have a melting point below 1450" C. if it contains at least 1% to 2% carbon and will remain molten at a temperature between about 1150" and 1200 C. if it contains about 4% carbon. If the supernatant slag is not unduly o xidic, the metallic iron product will remain carburized to the extent necessary to maintain it in the molten condition. Inasmuch as the presence of iron oxide in the slag imparts to the slag a certain oxidic character with respect to the carburized metallic iron product, we have found it to be necessary to limit the iron oxide content of the slag to a maximum of about 6% by Weight (calculated as Fe, which corresponds to 7.7% FeO) in order to maintain the metallic iron product in a suitably Vtappable molten condition, whereby the iron product and the slag will be separably tappable, while maintaining thereabove a smelting operation within the temperature range of l250 rto 1450" C. The iron oxide content of the slag can be readily adjusted, as is well known in the art, by proper correlation of the amounts of ore and carbonaceous reducing materialpresent in the furnace charge.

The smelting method of our invention is applicable to any oxidic zinciferous material,` zinciferous ore or ore concentrate, whether naturally occurring in the oxidized state or obtained by roasting or the like.V We' have successfully smelted such representative zinciferous materials which varied from one extreme to the other in their zinc content. For example, such oxidic zincifer-` ous materials ascalcined Sterling Hill ore having a 20% zinc content, sintered Eagle ore concentrate containing about 55% zinc, -sintered Waelz oxide containing 'about 70% zinc, and off-grade Zinc oxide pigment containing about 80% zinc,

have been smelted and metallic zinc vapor condensed therefrom with excellent condensing efficiencies. Inasmuch as our smelting method is applicable to smelting either the naturally occurring voxidic ore orV roasted sulfide ore, or Such ores which have been subjected to bulk concentration, it will be observed that the smelting procedure of our invention has Yan unlimited applicability as distinguished from the more limited versatility of other zinc smelting processes. Accordingly, any beneflciation to which the ore may be subjected prior tosmelting by the method of our invention need be only for the purpose of removing the easily separated gangue material by a simple bulk concentration process, Whether by flotation procedure or'otherwise. Y

In smelting pursuant toV ourA invention, the electric arc'should be s0 controlled as to obtain a favorable temperature distribution from top to bottom of the furnace. The temperature at the top of the furnace is limited by the refractories used for the furnace structure, but,as a guide it can be stated thatl furnace atmosphere temperatures of 1200-1350 C. are satisfactory with temperatures of 12701300 C. being presently preferred. .The temperature adjacent'the smelting zone at the interface betweenV the fiuid slag and floating zinciferous charge should not exceed about 1450" C. as pointed out hereinbefore and preferably ranges between 1350 and 1400 C.

The temperature prevailing through the slag body should be sufficient, however, to maintain both smelting conditions at its surface and molten iron-product conditions below the slag body, taking into account the chilling effector the relatively ccol charge delivered to the surface of the slag as well as the endothermic reaction characteristics of this charge. The position of the 4electric arc required to Aproduce this result will depend upon the number of electrodes, furnace size, and other related variables. The electrodes should not be so deeply immersed as to provide simply Vslag-resistance heating for this type of heating has been found by us to `be wholly incapable of counteracting the chilling (and slag freezing) eifect of the floating charge. When the furnace heating iseifected pursuant to the aforementioned specications, Zinc elimination from the charge, as evidenced by the zinc content of the slag, will be at least 98 When a zinciferous ore is smelted as described 'hereinbefora the resulting slag consists of lime,

silica and other slag components such as magnesia, manganese oxide, alumina and any unreduced iron oxide and zinc oxide. Of these numerous slag-forming constituents, originating both in the ore and in the ash of the carbonaceous reducing material, the lime and silica together usually comprise the major portion with the remainder being made up of the aforementioned other slag-forming constituents. In general, the lime and silica together comprise from about 40% to about 85%, and more commonly from about 60% to about 80%, of the slag. The other slag-forming constituents, including the maximum of 6% iron oxide (calculated as Fe) which may be permitted to remain in the slag as described hcreinbefore, comprise the remaining to 60%, and more commonly 20 to 40%, of the slag. We have found ythat the slag may be provided with the requisite uidity for the practice of our sinelting method if a certain `definte -relationship be maintained between the lime-silica ratio and the amount of `said other slag-forming constituents. This relationship is represented graphically in Fig. 1 of the drawing.

As indicated in Fig. l, we have found that the aforementioned critical relationship holds `true for lime-silica ratios as low as 0.921 and `as 4high as 12:1. The minimum lime-silica ratio of0.9:1 represents the lowest ratio at which it is cornmercially feasible to .recover reduced lzinc and iron from the furnace charge. At lime-silica ratios lower than 09:1, it becomes so dicult to reduce the zinc and iron that suitable Yrecoveries cannotl be obtained at smelting temperatures ranging as high as 1450 C. The upper limit of 1.2:1 for the lime-silica ratio is dictated by .slag viscosity; we have found that slags characterized by a lime-silicaratio in excess of 1.2:11 have such viscosity as to render them unsatisfactory for smelting operations pursuant 'to our invention. Within these limits, the aforementioned critical relationship between the lime-silica ratio and the amount of the other slag-forming constituents is represented by the curve AB, the 'lime-'silica ratios on or below the line AB leading to the vproduction of slags havingsuitable fiuidity for practice of our smelting method.

The relationship represented by curve ABin Fig. 1 comprises theprescription ofthe `maximum lime-silica ratio that can be used with amounts of the other constituents of the slag ranging upwardly from about 15% of the slag composition, the lime plus silica constituting the balance of the slag composition. Although oxidio zinc ores contain calcareous and siliceous materials in a wide range of proportions, we have observed that it is generally necessary to add lime or silica to the charge so as to obtain a limesilica ratio in the slag, including the silica content of the coal ash, coming within the range of 0.9:1 to 1.2:1. In order to obtain such a limesilica ratio, extraneous lime or silica may be added as required, or the requisite lime-silica ratio can be obtained by the conjoint use Aof two or more znciferous ores of different calcareous and siliceous contents.

The production of a slag having the requisite fluidity for practice of our invention can be readily achieved by adjustment of the lime and silica contents of the sla-g pursuant to a maximum represented by the line AB in Fig. 1. In the case of a lime-base ore, in which the lime-silica ratio exceeds 1.2:1, the silica component of the slag of silica or lime which must be added will, of

course, depend -upon the amount of said other slag constituents, as indicated in Fig. 1. lI-Iowever, as clearly shown by the line AB, the addition of lime or silica, as the case may require, must be made in such manner as to insure the presence in the resulting slag of at least 15% of other slag constituents. That is, in no event should the lime plus silica content of the slag exceed about of the total slag composition. The presence of at least 15% other slag constituents can lbe attained by controlling the smelting conditions so as to leave more iron oxide in the slag, by adding extraneous slag-forming constituents such as alumina, magnesia, iron oxide, soda, or the like, or by a combination of such procedures. When using a lime-slag ratio within the limits prescribed for line AB, we have found that amounts of the other slag constituents less than about 15% by weight of the slag preclude the attainment of suitable slag fluidity. The upper limit of the amount of the other slag constituents is prescribed only by their naturallyoccurring proportions in the ore.

Although the establishment of a slag composition characterized by a lime-silica ratio not in excess of that represented by the line AB in Fig. l will result ina slag fluidity suficiently high to permit satisfactory smelting in accordance with our invention, we have found `that a more lnarrowly circumscribed relationship Abetween the lime, silica and other slag components leads to the attainment of slags of maximum fluidity and maximum utility. rlChese slag compositions which are characterized by optimum smelting properties are defined by the line CD in Fig. 1. Theline CD represents the maximum lime-silica Aratio within the range of 1.05:1 to 1.15:1,'with respect to the total of the other constituents of the slag, which is compatible with optimum zinc and iron recoveries. In obtaining optimum slag uidity pursuant to the relationship represented by line CD, the amount of "other slag constituents should total at least 17% of the vslag composition.

Slag compositions coming within the vprescription of line AB in Fig. 1, characterized by fluidity `conducive to effective `reduction ofthe zinc and iron componentsofthe ore pursuant to our invention, make possible the attainment of an efciency of at least 85% in the condensation of the evolved zinc vapor as a result of more complete elimination from the vapor of the aforementioned dust-forming impurities. Slags having compositions outside of the prescription of line AB in Fig. 1, on the other hand, are characterized by such inadequate fluidity as to result in zinc vapor condensation eiciencies (i. e. the proportion of zinc vapor introduced into the condenser which is converted to molten metal) materially below 85%. This effect is readily appreciated in the following examples in which smelting conditions pursuant to our invention were substantially identical, the difference in each case bein-g essentially a difference in slag composition which, in turn, represented a difference in slag fluidity and consequently in heat dissemination throughout the slag. A slag having a lime-silica ratio of 1.2711, and in which the slag constituents v,other than lime and silica constituted 20% of the slag composition, resulted in a condenser eiiiciency of approximately 80 By increasing the silica content of the slag to obtain a lime-silica ratio of 1.14z1, with a resulting lowering of the amount of other slag constituents to 17% of the slag composition, the condenser eciency was somewhat better than 85%. A further lowering of the limesilica ratio to 104:1 While maintaining the other slag constituents at 17% raised the condenser eiiiciency to about 90%. Based upon similar experimental data, we have ascertained that under smelting conditions pursuant to our invention, as previously set forth, slag compositions falling within the prescriptions of line AB in Fig. 1 lead to zinc vapor condensation eiciencies in excess of 85% whereas those compositions outside of the prescriptions of line AB result in condenser eiiiciencies that drop off rapidly to the level of 30% and below. On the other hand, slag compositions coming within the more circumscribed prescriptions of line CD in Fig. 1 make possible zinc vapor condensation efciencies of at least 90%. To the best of our knowledge, the high condenser emciencies characteristic of smelting operations carried out pursuant to our invention and featuring slag compositions within the limits Ydiscussed hereinbefore are obtained as the result of the exceptionally uniform temperature which are established throughout such iluidly mobile slags.

The relatively uniform temperature prevailing throughout the fluid slag layer is taken advantage of, in accordance with our invention, as a means of imparting to the fresh charge the necessary smelting heat within the specied temperature range. To this end, we have found it advisable to deliver the charge to the furnace in such manner, advantageously through the furnace roof, as to provide in the vicinity of the electrodes a mass of the charge floating on the slag. Smelting thus takes place primarily at the interface between the fluid slag and the floating charge. From time to time an additional charge may be introduced with advantage adjacent the furnace walls in such manner as to provideY a downwardly and inwardly sloping bank of charge which not only protects the furnace walls but supplies an additional quantity of fresh charge available for absorbing heat from the slag.

The only requirement for the physical form of charge used in practicing our invention is that it be loose and dry. By loose we mean that the charge should not be introduced in massive form,

conditions say,` as a single large sintered block. The charge shouldA be loose so that it will fall freely on the surface of the molten slag and spread out thereupon to an extent commensurate with the angle of repose of the charge particles. By specifying that the charge should be dry .We mean that it should not be added in the molten condition. It is a characteristic feature of the smelting method of our invention that the charge be smelted on the surface of the hot uid furnace slag, and this Icondition can be met only when the 'charge is introduced into the furnace in the aforementioned loose dry form. The'degree of subdivision of the ore component of the charge is not critical. In general, we prefer to limit the maximum particle size of the ore in the charge to about 1/2 inch in diameter. Except for the problem of dusting there is no critical lower limit to the size of any of the charge particles.

We prefer to mix the charge components prior to their introduction into the furnace either in the form of a simple physical admixture or in the form of nodulized or otherwise agglomerated particles. For example, the zinciferous material (such as Waelz'oxide sinter) coal and lime may be mixed together and charged directly to the furnace, or the mixture may be moistened with water and further mixed with 2% bentonite as a binder, then briquetted, dried and crushed before charging to the furnace. Alternatively, the zinciferous material alone may be briquetted, using sulfite liquor as a binder, the briquettes being fired, crushed to suitable size and then mixed with the coal and lime for direct charging to the furnace. N o significant differences that could be attributed to the method of charge preparation were observed in either the smelting operation or condenser performance. In each instance, excess nes or the dust fraction may be removed by rough air separation. The charge may also be preheated with advantage to temperatures of the order of 400800 C. in accordance with conventional electric furnace practice. Any suitable preheating apparatus may be used for this purpose, the heat being supplied by an oil or gas flame or by the heat of combustion of the exhaust gases from the zinc condenser.

The amount of coal used in practicing our invention should advantageously somewhat exceed that amount theoretically required for the zinc and other oxides readily reduced by carbonaceous material. In general, We have found it satisfactory to use an amount of carbon (coal or coke) calculated to be of that required to reduce the readily reducible oxides otherthan zinc and about to 125% of the amount theoretically required to reduce the zinc componentV of the charge. Amounts of carbon in excess of of that theoretically required to reduce all readily reducible oxides in the charge have been found to be ineffective in increasing either the total amount of zinc reduced or its rate of reduction. Amounts of carbon in excess of about 125% of that theoretically required are undesirable both because of uneconomical waste thereof and because of the heat-insulating effect of the excess carbon floating on the slag. Where coal or coke is used as the reducing material, we have found it advantageous to crush this component rather than use it in a relatively coarse form. For exf ample, No.v 3 coal tends to segregate in a zinciferour charge and leads to less effective reduction than No. 3k coal roll-crushed to through lV mesh (Tyler Standard). v

Condensation of the zinc vapor-bearing smelt- 9 i ing gases produced in accordance with our invention poses no unusual problems. Although a stationary baffle-type condenser such as that described in the United States patent to Bunce No. 1,873,861 can be used for condensing the zinc vapor, maximum usufruct of our invention is realized when condensation is effected in a condenser of the type wherein the zinc vapor is brought into intimate contact with a relatively large freshly exposed surface of molten zinc. The latter type of condenser is represented by that wherein the zinc vapor-bearing gases are passed through a shower of molten vzinc forciblyhurled through a confined condensing zone as described in United States Patents Nos. 2,457,544 through 2,457,551 and 2,494,551. This latter type of zinc condenser is capable of removing and condensing to molten metal all of the zinc vapor contained in the smelting gases except for that amount of the vapor corresponding to the vapor pressure of g molten zinc at the temperature of the exhaust condenser gases.

The smelting method of our invention may be illustrated by the following specific examples. A charge mixture was prepared consisting of about 17.8 parts by Weight of anthracite coal (in which silica comprised 65% of the ash) and 1.3 parts by weight of burned lime as an extraneous flux per 100 parts by weight of a roasted zinc ore concentrate consisting of discrete particles of 1A diameter and finer. The zinc concentrate had the following analysis:

Per cent Zn. Pb 0.05 Fe 2.9 Cu 0.02 CaO 2.5 S102 1.6 C 0.80 Mn 1.6

S 0.20 CO2 0.32 Cd 0.002 MgO 0.27 A1203v 0.57

This charge mixture contained sufficient iron oxide and other slag forming gangue constituents to meet the operating requirements of our smelting method as set forth hereinbefore as can be seen from the following slag analysis:

Per cent 34.8 SiOz 32.3 MgO 2.

- 9.7 CaOzSiOz 1.08 CaO-I-SOz 67.1 Other slag constituents 32.9

The charge materials, thoroughly mixed, were fed to the furnace at intervals of 21/2 minutes successively through a series of six charging hop- 6 trodes, nominally rated at 100 kw. but operated."4

during our smelting operation at only about The two electrodes were so positioned as to produce arcs adjacent the surface of the slag, the size of the arcs at the aforementioned rate of..

power consumption being suflicient to maintain the desired temperature conditions in the bath and in the furnace chamber above the bath. The furnace chamber temperature was maintained in the range of l25G-l300 C. during the operating period. The slag temperature, measured as the slag was tapped from the furnace, averaged about 13.50 C. rEhe zinc Vapor-bearing smelting gases were passed through a splash-type condenser of the type referred to hereinbefore and the zinc vapor was condensed therein to molten metallic Zinc. After correcting for handling losses, for losses of Zinc vapor resulting from furnace leaks, and for the amount of fume passing through the condenser and other similar factors, the condensation efficiency in this operation appeared to be substantially 90%. The inhibition of volatilization of the refractory oxides was evident from the fact that the lime-silica ratio in the small amount of blue powder formed in the condenser was about one-third to one-half of the lime-silica ratio prevailing in the slag.

A second example of the smelting of a zinciferous material in accordance with our invention comprises the smelting of a furnace charge mixture of zinc oxide, burned lime and anthracite coal under virtually the same smelting conditions as those in the previous example. The zinc oxide was essentially 100% ZnO (79.9 Zn) and comprised a zinc oxide pigment classified as offgrade because of its failure to meet certain color specifications. Coal additions were made to the extent of 20.1 parts by weight per 100 parts of the zinc oxide to provide the required carbonaceous matter for substantially complete ZnO reduction; the lime was added to the extent of 1.2 parts by Weight per 100 parts of the zinc oxide to balance the S102 present inthe coal ash and thereby maintain a C'aOzSiOz ratio in accordance with our invention. The slag was initially compounded synthetically to produce in the furnace a molten slag having the following composition which, it should be noted, was maintained by the charge composition:

Per cent Fe 1.4 CaO 42.5 SiOz 40.7 MgCll 7.6 CaO:SiO2 1.04 CaO+SiC2 83.2 Other slag constituents 16.8

The iron present in the slag was picked up from the furnace refractory and from the coal ash, and the magnesia was introduced both as a component of the lime addition and by a deliberate ad- .dition to make up an appropriate amount of other slag constituents.

inoe the zinc oxide pigment in the aforementioned smelting mixture required densifying be fore being charged to the smelting furnace, it was briquetted and the briquettes were crushed to 1/4" particles before being mixed with coal and lime for furnacing. Equally satisfactory charge preparation was obtained by mixing together and hriquetting the zinc oxide and lime, then crushingl the briquettes, mixing the crushed briquettes with coal, and rfeeding the resulting mixture to the furnace. The zinc vapor in the resulting snielting gases was condensed with an efficiency cf about 88% in a splash-type condenser. It will he noted that successful smelting of this zincifcrous charge and effective condensation of the resulting Zinc vapor were obtained without produc- 1 1 ing a metallic iron product as one of the products of the smelting operation. In the smelting of such zinciferous material in accordance with our invention, it must be borne in mind that, with the exception of the requirement with respect to the permissible amount of iron oxide in the slag, each of the other prescribed smelting conditions must be observed.

It will be seen, accordingly, that our vinvention offers a commercially attractive method of smelting oxidic zinciferous materials such as refuse zinc oxide, blue powder and zinc ores, and the like, in an electric furnace. The method does not require anything other than conventional electric furnace equipment for the smelting operation and, in the case of zinciferous ores, not only produces zinc metal but also a pig iron product both of which function as collectors for valuable metal by-products which can be readily recovered by conventional means. UnderV normal operating conditions in a commercial scale furnace there appears to be every reason to expect a recovery of S-97% of the zinc component of the ore in the form of condensed molten zinc containing only those impurities which are now removed therefrom by conventional rectification. In the smelting of zinc ores and the like, substantially complete recovery of the lead, cadmium, copper, silver and gold contents of the ore can be realized in accordance with our invention, .the

lead, cadmium and some of the silver and gold being recovered from the condensed metallic zinc and the copper and the balance of the silver and gold being recoverable from the iron product. Only very small amounts of these by-product metals are lost to the slag.

We claim:

1. The method of smelting an oxidic zinciferous material with solid carbonaceous reducing material in an electric arc furnace with the resulting production of a molten slag and metallic zinc vapor substantially free from dust-forming impurities which comprises charging to the furnace the zinciferous material and an amount of the carbonaceous reducing material exceeding but not more than about 125% of the amount of said reducing material stoichiometrically required for reduction of the zinc and other oxides in the charge readily reduced by carbonaceous material and thereby precluding the presence in the molten slag of more than 6% by weight of iron oxide (calculated as Fe), further incorporating in the charge an amount of an extraneous fluxing material selected from the class consisting of lime and silica such that upon smelting of the zinciferous material the resulting molten slag derived from calcareous and siliceous components of the zinciferous material and from said extraneous fluxing material will contain not more than 85% by weight of lime (CaO) and silica (Si02) in a ratio between 0.9:1 and 1.2:1 and not exceed-y ing, with respect to the total amount of the other constituents of the slag, the ratio represented by the line AB in Fig. 1 of the drawing, whereby the fluidity of the slag within the temperature range of 1250 to 1450u C. is sufficient to permit heating of the slag by an electric arc without local overheating of the charge in contact therewith to a temperature in excess of 1150D C., heating said body of slag by electric arc heating to a temperature within the range of 1250 to 1450 C., and delivering the Ycharge to the furnace in such manner as to provide a mass of the charge floating on the slag, smelting of the charge thus taking place primarily at the inl2 Y terface between the fluid slag and the floating charge by the heat imparted to the charge by the slag. f Y f 2. The method of smelting an oxidic zinciferous material with solid carbonaceous reducing material'in an electric arc furnace with the resulting production of a molten slag and metallic Zinc vapor substantially free from dust-forming impurities which comprises charging to the furnace the zinciferous material and an amount of the carbonaceous reducing material exceeding but not more than about 125% Yof the amount of said reducing material stoichiometrically required for reduction of the zinc and other oxides in the charge readily reduced by carbonaceous material and thereby precluding the presence in the molten slag of more than 6% by weight of iron oxide (calculated as Fe), further incorporating in the charge an amount of an extraneous iiuxing material selected from the class consisting of lime and silica such that upon smelting of the zinciferous material the resulting molten slag derived from calcareous and siliceous components of the zinciferous material and from said extraneous fluxing material will contain not more than 83% by weight of lime (CaO) and silica (SiOz) in a ratio between 1.0511 and 1.l5:1 and not exceeding, with respect tothe total amount of the other constituents of the slag, the ratio represented by the line CD in Fig. 1 of the drawing, whereby the fluidity of the slag within'the temperature range of 1250 to 1450 C. is sucient to permit heating of the slag by an electric arc without local overheating of the charge in contact therewith to a temperature in excess of 1450" C., heating said body of slag by electric arc heating to a temperature within the range ofV 1250 to 1450 C., and delivering the charge to the furnace in such manner as to provide a mass of the charge floating on the slag, smelting of the charge thus taking place primarily at the interface between the uid slag and the floating charge by the heat imparted to the charge by the slag. Y, Y

3. The method of smelting an iron-bearing oxidic zinciferous material with solid carbonaceous reducing material in an electric arc furnace with the resulting production of a molten iron product, a molten slag and metallic zinc vapor substantially free from dust-forming impurities which comprises charging to the furnace the zinciferous material and an amount of the carbonaceous reducing material exceeding but not more than about 125% of the amount of said reducing material stoichiometrically required for reduction of the zinc and other oxides in the charge readily reduced by carbonaceous material and thereby precluding the presence in the molten slag of more than 6% by weight of iron oxide (calculated as Fe), further incorporating in the charge an amount of an extraneous fluxing material selected from the class consisting of lime and silica such that'upon smelting of the zinciferous material the resulting molten slag derived from calcareous and siliceous components of the zinciferous material and from said extraneous uxing material will contain not more than by weight of lime (CaO) and silica (SiOz) in a ratio between 0.9:1 and 1.2:1 and not exceeding, with respect to the total amount of the other constituents of the slag, the ratio represented by the line AB in Fig. 1 of the drawing, whereby the uidity of the slag within the temperature range of 1250 to l450 C. Yis sufcient to permit heating of the slag by an electric arc Without local overheating of the charge in contact therewith to a temperature in excess of 1450 C., heating said body of slag by electric arc heating to a temperature within the range of 1250 to 1450 C., and delivering the charge to the furnace in such manner as to provide a mass of the charge floating on the slag, smelting of the charge thus taking place primarily at the interface between the fluid slag and the floating charge by the heat imparted to the charge by the slag.

4. The method of smelting an iron-bearing oxidic Zinciferous material with solid carbonaceous reducing material in an electric arm furnace with the resulting production of a molten iron product, a molten slag and metallic zinc vapor substantially free from dust-forming impurities which comprises charging to the furnace the zinciferous material and an amount of the carbonaceous reducing material exceeding but not more than about 125% of the amount of said reducing material stoichiometrically required for reduction of the zinc and other oxides in the charge readily reduced by carbonaceous material and thereby precluding `the presence in the molten slag of more than 6% by weight of iron oxide (calculated as Fe), further incorporating in the charge an amount of an extraneous fluxing material selected from the class consisting of lime and silica such that upon smelting of the zinciferous material the resulting molten slag derived from calcareous and siliceous components of the zinciferous material and from said extraneous fluxing material will contain not more than 83% by weight of lime (CaO) and silica (S102) in a ratio between 1.05:1 and 1.15:1 and not exceeding, with respect to the total amount of the other constituents of the slag, the ratio represented by the line CD in Fig. 1 of the drawing, whereby the fluidity of the slag within the temperature range of 1250 to 1450c C. is sufcient to permit heating of the slag by an electric arc Without local overheating of the charge in contact therewith to a temperature in excess of 1450 C., heating said body of slag by electric arc heating to a temperature within the range of 1250 to 1450 C., and delivering the charge to the furnace in such manner as to provide a mass of the charge floating on the slag, smelting of the charge thus taking place primarily at the interface between the fluid slag and the floating charge by the heat imparted to the charge by the slag.

5. The method of smelting an iron-bearing oxidic zinciferous material with solid carbonaceous reducing material in an electric arm furnace with the resulting production of a molten iron product, a molten slag and metallic zinc vapor substantially free from dust-forming impurities which comprises charging to the furnace the zinciferous material and an amount of the carbonaceous reducing material exceeding but not more than about of the amount of said reducing material stoichiometrically required for reduction of the zinc and other oxides in the charge readily reduced by carbonaceous material and thereby precluding the presence in the molten slag of more than 6% by weight of iron oxide (calculated as Fe), further incorporating in the charge an amount of an extraneous uxing material selected from the class consisting of lime and silica such that upon smelting of the zinciferous material the resulting molten slag derived from calcareous and siliceous components of the zincifero/us material and from said extraneous fluxing material will contain not more than 83% by weight of lime (CaO) and silica (SiOz) in a ratio between 1.05:1 and 1.15:1 and not exceeding, with respect to the total amount of the other constituents of the slag, the ratio represented by the line CD in Fig. 1 of the drawing, whereby the fluidity of the slag within the temperature range of 1250o to 1450 C. is suiclent to permit heating of the slag by an electric arc without local overheating of the charge in contact therewith to a temperature in excess of 1450 C., heating said body of slag by electric arc heating to a temperature within the range of 1250 to 1450* C., delivering the charge to the furnace in such manner as to provide a mass of the charge oating on the slag, smelting of the charge thus taking place primarily at the interface between the uid slag and the oating charge by the heat imparted to the charge by the slag, and condensing the resulting zinc vaporbearing gases in a condensing zone by subjecting them to intimate contact with a relatively large freshly exposed surface of molten zinc metal.

ROBERT K. WARING. LUTHER D. FETTEROLF. THOMAS L. HURST.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 834,644 Snyder Oct. 30, 1906 859,132 SnyderV July 2, 1907 933,133 Snyder Sept. 7, 1909 1,738,910 Lepsoe Dec. 10, 1929 2,457,544 Handwerk et al. Dec. 28, 1948 2,509,326 Weaton et al. May 30, 1950 OTHER REFERENCES The Art of Electric Zinc Smelting by Johnson, published 1913 by Continuous Zinc Furnace Co., Hartford. Conn.

Claims (1)

1. THE METHOD OF SMELTING AN OXIDIC ZINCIFEROUS MATERIAL WITH SOLID CARBONACEOUS REDUCING MATERIAL IN AN ELECTRIC ARC FURNACE WITH THE RESULTING PRODUCTION OF A MOLTEN SLAG AND METALLIC ZINC VAPOR SUBSTANTIALLY FREE FROM DUST-FORMING IMPURITIES WHICH COMPRISES CHARGING TO THE FURNACE THE ZINCIFEROUS MATERIAL AND AN AMOUNT OF THE CARBONACEOUS REDUCING MATERIAL EXCEEDING BUT NOT MORE THAN ABOUT 125% OF THE AMOUNT OF SAID REDUCTION MATERIAL STOICHIOMETRICALLY REQUIRED FOR REDUCTION OF THE ZINC AND OTHER OXIDES IN THE CHARGE READILY REDUCED BY CARBONACEOUS MATERIAL AND THEREBY PRECLUDING THE PRESENCE IN THE MOLTEN SLAG OF MORE THAN 6% BY WEIGHT OF IRON OXIDE (CALCULATED AS FE), FURTHER INCORPORATING IN THE CHARGE AN AMOUNT OF AN EXTRANEOUS FLUXING MATERIAL SELECTED FROM THE CLASS CONSISTING OF LIME AND SILICA SUCH THAT UPON SMELTING OF THE ZINCIFEROUS MATERIAL THE RESULTING MOLTEN SLAG DERIVED FROM CALCAREOUS AND SILICEOUS COMPONENTS OF THE ZINCIFEROUS MATERIAL AND FROM SAID EXTRANEOUS FLUXING MATERIAL WILL CONTAIN NOT MORE THAN 85% BY WIEGHT OF LIME (CAO) AND SILICA (SIO2) IN A RATIO BETWEEN 0.9:1 AND 1.2:1 AND NOT EXCEEDING, WITH RESPECT TO THE TOTAL AMOUNT OF THE OTHER CONSTITUENTS OF THE SLAG THE RATIO REPRESENTED BY THE LINE AB IN FIG. 1 OF THE DRAWING, WHEREBY THE FLUIDITY OF THE SLAG WITHIN THE TEMPERATURE RANGE OF 1250* TO 1450* C. IS SUFFICIENT TO PERMIT HEATING OF THE SLAG BY AN ELECTRIC ARC WITHOUT LOCAL OVERHEATING OF THE CHARGE IN CONTACT THEREWITH TO A TEMPERATURE IN EXCESS OF 1450* C., HEATING SAID BODY OF SLAG BY ELECTRIC ARC HEATING TO A TEMPERATURE WITHIN THE RANGE OF 1250* TO 1450* C., AND DELIVERING THE CHARGE TO THE FURNACE IN SUCH MANNER AS TO PROVIDE A MASS OF THE CHARGE FLOATING ON THE SLAG, SMELTING OF THE CHARGE THUS TAKING PLACE PRIMARILY AT THE INTERFACE BETWEEN THE FLUID SLAG AND THE FLOATING CHARGE BY THE HEAT IMPARTED TO THE CHARGE BY THE SLAG

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