US1961424A - Zinc refining process - Google Patents

Description

June 5, 1934. C Q VMAlER ZINC REFINING PROCESS Filed Dec. 26. y19751 2 Sheets-Sheet l- All/wein@ Patented June 5, 1934 UNITED STATES PATENT OFFICE 3 Claims.

My invention relates in general tothe reduciion of zinc ores. It relates more in particular to improved means and the process for reducing zinc oxide to metallic zinc, and is a con- 5 tinuation in part of my prior application, Serial Number 513,458, flied February 4, 1931.

In the known art, the reduction of zinc oxide yto metallic zinc is accomplished by the addition to the zinc oxide of some reducing agent as carbon,

l0 a hydrocarbon or other organic compound which at elevated temperature combines with the oxygen of the' zinc oxide to form zinc vapor and various proportions of water, carbon monoxide and carbon dioxide. These proportionsare limited by the respective chemical equilibria involved, and are determined by chemical reaction rates in approaching these equilibria. The zinc vapor is then condensed from this mixture by cooling. By this process as now practiced, it is not possible to obtain zinc entirely in a coherent form, but some blue powder is always formed due to reversal at condensation temperatures of the above mentioned equilibria. This finely divided zinc and zinc-oxide mixture or blue powder is well known to owe its origin to mechanical aswell as chemical factors, (l) the rapid condensation of the zinc from the vapor to the solid or spheroidal state and (2) the reaction of the carbon dioxide, water or other oxygen compound formed by the reduction reactions at higher temperatures with ihe zinc vapor or finely divided solid or spheroidal zinc metal particles when the temperature is lowered from the reduction temperature, to form zinc oxide which coats the zinc particles. and prevents coalescence. .It will be seen from this discussion that if condensation takes place slowly the amount of powder due to factor one will be decreased but that formed by factor two will bev increased so that the best that can be hoped for 40 is a compromise with the formation of considerable blue powder.

Other methods known to the prior art require the mixture of gases from the reduction of zinc oxide as above described to be passed over incandescent coke to remove the oxidizing gases before condensing the zinc. This method, however, requires the use of expensive solid fuel' and requires two steps, the second of which must be carried out at a temperature considerably above 1000 degrees centigrade which Oilers difficulties of operation and produces less pure zinc than processes carried out at lower temperatures.

In still another method known in the prior art reducing gases such as carbon monoxide y and hydrogen are added to the condensing chamber.

This involves an additional step in the preparation of the reducing gas and cannot substantially reduce the amount of oxidizing gas present without too much dilution of the zinc vapor which causes difficulty in condensation. Carbon monoxide and hydrogen in some instances have been used directly as reducing agents, but, such gases can reduce only insignificant amounts of zinc at temperatures below 1000 C. and the carbon dioxide or watervapor formed in the reaction readily reoxidizes the metallic zinc when the mixed gases are cooled in the condenser to separate liquid zinc. In this process excessive amounts of blue powder invariably are obtained.

The principal object of my invention is the provision of an improved process for producing metallic zinc.

Another object is to reduce zinc oxide by a process in which no re-oxidizing vapor such as carbon dioxide or steam can be formed.

Another object is to reduce zinc oxide at lower temperatures than have been used successfully heretofore.

Another object is to reduce zinc oxide to metallic zinc with the production of a minimum amount of blue powder.

Another object is to reduce zinc by a process which can be carried out continuously and economically.

Other vobjects and features of the invention will be apparent from the following detailed description taken with the accompanying drawings.

In the accompanying drawings I have illustrated in diagrammatic form two modifications of.

apparatus suitable for use in the practising of my process. In this showing,

Figure 1 illustrates a suitable form of apparatus to be used when the pretreatment process hereinafter'described is not employed in the practice of the invention, and

Figure 2 illustrates a modified form of apparatus for use when the pretreatment process hereinafter described is employed in conjunction with my process.

In general, my invention comprises the reduction of zinc oxide to metallic zinc by means of methane as a reducing agent. the source of the methane being preferably a natural gas which contains methane in amounts greater than approximately 80%. Other sources of methane can, however, be employed.

In carrying out my process, I control all the factors so that the' zinc oxide is reduced by the methane substantially according to the reaction This reaction may be carried out in any suitable type of retort heated in any suitable manner to which the zinc oxide is supplied in relatively finely divided form. The gaseous products of thereaction are carried from the retort into'a condenser where the zinc vapor is condensed 'and run off into suitable molds for further disposition in the usual way.

Iam aware that the use 0f methane and other As previously disclosed, when such side reactions occur, it is impossible to prevent the formation of considerable amounts of blue powder, so much so.

in fact that the methane reduction of zinc oxide heretofore has been impracticable.

I have discovered, by the application of thermodynamic chemical methods to the equilibria involved in the reduction process, that by controlling certain factors including especially temperature and pressure, the disposition of the zinc oxide with respect to the methane and theabsence of catalytic iron, in the condensing range, that I can substantially inhibit side reactions and the products of the reaction will be substantially zinc, carbon monoxide and hydrogen, with the formation of substantially no carbon dioxide or water vapor, with a concomitant decrease in the amount of blue powder formed. One of the most important characteristics of my invention is the discovery that methane will not decompose in the presence of zinc oxide. under the conditions herein described (but under other conditions methane is incapable of practical use for zinc reduction due to excessive decomposition into carbon and hydrogen). In other words, the rate of decomposition of methane in intimate contact with zinc oxide, within certain temperature ranges, is small as compared with the rate of reaction with zincoxide. By making use of this discovery, and controlling the disposition of the methane with respect to the zinc oxide so that there is always an intimate contact between the two, the methane will be available in the form of CI-I4 for direct reaction with the zinc oxide. This, with a proper control of temperature, substantially completely avoids a possibility of side reactions ofthe character hereinabove referred to.

As to temperature, I have found that a temperature range of approximately 800 to 1000" centigrade produces the best results, with the very best temperature range between 925 to 990 centigrade. The following analysis will illustrate the results obtained in the preferred temperature range.

centrigrade) CH4 C O K C Or Hz H2 0 Percent Perce Percent Percent 3 25. 0 0. 5 5l. 2 0. 7 29. 2 0. 4 50. 2 0. 6 32. 5 0.8 66. 7 0. 9

Above this temperature range `I have found that the carbon dioxide content increases and below it the methane decomposes with the deposition of carbon with consequent loss of efficiency.

By accomplishing the reduction within the= above range and maintaining the methane throughout the heating range in intimate contact with the zinc oxide, I have secured excellentresults in producing metallic zinc with .u

minimum amount of blue powder, substantially less than is customarily formed in the most satisfactory present commercial processes. The best results are obtained by a proper control of the condensing step of the process, particularly avoiding the presence of catalytic iron within the condensing range, and cooling at a controlled rate which is not too rapid, this being possible as hereinabove explained due to the fact that the gases entering the condenser consist substantially entirely of zinc, carbon monoxide and hydrogen. In actual practice, good results attend the practice of impinging the vapor streamy directly upon the liquid zinc surface.

It is evident that the desirable reaction occurring between zinc oxide and methane is reversible but at the desirable temperatures set out the rate of the reducing reaction is so very much greater than the rate of speed of the reverse reaction that very high eciency results and the yield of zinc vapor is relatively very large. It is, of course, true also that zinc oxide and carbon monoxide will react together in accordance with the following reaction This reaction is undesirable as the presence of relatively large amounts of CO2 in the condensing range will result in the formation of blue powder. This reactionalso is reversible, however, and, at the temperatures which I select, chemical equilibria in this reaction are such as to prevent the formation of harmful amounts of carbon-dioxide. I have found that when the carbon dioxide content is kept below approximately 1% the rate of reoxidation reaction is so low that zinc vapor may But, at the temperatures which I employ, only very slight traces of Water vapor can be formed according to the equilibria involved, and slight traces of water vapor are without effect in the condensing range, as in the case of traces of carbon dioxide.

I shall now refer to the drawings in which I illustrate one type of apparatus with which my process may be carried out. I employ a retort l0 suitably supported within a furnace 11 into which retort the ore is adapted to be inserted through an ore delivery pipe 12.. A gas delivery pipe 13 is provided adjacent the ore delivery pipe. The retort rests upon a condenser 14 and between .the retort. proper and the condenser I support in any suitable manner pieces of refractory material 16. Near the bottom of the retort proper and above the pieces of refractory material I insert an ore discharge pipe 17 through which the residue of the ore, after the reduction of the zinc oxide, is passed. 'Ihe gases, including the products of the reaction, pass through the refractory material 16 (which material holds back the ore) down into the condenser, its path through the condenser being relatively long due to the presence of horizontal baffles 18 and vertical bafes 19'. The gases are cooled in their path through the condenser and portions of the metallic zinc, becoming liquid, flow to the bottom of they condenser and out through a tap '21. The gases, however, impinge directly onto the liquid metal- `lic zinc in the bottom of the condenser, andthe be used to pre-treat the zinc ore, or can be used in other processes for example to remove sulphur from certain ores. It will be seen that with this form of apparatus the ore and gas may be fed continuously and concurrently or the retort may be charged with the ore and then reduced in a batch. I prefer, however, the continuous treatment for several reasons which readily suggest themselves, but principally because the intimate contact is more easily maintained -with the continuous treatment.-

One. of the advantages of the concurrent downward movement of the ,ore and gas is that the intimate contact between the gas and ore is maintained. I have found that if this intimatecontact is maintained, a metal retort can be used with satisfaction (the relatively low temperature also makes this possible) since whatever'cata-4 gas, I have found that the reduction is ineifective. Below 1000 C. substantially only carbon is formed by thermal decomposition of the hydrocarbon gas, and, at these temperatures, the carbon so formed will not subsequently react with zine oxide to reduce the same. formed would disappear at high temperatures by reaction with the zinc oxide in this way, but in doing so would produce reoxidizing carbon dioxide and the effect of my invention thus could not be obtained.

Methane gas is unstable at temperatures from 8001000 C., but is effective for. my purpose as controlled according to my teachings. I explain my results by the following hypothesis which seems to t all of my findings. If attention is focused on an individual molecule of this gas, it is found that a certain critical time period after its temperature is raised (which in general is a major fraction of a second) is necessary to permit the reaction of decomposition to occur. If, however, this individual particle may be caused to come into contact with a molecule of zinc oxide before the critical time period has elapsed, the faster reaction of methane with. zinc oxide takes place to the exclusion of thermal decomposition. I accomplish this conjunction of methane and zinc oxide in less than the critical time for thermal decomposition by maintaining it in intimate contact with the zinc oxide, preferably passing the gas downwardly through a nely granular bed of ore containing no large voids. Thus, if using briquetted charges, I partly ll the'interstitial spaces with granular material passing through screens 8 meshes per inch, but remain- The carbon so ing'upon screens 40 or 60 meshesv per inch.. In thisway, the molecules of gas are never absent from ore particles for a great enough time to allow thermal decomposition.

The downward flow of gas makes the process relatively easily controllable since I have found that upward flowing gas tends to form channels in the ore charge, producing the voids which must be eliminated to prevent carbon formation.

The condenser is preferably made of a suitable refractory material substantially devoid of catalytic iron in its constitution. Many of the ordinary refractory materials of commerce are satisfactory for the purpose. The material indicated at 16 is, for example, broken pieces of refractory material. By saying that zinc oxide has an anti-catalytic effect, I mean that al'- though methane is unstable at temperatures between 800 and 1000 centigrade, in the presence of zinc oxide and with sufficiently intimate contact, the reduction reaction may be made to occur preferentially as indicated above.- For this reason, care should -be taken to avoid raising the temperature of the methane to the reaction temperature in the absence of the anti-catalytic zinc oxide either before it enters the retort or even after entrance, and it should enter the retort at a point where the temperature is below 800 degrees for best results, although some slight preheating of the methane may take place without serious results; If care is not taken to maintain the methane in intimate contact with the zinc oxide, the carbon formed by the breaking down of the methane will be deposited in such a way as to substantially choke up the retort and prevent the proper movement of the gas therethrough. I found that once carbon starts to form it has a tendency to form relatively quickly but it will not initially begin ito form if an ample supply of zinc oxide is present in substantially all portions of the retort in a condition to contact intimately with the methane.

Other ways of maintaining this intimate contact may, of course, be utilized in place of the down flow principle employed in the apparatus described. the simplest means of obtaining the intimate contact and has been effective in producing very satisfactory results.

When I speak of the reduction of zinc oxide. it

is understood that this term zinc oxide is employed in the usual manner customary in the industry. It includes roasted blend which has been substantially freed from sulphur or any nat- This is probably, however, one of ural or processed ore consisting essentially of zinc oxide.

One very satisfactoryand novel way of practicing my invention is to employ the exit gases of the zinc oxide reduction process as they leave lthe retort for the pretreatment of the ore, as for -which mingles with the effluent gases of the retort and, coming in contact with liquid zinc in the condenser, recombines 'to-form a fouled condensate containing infusible zinc sulphide. The m lcerial se formed is analogousto blue powder.

To avoid this trouble, I have found that I can remove the harmful sulphur impurities by pretreatment of the ore at temperatures slightly lower than 800" C., using the exit gases from the retort, after condensation of the zinc therefrom. 'Ihis pretreatment may be accomplished in another i'etort similar to the one used for reduction, but without the condenser, which is replaced by a settling chamber or dust separator to collect the impurities driven off, and operated at temperatures approximately from about 600 to 800 C. vUnder these circumstances, practically no zinc is reduced. Some other substances may be affected by such pretreatment such as cadmium, arsenic (if present), and some other volatile impurities.

I regard the pretreatment of ore by exit gases or other reducing gases of similar composition as an essential p art of my process and constituting a part of the invention whereby blue powder formation is minimized., and entirely fusible and coherent substantially pure metallic zinc produced.

The o re is preferably fed to the retort at substantially the temperature required to obtain the desired reaction described. In actually carrying out the process, the ore after roasting can be fed directly to the pre-treating retort and then to the reducing retort and this can be accomplished so that it will be at a temperature only slightly below the reduction temperature when delivered to the reduction retort. If the pre-treatment step with the exit gas is not employed, any other suit-- able means may be employed for assuring that lthe ore is preheated before delivery to the reduction retort. The process can be carried on by supplying substantially cold` ore to the reduction retort. and depend on the heat supplied to the retort to raise it to the reduction temperature, but this is not the preferred practice.

The reducing apparatus should be regulated to have a pressure slightly above atmospheric pressure to prevent the entrance of air at any point and it may be necessary under certain circumstances to provide a small counter-current of the exit gas through the' orc discharge pipe to prevent diffusion of zinc into such pipe.

In Figure 2 of the drawings, I have shown a modified form of apparatus for use when the pretreatment process described above is employed in conjunction with my process. In this form of apparatus the construction is similar to that' shown in Figure 1 with the exception that a pretreatment retort 30 is arranged above the retort 10 and is separated therefrom by a partition 31 in which a bell hopper 32 is arranged whereby pretreated ore may be delivered `from the retort 30 to the retort l0 by operation of the lever 33. As` will be apparent from Figure 2, in the modied form of apparatus shown the ore inlet pipe 12 is adapted to deliver ore into the retort 30 instead of directly into the retort 10, as in Figure 1. The gas outlet pipe 22, provided with a valve 34, is connected to the inlet side of a gas pump or fan 35 which is adapted to force the exit gases from the condenser through the pipe 3 6 to the gas inlet pipe 37 and into the pretreatment retort 30.

Preferably at the lower end of the retort 30 and opposite the inlet pipe 37 is arranged a gas outlet pipe 38. As shown, the pipe 3.6 is also connected with the pipe 39, having a valve 40, which connects with a gas inlet pipe 41 arranged in the ore discharge pipe 17, whereby a small countercurrent of exit gas is provided in the ore discharge pipe 17 to prevent the diffusion of zinc into such pipe.

Those skilled in the art will understand that the explanation of the manner in which the proccss is carried out and in which the apparatus is constructed and operated is subject to considerable variation and modification in accordance with specific conditions.' I do not restrict myself, therefore, in any sense by the details disclosed but the invention is limited only by the scope of the appended claims.

What I claim as new and desire to protect by United States Letters Patent is:

1. A process for reducing zinc oxide in ore to metallic zinc, which comprises continuously feeding zinc ore into the top of a vertically disposed retort, heating the ore to a temperature of between 800o C. and 1000 C., continuously supplying methane to the top of the retort and passing the methane vertically downwardly while maintaining intimate contact between the methane and zinc oxide in the heating range, whereby the methane and zinc oxide react to produce gaseous reaction products, including principally Zinc vapor, carbon monoxide and hydrogen, passing the gaseous products into a condenser, condensing the zinc vapor and continuously removing residue ore material from the bottom of the retort. f

2. A process for reducing zinc oxide to metallic Zinc, which comprises continuously feeding the zinc oxide in relatively ilnely divided form into one end of a retort, heating the zinc oxide to a temperature between 800 C. and-1000 C., continuously passing methane into the same end of the retort as that into which the Zinc oxide is delivered, whereby a concurrent and confluent movement of the zinc oxide and methane takes place through the retort, maintaining the zinc oxide and methane in intimate contact with each other in the heating range, whereby the methane is maintained stable with the exception of a reaction with zinc oxide toproduce gaseous reaction products, including principally zinc vapor, carbon monoxide and hydrogen, passing the gaseous reaction products into a condenser to condense the zinc vapor and continuously removing solid residue material from the other end of the retort.

3. A process for reducing zinc oxide in ore to metallic zinc, which comprises continuously feeding zinc ore into the top of a vertically disposed retort, heating the ore continuously to a temperature between 800 C. and 1000 C., continuously supplying methane to the top of the retort and passing the methane vertically downwardly while maintaining intimate contact between the methane and zinc oxide in the heating range, whereby the methane and zinc oxide react to produce gaseous reaction products, including principally zinc vapor, carbon monoxide and hydrogen, passing the gaseous reduction products into a condenser substantially free of catalytic iron, condensing the zinc vapor, and continuously removing residue ore vmaterial fro-m the bottom of the retort.

CHARLES G. MAIER.

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