US1661813A - Process of sintering zinc ores - Google Patents

Description

Patented Mar. 6, 1928.

UNITED STATES,

HENRY J. STEHLI, OF CEDAR GROVE, NEW JERSEY.

PROCESS OF SINTERING ZINC OBES.

No Drawing.

Heretofore it has been the usual practice in the production of metallic zinc from zinc sulphide ores to charge fine ores or concentrates into a roasting furnace where the ores are roasted for the elimination of sulphur before being subjected to a distillation process. While the sulphur can be almost entirely eliminated by prolonged roasting at high temperatures, the cost of doing this is commercially prohibitive and in practice there still remains in the roasted ores, after their removal from the furnace, somewhere in the neighborhood of 2% or more of sul phur. Such ores are commonly known as dead-roasted ores.

The ores after roasting are mixed with coal, coke or other fuel and sometimes with small amounts of salt and other ingredients. The charge is'then placed in the retorts of a retort furnace. To each retort there is attached a condenser. When the furnace containing the retorts is heated in a suitable manner, a reaction takes lace between the carbon and the zinc, whic latter exists in the roasted ore mainly in the formof zinc oxide, forming metallic zinc vapor and carbon monoxide. The zinc vapor passes over into the condenser molten metallic zinc, and the carbon monoxide passes oil throu h avent in the condenser and burns. fn addition to the zinc present in the roasted ore in the form of zinc oxide there is, due to;imperfect roasting, always an appreciable amount of sulphur which exists in the roasted ore in the form of zinc sulphide and zinc sulphate.

The material remaining in the retorts after the distillation is completed and known as zinc residue is drawn out and carried out to the dump. Such zinc residue comprises some earthy matter originally contained in the zinc ore and a very appreciable amount of zinc which has not been reduced and driven off because it existed in'the roasted ore in the form of sulphide or sulphate of zinc; There is also present a relatively great amount of unburned carbon, and often small values in silver and gold of the original ore contained therein.

A typical analysis of zinc residue would show something llke 10% zinc and 30%carbon. Heretofore such zinc residues have been almost invariably waste, as no cheap where it is condensed into Application filed October 9, 1928. Serial No. 140,681.

and efiicient method has been discovered so far for utilizing them.

The foregoing description refers particu: larly to preliminary roasting of zinc ores in a well known way.v Of recent years it has been proposed to treat pre-roasted zinc ores by the sintering process because of its being more economicaland giving a better roasted product. Under such process after the zinc ores have beenprelim'inarily roasted there has been a SllifiClGIll) amount of sulphur left to be used for fuel in the sintering process, say, 8% to 12%, or if the preliminary roasting has been carried beyond such point green zinc sulphides have been added to the mixture to increase the sulphur content as fuel for sintering. To be certain of having enough sulphur for carrying on the sintermg operation it has been desirable to err on the high side and have invariably a little too much sulphur in the charge, as when too'small an amount of sulphur is in a charge it will not ignite or sinter at all. A disadvantage of this is that because of using sulphur itself as a fuel for sintering and ecause of having a slight excess of sulphur it has not been practical to entirely burn out the sulphur in the sintering operation, so that, at best, the sintered product would have a proportion of sulphur remaining in it, say between 1% and 1 This remaining sulphur holds up a certain amount of zinc in chemical combination and complicates the reactions. in and satisfactory operation of the retorts in the subsequent process for the recoveryof the zinc from the sinter. Furthermore, the furnaces referred to for pre-roasting, or dead-roasting, zinc ores are of such a character that they convert a considerable amount of the sulphur present in the ore from the sulphide into the sulphate form, and such pre-roasted charge going into the sintering machine invariablycontains such sulphate sulphur,

which greatly complicates and hinders the sintering operation, because the decomposition of sulphates is an endothermic reaction and requires considerable heat. It is, therefore, necessary where sulphur is used as the sintering fuel to not only leave enough sulhide sulphur in the charge to furnish the eat for sintering the ore, but also to leave a certain additional amount in the charge to furnish the heat required to break up the sulphates and expel the sulphur from them, which further aggravates the conditions previously referred to and makes it difficult to expel all the sulphur from the sintering charge so as to leave a sintered product sufiiciently low in sulphur to give the best results.

The objects of my invention are to desulphurize zinc ores and to produce a roasted or sintered zinc product substantially free from zinc sulphide and entirely free from zinc sulphate and of such a porous physical character as to permit rapid reduction and distillation of its zinc contents in subsequent treatment, as for the recovery of metallic mm.

A further object of my invention is to utilize the residues, from the retorts in which the pro-roasted and sintered zinc ores have been treated to obtain metallic zinc, as the fuel to be added to the pro-roasted ore in the subsequent sintering process thereof. Such residues are commonly known as zinc residues and contain more or less unburned carbon which I utilize to furnish the heat necessary for carrying onthe sintering of the preroasted zinc ores whereby sulphur can be substantially completely eliminated from the sintered product. A further advantage of my invention is that the zinc present in the zinc residues used in the sintering step 18 so acted upon during the sintering process as to lose its refractory character and become again amenable to reduction and distillation in the retorts.

In carrying out my invention I pre-roast fine zinc sulphide ore in any known way and by preference down to as low a point as is commercially feasible, preferably without the use of fuel in the preroasting furnace other than the sulphur in the ore itself, in other words autogenous roasting. The zinc ore may be roasted in this way economically to a sulphur content which would be too low to furnish sufficient fuel to alone carry on subsequent sintering or blast roasting of such ore, say to 3% or &% sulphur, but too high to enable the roasted product to be used directly in the distillation furnaces. To such pre-roasted ore I add water in the usual way for sintering and also carbonaceous material and ignite the charge and cause the carbon to burn and the charge to sinter by passing air therethrough. The sulphates present in the roasting material are very readily and efficiently decomposed and broken up by the carbon, and under such conditions the material or charge sinters very rapidly and makes an unusually excellent sinter from a physical viewpoint, and most important, because of the comparatively low sulphur content the material has as fed to the sintering machine, and because of the use of carbon to furnish the heat necessary for carrying on the sintering the sulphur can be entirely eliminated from the sintered product. The roasting or sintering of the charge of preroasted ore containing carbonaceous material can be carried on in the well known sintering machines.

The carbonaceous material I use with the preroasted ore to furnish the necessary heat to reduce the sulphates and sinter the material comprises zinc residues before referred to. After the material comprisin a preroasted zinc ore and zinc residue Iias been sintered, it may be mixed with the usual coal or coke, salt, etc., and placed in the retorts and heated in a usual or desired way, whereby the oxidized zinc in the sinter will be reduced to metallic zinc and volatilized. The volatilized zinc will pass into the condenser and be condensed in the form of a molten metallic zinc and the carbon monoxide will be carried off throu h the opening in the condenser as usual. ecause the sintered zinc ore is free from sulphur and highly porous, the distillation of the zinc vapor will be found to be greatly facilitated by my invention, and, also, I am enabled to recover a higher percentage of the zinc contents of the ore than is possible by the former method.

The zinc residues (from the retorts) which contain carbon and have heretofore been looked upon largely as a waste product at zinc smelters, furnish an ideal cheap material for supplying the carbon necessary for carryin on the above-mentioned reducing of sulp iates and sintering of'the ore. Zinc residues as produced at the zinc smelters may vary considerably in composition and all zinc residues in the exact condition that they are produced may not be suitable for my purpose. It is well known that by screening, by water concentration, or by magnetic concentration, zinc residues in certain cases may be so screened or concentrated as to produce two or more products, one of which will contain larger proportions of carbon and less of the earthy material than the other and may be more suitable for use as fuel in the sintering charge by my process than unscreened or unconcentrated residues, and the" other or others of which may be suitable for use for other purposes but may not be desirable in my process. As an illustration, some zinc residues contain an excessive proportion of silica and alumina which it may be undesirable to return into the sintered charges. By one of the above described methods it will fre uently be possible to eliminate a considcra le proportion of this silica or alumina, thus rendering the zinc residues much more suitable for my purpose. The use of the zinc residues provides advantages since it not only furnishes an easily available and cheap source of carbon but puts into the charge (com rising preroasted zinc ores and zinc residues) a certain amount of undistilled zinc which is contained in such residues, which zinc becomes available for extraction by distillation in the retorts. In other words, the zinc contained in the waste zinc residues from retorts can be recovered when such residues are used in a mixture with preroasted zinc ores and sintered, since in the sintering process the refractory undistilled zinc in the residues changes in such a way that it will then volatilize in a retort in which the sintered product containin such previously refractory zinc is treate A further advantage of using the zinc residues from the retorts in sintering preroasted zinc ores is that zinc ores frequently contain small amounts of gold, silver, copper and other metals which have a definite value, and which remain in the zinc residues after the zinc has been distilled, as described in the distilling process above. Such zinc residues as commonly produced, have gold, silver, copper and other metals in quantities usually too small to pay the cost of recovering them. In accordance with my invention of using such zinc residues and mixing them into a fresh charge of preroasted zinc ore for sintering, the gold, silver, copper and other contents will gradually become built up in the residues as they are used one or more times to a point where residues will finally contain suflicient precious and other metal to pay for their recovery. At such time the enriched residues from the retorts would be taken out of the cycle for suitable treatment to recover their values, and a new lot of residues from the dumps on hand would be used to take their place in the charges for the sintering machines.

lVhile I have particularly described the desulphurized sintered zinc ore as being adapted to be used as charges in retorts for recovering the zinc therefrom, it will be u11- derstood that such material may be used for other purposes than for recovering the metallic zinc therefrom in the manner stated, such as for the production of zinc oxide from the desulphurized sinter described, or for the recovery of the zinc as the metal or its salts by other methods, such for example as by hydrometallurgical methods of treatment.

Having now described my invention, what I claim is:

1. In the art of desulphurizing zinc sulphide ores the process consisting in roastin the ore down to a point where the residue sulphur in the ore, as sulphide, will be insuflicient to carry on sintering of the ore, mixing the roasted ore with sufiicient zine residues to supply the carbon required to furnish the heat and reducing agent to decompose the sulphates in the ore, and causing internal combustion of said sulphur and carbonaceous material contained in the zinc residues by blast roasting in an oxidizing atmosphere to eliminate the sulphates in the ore and sinter the mass.

2. In the art of despuphurizing zinc sulphide ores the process consisting in roasting the ore down to a point where the residual sulphur in the ore, as sulphide, will be insufiicient to carry on sintering of the ore, mixing the roasted ore with sufiicient zinc residue to supply the carbon required to furnish the heat and reducing agent to decompose the sulphates in the ore, and causing internal combustion of said sulphur and carbonaceous material contained in the zinc residues by blast roasting in an oxidizing atmosphere to eliminate the sulphates in the ore and sinter the mass, the heat of combustion of the carbon causing such a change in the refractory undistilled zinc in the said zinc residue as to permit volatilization of said zinc when the sintered product is sub sequently treated in a retort.

3. In the art of desulphurizing zinc sulphide ores .the process consisting in roasting the ore down to a point where the residual sulphur in the ore, as sulphide, will be insufficient to carry on sintering of the ore, mixing the roasted ore with sufficient zinc residues to supply the carbon required to furnish the heat and reducing agent to decompose the sulphates in the ore, and causing internal combustion of said sulphur and carbonaceous material contained-in the zinc residues to eliminate the sulphates in the ore, the heat of combustion of the carbon causing such a change in the refractory undistilled zinc in the said zinc residues as to permitvolatilization of said zinc when the I sintered product is subsequently treated in a retort, and causing reduction and volatilization of the zinc in the ore and of the zinc contained in the zinc residue.

4. In the art of desulphurizing zinc sulphide ores the process consisting in roasting the ore down to a .point where the residual.

sulphur in the ore, as sulphide, will be insufficient to ciirry on sintering of the ore, mixing the roasted ore with sutficient zinc residue to supply the carbon .required to reduce the sulphates in the ore and sinter it, and sintering the ore by combustion of the carbon in the zinc residue and simultaneously causing air to pass through the ore to support combustion and carry ofi' sulphur from the ore.

5. In the art of desulphurizing zinc sulphide ores the process consisting in roasting the ore down obtained by autogenous roasting by combustion of the sulphur inthe ore, mixing the roasted ore with zinc residues suflicient to provide the carbon required to furnish the necessary heat and reagent to react on the to as low a point as can be sulphates in the ore and sinter it, and sintering the mixture by causing combustion of the carbon in the zinc residue and simultaneously causing air to pass through the charge to support combustion and carry off sulphur from the ore.

6. In the art of desulphurizing zinc sulphide ores the process eonsisting in roasting the ore down to as low a point as can be obtained by autogenous roasting by combustion of the sulphur in the ore, mixing the roasted ore with zinc residues sufiioient to provide the carbon required to furnish the necessary heat and reagent to react on the sulphates in the ore and sinter it, sintering the mixture by causing combustion of the carbon in the Zinc residue and simultaneously causing air to pass through the charge to support combustion and carry oil sulphur from the ore, the heat of combustion of the carbon causing such a change in the refractory undistilled zinc in the said zinc residue as to permit volatilization' of said zinc when the sintered product, is subsequently treated in a retort, and causing reduction and volatilization of the zinc in the ore and of the zinc contained in the zinc residues.

7. In the art of desulphurizing zinc sulphide ores the process consisting in roasting the ore down to a point where the residual sulphur in the ore, as sulphide, will be insufficient to carry on sintering of the ore, mixing the roasted ore with suiiicient zinc residues to supply the carbon required to furnish the heat and reducing agent to decompose the sulphates in the ore, and cansing internal combustion of said sulphur and carbonaceous material contained in the zinc residues to eliminate the sulphates in the ore, the heat of combustion of the carbon causing such a change in the refractory undistilled zinc in the said zinc residue as to permit volatilization of said zinc when the sintered product is subsequently treated in a retort, and causing reduction and volatilization of the zinc in the ore and of the zinc contained in the zinc residue, adding residues from the reduction and volatilization to preroasted ore and sintering the mixture to buildup precious metal Values in such sinter.

8. In the art of desulphurizing zinc sulphide ores the process consisting m roasting the ore down to as low a point as can be obtained by autogenous roasting by combustion of the sulphur in the ore, mixing the roasted ore with zinc residues suflicient to provide the carbon required to furnish the necessary heat and reagent to react on the sulphates in the ore and sinter it, sintering the mixture by causing combustion of the carbon in the zinc residue and simultaneously causing air to pass through the charge to SHPPOI'ECOH'I- bustion and carry otl' sulphur from the ore. the heat of combustion of the carbon causing such a change in the refractory undistilled zinc in the said zinc residue as to permit vol atilization of said zinc when the sintered product is subsequently treated in a retort, and causing reduction and rolatilization of the zinc in the ore and of the zinc contained in the zinc residues, adding residues from the reduction and volatilization to prcroasted ore and sintering the mixture to build up precious metal values in such sinter.

9. In the art of desulphurizing zinc sulphide ores the process consisting in roasting the ore down to a point where the residual sulphur in the ore, as sulphide, will be insullicient to carry on sintering of the ore,

mixing the roasted ore with sullicicnt carbonaceous material derived from zinc dcsidues by screening and containing some zinc to supply the carbon required to furnish the heat and reducing agent to decompose the sulphates in the ore, and causing internal combustion of said sulphur and carbonaceous material contained in the zinc residues to eliminate the sulphates in the ore.

10. In the art of desulphurizing zinc sulphide ores the process consisting in roastin the ore down to a point where the residua sulphur in the ore, as sulphide, will be insufiicient to carry on sintering of the ore, mixing the roasted ore with suflicient carbonaceous material derived by concentrating zine residues and containing some zinc to supply the carbon required to furnish the heat and reducing agent to decompose the sulphates in the ore, and causing internal combustion of said sulphur and carbonaceous material contained in the zinc residues to eliminate the sulphates in the ore.

HENRY J. STEHLI.