Jàn. 7, 1947. G. GUTZEIT ETAL TIN ORE ‘I‘REATMENT F‘íle‘d Jan. 23, 1943 2 Sheets-Sheet’ 1 ATTORNEY Jan; 7, 1947. G. GUTZEIT ET A‚L TIN ORE TREA'I‘MENT Filed Jan. 23, 1945 2 Sheets-Sheet 2 TEMPERATURE OF THE REDUGING ROAST(°F) FIG.2. INV\ENT.ORS GREGOIRE GUTZEIT, ELLIOTT J.R'OBERTS a. ROBERT B.THOMPSON BY Patented Jan. 7, 1947 UNITED STATES PATENT OFFICE 2‚413,762 TIN ORE TREATMENT Gregoire Gutzeit and Elliott J. Roberts, Westport, and. Robert B. Thompson, Wilton, Conn, as signors to The Dorr Company, New York, N. Y., ‘a corporation of Delaware Application J anuary 23, 1943, Serial No. 473,406 6 Claims. (01. 23-53) This invention relates to the recovery of tin and metals of the same class Írom ores and mill products. It renders specìally possible the ex traction of tin trom refractory ores (complex cassìterite-sulphide ores and ores containing so called colloîdal tin) as well as from the tail ings of tin concentrators, although it is not ap plicable to ores with a carbonate gangue. Tin occurs principally as the mineral cassit 2 ing conditions, and the third stage comprises a leaching operation whereby a tin-hearing solu tion is selectively obtained that can be separated from the gangue residue. Another stage com prises precipitating the tin out of its mother liquor under criti'cal conditions. Since it is the tin that is the value to be ‚recovered and since the proportion thereof in the ore or other material is ‘small, a high per centage of extraction of the tin is a feature of erite, S1102. As is well known to mining en advantage of this invention. Since tin ore is gineers, there are two main types of tin ores: normally Íound only in out of the way places in the ?rst one is the so’called stream tin, i. e., an the world, where freight rates are high, the alluvial deposi‘t where the cassiterite particles are availability of chernical reagents is very impor generally tree; the other type is the 10de tin, in tant. Another feature of advantage in this in 15 which the tìn oxide is mcstly associated with vention is the possibility of regeneration of the stannite (tin copper sulphide), pyrites, arseno reagent for reuse. pyrites, bismuth, antimony, and arsenic sulphides, As the roasting stage: If one attempts to re and often with wolframite. A rarer occurrence duce the cassiterite of a tin ore which has a sub is the eluvial tin (French Indo-China, Belgian Congo). The ores of the ?rst types generally 20 stantial content of iron compounds as well as other base metals in some proportion, the dif? yield clean concentrates and fairly good tailings. culty is faced in that in the process of reduc These are principally found in the Malaya Pen ing the tin, iron and/or certain other elements insula and in the Netherland Indies (Banka and are likely to be reduced to the metallic state. A1 Billiton). The second type is more scattered loys of the tin and these metals then form and over the world, and is found mainly in Bolivia, no economicai method has yet been found Íor Nigeria, Belgian Congo, Cornwall (E’ngland), leaching out tin so alloyed. Therefore, the ?rst Brittany (‘France) , China, and Indo-China. This object of the roast of this ínvention is to reduce type of ore is dif?cult to concentrate. The l0sses the tin compound while not reducing the iron, in the concentration tailings are alWays very high, 230 so that the reduced tin compound is made avail especially in the slime fraction where grades of 2.5% Sn and. over are not rare.’ The sulphîdes are often ?oated out of the crude ore, but even so the tailings and concentrates stil1 contain more or less base-metal sulphides. In the smelt ing plant, the concentrates have to be roasted, “ sometimes recleaned by‘ gravity methods, and leached with hydroc‘nloric acid in order to remove the impurities (excess of iron) prior to the smelt ing operation proper. The object of this invention is to recover the tin lost in these concentrator tailíngs, or even to treat directly the refractory ores as well as any suitable mill product. ‘ ’ Many attempts have been made to leach the tin directly from its ores or Írom concentrates but these have all failed due to the fact that cassiterite is completely insoluble in all aqueous solutions. (Mantel’l, “Tin” (1929) A. C. S. Mono graph Series, pp. 131-138.) 'I'his invention pro poses to make use of severa1 important and. con joint stages of treatment of which the ?rst is a reducing sulphidizing raast which results, under cr‘itical conditions, in the formation of a solu ble tin compound. The secohd stage comprises cooli‘ng the roasted material under non‘oxidiz- ‘ able Íor subsequent leaching. T0 that end this invention proposes to use a sulphidizing atmos phere whereby the tin oxide of the ore or ore ma terìa1 is converted Írom tin oxide principally to stannous sulphide (SnS) which is soluble in a subsequent leaching process. What is perhaps of eq1lal importance, is that the iron and certain other elements are held as, or are convertedto, sulphides, trom which combination they do not so readily reduce to the metallic state. The requirements of this suiphidizing roast are that ?rst it must be carried out in an atmosphere of reducing gas such as hydrogen and enough sulphur should be present to convert the tin and possible interfering elements to their sulphides. The sulphur needed is preferably evolved in situ, although it may be added as a gas if desired. ‘The temperature of the Toast is important, be cause if too 10W, the requisite ‘reactions do not take place, while if too high, tin may be lost through volatilization. Suiî1cient time must be allowed to convert the major part of the tin into the sulphidcè, but too long a reaction time should be avoided due to the danger of Íormation of metallic iron or other metal which would alloy with any metallic tin which may be formed dur ing the process. This is especially true when the sulphur supply is marginal or inadequate. It is 4 as tailings, is submitted, properly subdivided as‘ to particle size, to the treatment as summarized above, and if the roasted products are leached in practically impcssible and is also unnecessary to entirely prevent the r‘ormation of metallic tin a waak sodium hydroxide solution, the curve of recovered ‚tin plotted as a function of the roast as long as the alloying metals are prevented Írom ing temperature for a speci?c time interval wil] show a maximum indicating a critical tempera ture range, which curve falls away and also shows that undesirable reactions result if the tempera ture is too high. As already stated, the tin-hearing 10de ores al ways contain sulphides, and mainly pyrite. appearing. _ Thus the requirements of the roast include the presence of a proper amount of sulphur and a reducing gas as well as temperatures within a critical range and a certain control of the time interval of heating. The ore or mi1l product to be so roasted should be properly sub-divided Pyrite (FeS2) decomposes into pyr1‘hotìte (FeSw) ‘as to particle size, which should preferably not at a temperature a'oove 575° C. The reaction is be in excess of 65 mesh and in general the smaller 15 very rapid at 665° C. In the presence of hydro« the particle size the better if the process is to be gen, .however, the decomposition of pyrite starts e?ìcient. at a lower temperature. Thus, the sulphur for The temperature range should be a‘oove 55€)“ the reaction is furnished in Bolivian tin ore by C. and below 900° C.‚ with optimum between 650° the alteration of pyrite to pyrrhotite which pyrite C. and 830° 0. The amount of sulphur present 20 or its sulphur-hearing equivalent is added to the should be substantially over the theoretical chem ore to supply any lack of natural constituent ical requirements thereof to accomplish the re . frorn which sulphur vvill evo1ve in situ, and the action and the amount of hydrogen or other series of reactions involved may be represented strongly reducing gas should always be su?î by the follovving equations: cient to assure the reducing action. As to the cooling stage, the roasted, treated and converted. tin-hearing material should be cooled under nomoxidizing conditions. A variety of cooling methods can be used so long as the reduced constituents of the ore are not permitted 30 to have any substantial reversion due to oxida tion. As to the leaching stage, the essential charac teristic of this treatment is that the cooled, re duced and sulphidized ore material is subjected to the tin sulphicle leaching action of a caustic alkali such as caustic soda (NaOH) . If the reduction is performed with hydrogen in the presence of pyrite or other sulphur evolving constituent from which sulphur is volatilized at a temperature lower than the temperature of the main reaction, the equation may be written in summarized form as Íollows: The reason for this is that the tin sulphide and metallic tin The use of a large excess of pyrite is objectiona bie, so its use must be carefully controlled. rials, including the iron and copper compounds, 40 These reactions occur only abov_e 550° C. Above arenot. So the purpose of the leaching stage is 900° C. the tin sulphide volatilizes badly. Stan to dissolve selectively the tin compound out of the nous sulphide, as already stated, is soluble in ore material, whereupon the tin compounds in alkalis provided some oxidizing compound such solution are readily separable from the solid as air be present. Stannous su1phide is further gangue residue. 45 slowly reduced by the hydrogen to metallic tin As to the precipitation stage, the tin solution is SIIS’+H2=S?+H2S. Pure metallic tin is also subjectecl to the action of a precipitation agent soluble in sodium hydroxide in the presence of an such as calcium, barium, strontium or magne oxidizing agent. Hovvever, in the presence of re’ are soluble in alkali whereas the gangue mate sium sa1t but preferably lime. Here, however, a requisite is that precipitation shall take place in a relatively high concentration of already pre cipitated soiid phase of calcium-tin compounds, probabiy calcium orthostannate. Uniess this precaution is used, a much larger amount of 1ime is needed, indeeol as much as 30€) percent of theo duced metals (Cu, Bi, Fe, etc.) the metallic tin wil] form al1oys which are insoluble in alkalis. These reactiong are not separate as in the sys tematic description above, but take place simul taneously at different rates. In fact, just above the reaction temperature, SnSz is possibly formed besides SnS and some Sn. Moreover, the condi retical and then the precipitate yielded is 10W in tions in the furnace are also a function of the grade, very ?nely divided ancl dif?cult of separa time. At the beginning, there is an excess of tion. On the other hand, if the precipitation is sulphur present, and the sulphidizing reaction is carried out in the presence of a high concentra at its maximum; but when most of the sulphur tion or pulp blanket of already precipitated stan 60 has been evolved and has reacted or escaped as nate, the precipitated particles are coarse crys hydrogen sulphide, this atmosphere becomes tals with a rapid settling rate and the solution is more and more reducing. cornpletely stripped of tin with only the theoret If the sulphur available is marginal or inade ical quantity of lîme. 'I‘hîs means that they not quate, it is still possible to obtain fair results by only wil1 settle quickly in the mother liquor, but are readily ?iterable therefrom. The separated mother liquor or ?ltrate which is a harren solu tion, is then available for recirculation and reuse as a solvent in the leaching stage. After ?ltration ancl drying the stannate is pref erably calcined to drive out the combined water, forming anhydrous calcium orthostannate, since in this form the product may be stripped more eeonomically. . ‘ If a tin ore or a tin-hearing mill product such_ carefully controlling the degree of reductîon more particularly as exempli?ed by the time. By hold ing the time of contact at the reaction tempera ture to a minimum which may be as 10W as 10 minutes or so, a reasonable recovery may often be obtained, whereas langer times of 30 minutes may result in drastic lowering of the extraction olctainable with the NaOH solution. A possible explanation of this behavior is as follows; At 650° or 700° pyrite readily breaks down into pyrrhotite, FeSm, with the rest of the 2,413,762 5 sulphur being liberated as S2 or converted into I-I2S. These compounds carry out the other re actions indicated and the rest is carried away. If then all of the íron is in the form of pyrrhotite and the tin in the form of 5118, we have an ideal setup. IÎ more reducing gas is passed over the charge metallic tin may form, bui; metallic íron is prevented from Íorming for a long period be cause the FeS1.2 must be substantially converted to FeS 6 tains in the plant, but in special cases intentîonal temperature adjustment may be indicated by simple tests. The best means of recovering the tin is a‘ pre cipitation procedure which makes use of the ad. dition of lime. The tin is precipitated as hy— drated calcium orthostannate, containing alu mina and calcium carbonate as the principal im FGS1.2+Ü.ZH2*FGS+ÜZH2S purities. The precipitation of the Sn should preferably be carried out in the presence of pre viously precipitated Sn, i. e.‚ of a precipitate of calcium stannate produced in a Íormer cycle, for before any l5'eS is reduceol to metallic íron. It otherwise a large excess of lime would be neces takes a large excess of H2 to so reduce the FeSr.z sary, in order to strip the solutionof its Sn. If the theoretical amount of linie be added to the solution. it has been found that not more than 2Û% of the Sn is recovered, and it was neces sary to go to 300% of theoretical to e?îectively to FeS: of the order of 106-1000 mols of H2 per atom of sulphur removed, depencling on the tem perature. If only a small amount of this buffer ing pyrrhotite is present, practical considerations may cause enough hydrogen to be used to over come the margin of safety which the pyrrhotite affords and cause metallic íron to be iormed: precipitate the tin. But, iÍ the lime loe added to the solution in the presence of previously pre cipitated Sn, the calculated amount on the basis of CaSnO3 reduces the tin content of the solution which will then quickly alloy with any tin it to a very 10W value. As NazC0z builds up in the solutions through excess of CO2, a certain amount comes in contact with and tie this tin up as an of lime is used in precipitatìng,r CaC0s. alkali insoluble particle. Ïore, a slight excess of lime over tin is actually There If a hydrocarbon gas is ’used to provide the re used in practice. clucing atmosphere, the presence of steam as a Effecting the precipitation in the presence of a catalyst for the cracking of the hydrocarbon is 2%;% suspension of previous precipitates from a alrnost a requirement in order to yie1d high re 30 concentration of 10 to 12 grams of tin per liter of coveries. From the roasting furnace there es solution has given excellent results. Such a step capes volatilized S, H2S, ASH3, SìoSs, etc. In some seems to scrub the solution of its supersaturation cases it may be advantageous to remove the stan and lorings the solution into equilibrium. The nite prior to the reducing treatment, by flotation, stannate precipitate is sandy, it settles to a 10W acid leaching, or chlorine leach, as this mineral density of say 64% solids, anci ?lters easily, directly Íorms an alkali-insoluble compound. Since no excess lirne is used, the mother liquor The next step is the cooling, Which should be can be re-used without carbonation as it is al done to a temperature below the melting point of reacly substantially free of lime. However, if re tin (230° C.) and under careful control so that g‘eneration is necessary, it can be done by the use the ore is cooled and passed into the leaching so« 40 of sodium carbonate or carbon dioxide. lution in such a way as to prevent re-oxidation of A very satisfactory apparatus fcr accomplish the reduced ore. The leaching solution is best an ‚ing this precipitation step, is a machine made alkali, like sodium sulphide, potassium sulphide, and sold by The Dorr Company, In’c., of New potassium hydroxide, or sodium hydroxide, and York, as shown in United States of America Pat should not contain calcium ions (soft’water). ‚‚~ ent No. 2,259,221‚ to Darìoy, Roberts and Weber. From an econornical point of view, NaO-ï-I seems in this apparatus, the precipitant is added to the to be the best suited dissolvent. The ore is agi lower part of the machine wherein flocculation tatecl With a weak solution of sodium hydroxide takes place in a zone contaìning‘ a sludge blanket. (containing‘ an excess of reagent, say about ?ve The sludge bianket rises to a level above a sludge times the amount of tin to be leached)_ for a pe .5 o collecting pocket frorn which the precipitate is riod of time up to the order of 25 hours. A coun drawn oí’f, while clari?ed effluent overlies the ter-ourrent decantation system is advantageous. sludge blanket and over?ows from the upper sec The dissolution of tin compouncls in the caus tion of the machine. ‘The hydrated calcium or tic solution requires the presence of oxygen to thostannate precipitate, which runs about 41% convert the tin to the stannic condition in which (u) Sn, is then dewatered such as by ?ltering. It state it is soluble in caustic. This oxygen can be contains 5 molecules of crystallization water, ’ supplied by contacting the solution with atmos which are expelled at about 5Gil° C. or below pheric air. I-Iowever, if air loe also used for agi« yielding a calcined product with more than 50% tation purposes, care should be exereised to pre Sn. The latter may be direc-tiy smelted to re vent the formation of appreciable amounts of so cover the tin, or may be treated in various ways to arrive at the ?nal metallic tin. dium carbonate. O-ther methods of recovering the dissolVed tin The chemical reactions of the dissolution are as values from the alkaline leaching medium are follows: possible and in certain cases may be economically 6:3 more desirable. One of these is to carbonate the alkaline solution with a CÜ2 containing gas in the presence of an electrolyte such as NaCl if the Solutì0ns are rather dilute. In this case the" tin The strength of sodium hydroxide to be used is precipitated as hydrated stai1nio oxide which is not important, but the total amount of NaOI-I may be separated, dried and calcined to yield a present should be high enough to provide oom‘ high-grande product. »01’’ 892 may be used asÌ-a plete dissolu-tion in a reasonalole time. Seven to precipitating agent. — The remaining solution'is eight per cent by weight of NaOI-I on the basis of ’ regenerated with a controllecl amountoi lime. the calcined ore (containing 2% Sn) gave excel Direct ele‘ctrolysis of the solution is alsoa possi-ï lent results. The dissolution wil1 ordinarily be conducted atpwhatever temperature normally ob bility with the’productioh of electrolyti‘c‘ï metal‚l 2,413,762 7 which may be then melted down and cast into bars. The tin may also be precipitated from so lution in the metallic state by means of zinc dust, preferably after deaerating. In this case, zinc hydroxide will also be separate and may be recov ered separately. ‘ Example 1 Fine s1ime tailings from the 'I'elamayu (Ara mayo C‘ompany, Bolivia), containing 2.49% Sn (of which 10% as stannite and. the rest as cassi terite) as well as pyrites and other metallic sul phides, were heated in a rotary kiln to 799° C. The heating perìod was 22 minutes in an atmos 8 sho‘wn in the diagram inéludes an agitator. It is contemplated that this leaching station will con tain suitable agitators, together with a succession of thickeners for practicing what is commonly known as counter-current decantation or coun ter-current leaching and decantation. The reac tion product is subjectecl in the leaching station l9 ‘to action thereon by an alkaline leaching sol vent, such as sodium hydroxide, and in the leach 10 ing station the tin goes into solution. The solu tion pregnant with the dissolved tin passes as ef fluent from the leaching station I9 through pipe 20 into a, precîpitation station 2I which is housed within a machine to which there is also supplied phere of hydrogen and water vapor, provided by 15 through pipe 22 a precipitating agent, such as the saturation of the reducing gas in water heated to hydroxides, oxides or salts of calcium, barium, 51° C. The kiln was rapidly cooled and the ore strontium, magnesium, or the like. ieached at a dilution of 5:1 with an 0.8% sodium The precipitation station or machine 2I, in the hydroxide solution. After 20 hours, 70.04% of the tin contained in the ore was extracted. A 20 preferrecl form is described in the aiorementioned patent, and includes within its tank an assembly second leach with a fresh solution containing 23 of rotatable ?occulating paddles or blades, and 0.8% NaOH gave an additional recovery of 8.5%. a sludge receiving and collecting pocket 24 from Thus, the total recovery was 78.54 % . which sludge is withdrawn through a pipe 25 and Example 2 conducted to a ?lter 26 or other dewatering appa Composite taílings from the Telamayo Mine 25 ratus. The precip‘itating agent precipítates or strips the tin out from the pregnant solution that (Aramayo Company, Bolivia), containing 1.86% enters the machine or tank 2I, and this solid pre Sn and 4.5% S, prìncipally as pyrite, was batch cipitate is removed from the tank in the form of roasted in a rotary kiln to 7 99° C_ for 25 minutes in sludge that is dewatered on the ?lter 25. Filter a current of hydrogen, and cooled in the same gas. The calcine containing 1.94 Sn, was leached with 30 cake from ?lter 26 passes to a, calciner station 21 from which calcium stannate 28 emerges suitable 20 gpl. NaOI-I solutions in a. cyclic test procedure for smelting or other treatment for recovering its simulating counter-current leaching and decanta tin content. tion, The pregnant solutions were precipitated So‘ much for the direct steps. But the process with lime in the presence of previously precipi 35 of this invention can be carried out continuously tated material and the harren solutions reused. and cyclically. 'I‘o that end. the mud or pulp that The test was continued for 10 cycles and an aver forms a sediment in the leaehing station or thick age extraction of 83% and 9, tin recovery as ener 19 is raked to discharge whereupon it is con CaSn0s of around 80% was obtained. The ?nal ducted through pipe 29 to a, dewatering and wash product aiter calcíning containea 51.4% Sn. 40 ing or ?lter station 312. From thîs ?lter, there are Smelting tests on the latter product with the ad two discharges, namely, one by which the ?lter dition of SÌO2 and F8203 gave high reco-veries of ‚ cake of mud or pulp from which the solution has metallic tin. been substantially removed passes to discard or Drawings acco-mpany thîs speci?catio-n for aid waste as at 31, and the other is the alkali solu ing in an understanding of the invention, in which tion ?ltrate or leaching liquor containing some Fig. 1 shows a graph based upon tests showing tin which passes through pipe 32 on its way back the critical temperature factor for two different into the leaching station I9. Supernatant e?‘lu time intervals (combined of course with the ent from the precipitation station 2| over?ows proper reducing and sulphidizing conditions) ‚ from the upper section thereof in the form of an while Fig. 2 is a. diagrammatic ?owsheet to indi 50 alkali solution substantially harren of tin in solu cate in general the sequence of machines or appa tion, and is conducted through pipe 33 back to ratus usable to p1‘actice the invention on a com—. the leaching station IS]. mercial scale. Assuming now that an operator has a plant In Fig. 2 of the drawings, I I indicates a furnace ready to start up and he wants to determine the or kiln in which the ore is subjected to a reducing 55 quantities of materials to use; the non-carbonate sulphidizing roast, to which furnace the ore 12 is tin ore or material is analyzed to determine the fed together with a suitable reducing gas l3. The amount of volatilizable sulphur needed in the furnace may also have fed to it pyrite 15 or other kiln. The ore usually comprises tin compounds, source of sulphur, but ìf pyrite be used, an ex divided into a major portion of tin oxide and a cess of pyrite is detrimental in the leaching, so 60 minor portion of tin-sulphide complex. Other the quantity used should be carefully controlled. constituents are iron oxides and sulphides as well In the event that a tin ore to be treated has a as silicates of various kinds. The tin, sulphur su?îcient sulphur content naturally in it‚ of course pyrite need not be added. From the furnace H, gaseous products E6 of the reaction are emitted in the form mainly of sulphur and hydrogen-sul phide but these will also contain other products such as ASH3, SbS3, and the iike. The hot solid reaction products are discharged from the fur nace | I into a cooling apparatus or station i'l' so that they will be cooled quickly in a manner to prevent or minìmize their re-oxidation. From the cooling apparatus 11 the solid reaction product passes through pipe Iâ and is subjected to a leach ing treatment in a leaching station ’i9‚ which as and reactible iron content is determined analyti cally. Then there is calcuiated the quantity of sulphur needed chemically to be equivalent to the tin and iron. In such a calculation it is best to ?gure the tin as SnS and the iron as FGS1.2. IÍ insu?icient S is shown by the calculation, pyrite is added to make up the difference. It is appar ent that there is available for tin and oxide iron compounds 0.8 atom of sulphur per mol of FeS2. some excess of sulphur above that so calculated makes the reduction less critical as to timing but too much is undesirable. This then comprises the burden of the kiln, and hydrogen or hydrogen 9 containing reducing gas is continually passed through the kîln during the heat treatment stage when the tin oxide is converted to stannous sul phide while the sulphur minimizes reduction of iron compounds to metallic iron. As the resulting tin compounds are soluble in alkalis while the iron compounds are insoluble, an alkali leach causes the dissolving of the tin compounds, leaving the iron compounds as solid residue. Therefore, con joint requirements for the practice of this inven tion‚are exempli?ed, for instance, in the sulphidiz ing reducing roast treated at a reaction tempera ture within a speci?c range followed by alkali leaching, followed by a special precipitation stage 10 2. The process according to claim 1, character ized in that the period during which said ore ma terial is heated and the temperature at which it is heated are such that the formation of other metals adapted to alloy with tin is rninimizecl. 3. The process accordìng to claim 1, character ized in that the solution pregnant with tin is reacted with a metal oxide selected from the group consisting of calcium oxide and barium oxide to precipitate a tîï1 cümpollnd 4. The process according to claim 1, in that the solution pregnant with tin is reacted with a metal oxide selected from the group consisting of cal cium oxide and barium oxide to precipitate a tin compound, regenerating‘ at the Same time the if unusual ef?ciency is to be realized therein, and 15 alkaline metal hydroxide solution, separating the then calcination of the separate precipitated tin precipitate and the solution, and re-using the said compound or other tin recovery step, with the‘ solution to leach the cooled reduced ore. leaching operation being cyclic. 5. The process according to claim 1, character We claim: ized in that the solution pregnant with tin is 20 1. The process of treating tin ore, which com reacted with calcium oxide te precipitate calcium príses heating the ore during a controlled period stannate, the precipitation step being conducted of time above 550° and below 800° C. in a strongly in the presence of an eXcess of previously precipi reducing atmosphere and in the presence of tated calcium stannate. enough sulphur so that stannous sulphide and 6. The process according to claim 1, character metallic tin are Íormed and so that the iron in ized in that the solution pregnant with tin is sub the ?nal product is in the ‚form of its sulphide jected to neutralization by an acid reagent chosen stable at the reaction temperature, and under from the group consisting of CO2 ‘and S02 f0î‘ such conditions to insure that tin sulphide is not precipitating a tin ccmpound out of the pregnant volatilized; cooling said‘treated material under. separating the precipitate and solutîon, non-oxidizing conditions; leaching out; the treat 30 solution, and regeneratìng the solution with a controlled ed material with a solution of an alkaline metal hydroxide in the presence of enough oxygen to transform (1) substantially completely metallic tin into an alkali metal stannate salt, and (2) stannous sulphide into an alkali metal stannate 35 salt and. a salt of alkali metal, tin and sulphur; and precipitating a tin compound Írom tl_1_is solu tion. \ amount of lime. — GREGOIRE GU'I‘ZEIT. ELLIOTT J. ROBERTS. ROBERT B. THOMPSON.