Patented Sept. 2, 1947 2,426,914

UNITED STATES PATENT OFFICE 2,426,914 PURIFICATION OF HYDROCHLORIC ACID Walter S. Allen, Flushing, N.Y., assignor to Gen eral Chemical Company, New York, N. Y., a corporation of New York l No Drawing. Application February 6, 1945, Serial No. 576,510 Claims, (C. 23-154)

This invention relates to manufacture of so 2 called chemically pure hydrochloric acid. chloric acid gas containing the same are utiliza In production of “chemically pure’ hydrochlo tion of a suitable catalyst, the presence in the c acid, a major problem encountered is the re gas stream undergoing treatment of an adequate duction of the arsenic impurity content. While amount of CO and water vapor, and tempera chemically pure hydrochloric acid has been made ture conditions during the catalytic contacting directly from HCl gas produced by reaction of operation of not less than 350 F. Sulfuric acid and sodium chloride, the more com One commercial utilization of the principles mon and well known method is by redistillation of the invention is in connection with the re of commercial hydrochloric acid. In the distill distillation of commercial muriatic acid. Such lation of the commercial acid, some impurities 10 principles are applicable whether the primary remain largely in the still bottoms, while free purpose of the operation is to remove free chlo chlorine, SO2 and arsenical impurities mostly rine from a muriatic acid which already meets pass over with the distillate and appear in the “chemically pure' trade requirements as to low condensed product. It is known that, when dis arsenic content, or whether the object of redis tillation is carried out in the presence of free 5 tillation is to lower the arsenic content of a COm chlorine, the arsenic and SO2 contents in the mercial muriatic acid. Overhead of the still may be reduced consider In the case of purification of aqueous hydro ably, and a good deal of the arsenic held back in chloric acid which contains an objectionable the still bottoms. But in this situation, where amount of free chlorine but does not require distillation is effected in the presence of an ap 20 treatment primarily for arsenic impurity reduc preciable excess of free chlorine, much of such tion, such acid is run into a suitable vessel, dis free chlorine is carried over with the hydrochlo tilled, and there is formed a gas-vapor mixture ric acid vapor. The presence of free chlorine containing hydrochloric acid gas, free chlorine in the product acid is substantially objectionable, 25 and water vapor. Aqueous hydrochloric acids and to meet this requirement it has been pro include such an amount of water that the gas p0Sed to paSS hydrochloric acid contaminated vapor mixture formed by distillation always con With free chlorine thru activated carbon. In this tains more water vapor than is needed to support procedure, apparently acting as an adsorbent, catalytic reduction of free chlorine to HC1. the activated carbon collects the chlorine impu 30 Hence in usual practice, the gas-vapor mixture rity, with the result that, as operation progresses, evolved by distillation of aqueous hydrochloric activity of the carbon continually decreases be acid inherently contains more than enough water cause of increased chlorine content, and frequent vapor needed in the present process. replacement of the activated carbon, a relatively The carbon monoxide employed may be from expensive material, is required. Another defi any suitable source, and may be relatively pure ciency is that, to operate to best advantage, this 35 CO, or the CO of a gas mixture containing hy method requires the maintenance of the activat drogen and non-deleterious or inert diluents. ed carbon at elevated temperatures within rel The reducing material used may be introduced atively narrow limits. into the HCl gas-chlorine-water vapor mixture The major object of this invention lies in pro-. 40 at any suitable point ahead of a chamber con vision of a process for removing free chlorine taining the catalytic material. The amount of from hydrochloric acid gas contaminated with reducing material should be at least sufficient to this impurity. effect reduction of the free chlorine to HCl, al though it is preferred to use a substantial ex It has been found that free chlorine contained cess. An excess of CO in the catalytic reducing in hydrochloric acid gas may be removed by . zone may be maintained readily by supplying catalytically reacting such chlorine with car to the process such an amount of reducing agent bon monoxide in the presence of water vapor that the tail gas of the final apparatus unit in and at temperature not less than 350 F. It is which hydrochloric acid is condensed always has thought that the Overal removal of free chlorine an appreciable CO content. To insure substan is effected by reducing the free chlorine to hy- . 50 tially complete removal of free chlorine, excesses drochloric acid and oxidizing the carbon mon of Water vapor and reducing agent are needed, Oxide to carbon dioxide in accordance with the and one of the advantages of the instant proc equation, CO-Cl2--H2O->CO2--2HCl, ess is that the presence of excesses of water va Important operating factors of the instant por and reducing agents indicated does not re process for removing free chlorine from hydro 55 quire introduction of any reactants or reaction 2,426,914 4. 3 also in the manufacture of high grade hydro products which further contaminate the prod chloric acid by processes in which the product is uct, water being one of the necessary constitu made directly from hydrochloric acid gas formed ents of the final condensed product, and hydro by reaction of sulfuric acid and sodium chloride, gen, CO, and the CO2 formed by oxidation of which gas, because of the particular conditions CO, all being insoluble in aqueous hydrochloric of production, may contain free chlorine. In acid and removable from the system in the tail such an operation the hydrochloric acid gas leav 9.a.S.The catalysts employed are solid granular ma ing the sulfuric acid-sodium chloride reaction terials which are non-volatile at the temperature furnace may be passed first thru a bed of mate of operation and are free of iron and arsenic. It rial such as suitably-sized coke to effect removal has been found that activated carbon, purified of entrained solid impurities and Sulufric acid coke, purified Wood charcoal, and silica gel effec mist from the gas stream. Arsenical impurities tively catalyze the free chlorine reduction reac may then be removed in any way known in the tion. Examples of commercial types of activated art, for example as shown in Adamson U. S. P. carbon which may be employed are “Norite' car 5 1,936,078 of November 21, 1933. Usually such a bon and “Columbia' carbon. The granular forms gas stream contains more than enough water va of these carbons, as distinguished from the poW por to support the hereindisclosed catalytic re dered forms are preferred, the common "commer in Oval of free chlorine. The reducing agent may cial size of about thru 3 on 14 mesh being par be added to the gas stream at any suitable point, ticularly suitable. The coke may be such as that 20 and thereafter practice of the invention is the prepared by igniting metallurgical coke at a red same as described. - heat in the absence of air, boiling with hydro In the following three examples, the primary chloric acid, Washing and drying, the purpose of purpose.of operation was to remove free chlorine this treatment-being to get rid of volatile matter, from commercial muriatic acid, purification as to hydrogen sulfide and iron, all of which Would con 25 lowering of the arsenic content being secondary taminate the final product. Wood charcoal, if and incidental. The impure acids Contained used, may be purified in the same manner, Gran enough chlorine so that the gas-vapor mixture ular commercial silica, gel of sizes...ranging about formed by distillation contained free chlorine. thru 8 on 20 mesh may be used. It has been Eacomple it found that these materials, when used under the 30 Crude muriatic acid, containing by Weight 32% hereindescribed conditions, function as catalysts, HCl, .017% free chlorine, and 0.00003% As, was are not consumed, do not become contaminated, continuously run into a still, and distilled at ten and do not require revivification or replacement. perature of about 225 F. The hydrochloric acid Temperatures in the catalyst chamber are kept gas-free chlorine-water vapor mixture was passed at not less than 350 F., and usually at 400 F. and 35 upwardly thru a 6-inch deep bed of thru 8 on 14 higher. When using purified coke and Wood mesh. “Norite,’ maintained by external heating charcoal, and silica gel as catalysts, better results at temperature of about 400 F. The mixture are obtained when operating at temperatures of leaving the catalyst chamber was cooled to con 425-450° F. While temperatures as high as 470 dense hydrochloric acid, and the nearly colorless F. have been employed, higher temperatures af 40 product analyzed by weight 35% HCl, 0.0002% ford no substantial operating advantage. Pro free chlorine, and 0.00000005% As. At the end vided that the indicated minimum temperature of this run, covering a period of 2 hours, the ac is held in the catalyst chamber, temperatures may tivated carbon showed no appreciable weight loss. vary over a substantial range and need not be 45 In a parallel operation, substantially pure carbon closely controlled. The stated temperatures may monoxide was introduced into the hydrochloric be maintained by any suitable means, e. g.: ex acid gas-free chlorine-Water vapor mixture gen ternal heating - of the catalyst chamber, or by erated in the still, the amount of CO being such heat-exchange heating of incoming reducing that the tail gas of the condenser in which hydro agent, or both. . . 50 chloric acid was liquefied contained some CO. When, as is most often the case, the object of The condensed product obtained was water white, redistillation is to lower arsenic content, if the and analyzed by weight 35% HCl, 0.00000005% commercial muriatic acid to be treated does not As, and no free chlorine. On completion of this contain as an impurity a sufficient. quantity of run, covering a period of 2 hours, the activated free chlorine, redistillation of the crude acid is 55 carbon showed substantially no loss of weight carried out in the presence of enough oxidizing and contained no appreciable amount of chlorine. agent to insure the presence of an excess of free There was no detectable consumption of acti chlorine in the still. Arsenical impurities are held back in the still bottoms evidently because free wated carbon, thus indicating that its action is chlorine oxidizes arsenical impurities to less vola catalytic. tile form. While free chlorine itself may be in 60 Eacample 2 troduced into the distillation operation, ordi Crude muriatic acid, containing by weight 32% narily the free chlorine is provided by adding to HCl, 0.0016% free chlorine, and 0.00003% As, was the impure-acid prior to distillation a suitable run continuously into a still, and distilled at tem amount of oxidizing agent, such as alkali metal perature of about 225 F. The hydrochloric acid chlorates, permanganates and hydrogen perOX 65 gas-free chlorine-water vapor mixture was passed ide. In any given operation, the amount of oxi upwardly thru a 6 inch deep bed of thru 5 on 10 dizing agent should be enough to insure the pres mesh metallurgical coke purified as previously ence of readily detectable free chlorine in the hy described, maintained by external heating attem drochloric acid-gas mixture generated by distilla 70 perature of 425-475° F. The mixture leaving the tion. The reducing agent used is then added to catalyst chamber was cooled to condense hydro the hydrochloric acid gas-free chlorine-water va chloric acid, and the slightly yellowish product por mixture produced by the distillation step, and analyzed by weight 35% HC and 0.0002-0.0006% chlorine is catalytically removed as already de free chlorine. This run covered a period of 2 scribed. - - 75 hours. In a similar operation, substantially pure The principles of the invention may be utilized 2,426,914 5 carbon monoxide was introduced into the hy 6 drochilloric acid gas-free chlorine-Water Vapor Eacample 5 mixture generated in the still, the amount of CO The crude muriatic acid used contained by being such that the tail gas of the condenser in weight 32% HCl, 0.00003% As and no free chlo which hydrochloric acid was liquefied contained rine. To this material, there was added enough some CO. The condensed product analyzed by KClO3 to give a chlorine content of 0.0016%. weight 35%, HCl and less than 0.00000005% As, The liquor was run continuously into a still and was water white and contained no trace of free distilled at temperature of about 225 F. The chlorine. On completion of this run, covering hydrochloric acid-water vapor mixture contain a period of 1.5 hours, the coke showed no sub 0. ing some free chlorine was passed upwardly thru stantial Weight, loss and contained no appre a 6 inch deep bed of thru 5 on 10 mesh purified ciable amount of chlorine. There was no de coke maintained by external heating attempera tectable coke consumption. ture of about 450° F. The mixture leaving the Eacample 3 catalyst chamber was cooled to condense hydro 5 chloric acid and the yellow colored product ana The crude muriatic acid used contained by lyzed by weight 35% HCl, 0.00005% free chlorine weight 32%, HCl, 0.0018% free chlorine, and and 0.00000005% As. The run covered 2.5 hours. 0.0003% As. The impure acid was run contin In a similar operation, substantially pure carbon uously into a still, and distilled at temperature monoxide was introduced into the hydrochloric of about 225° F. The hydrochloric acid gas-free 20 acid gas-free chlorine-water vapor mixture gen chlorine-Water vapor mixture was passed upward erated in the still, the amount of CO being such ly thru a 6 inch deep bed of thru 10 on 20 mesh that the total gas of the condenser in Which hy silica gel, maintained by external heating at drochloric acid was liquefied contained some CO. temperature of 450-470° F. The mixture leaving The condensed product obtained was Water White, the catalyst chamber was cooled to condense hy 25 and contained by weight 35% HCl, and drochloric acid, and the yellowish product ana 0.00000005% As, and no free chlorine. On com lyzed by weight 35% HC1 and 0.001% free chlo pletion of this run, covering a period of 1.5 hours, rine. This run covered a period of 1 hour. In the coke showed no substantial Weight loSS and a similar operation, substantially pure carbon contained no appreciable amount of chlorine. monoxide was introduced into the hydrochloric 30 I claim: acid gas-free chlorine-water vapor mixture gen 1. In the process of purifying hydrochloric acid erated in the still, the amount of CO being Such gas containing free chlorine as an impurity, the that the tail gas of the condenser in which hy step comprising catalytically reacting said chlo drochloric acid was liquefied contained some CO. icine With carbon monoxide in the presence of wa The condensed product was Water white, and con 35 ter vapor whereby said chlorine is reduced to tained by weight 35% HCl and no trace of free HC. chlorine. On completion of this run, covering 2. The process of purifying hydrochloric acid a period of 1 hour, the silica gel contained no gas containing free chlorine as an impurity which appreciable amount of chlorine. comprises passing said gas, together with Water In the following examples, chief object of op 40 vapor and carbon monoxide, in contact with a eration was to reduce the arsenical impurity con catalyst of the group consisting of activated car tent of crude muriatic acid which did not contain, bon, coke, wood charcoal and silica gel, and main as an impurity sufficient free chlorine to effect taining temperature not less than 350° F. during arsenical impurity content reduction in the dis Said contacting operation. tillation step. 45 3. The process of purifying hydrochloric acid Eacample 4 gas containing free chlorine as an impurity The crude muriatic acid used contained by which comprises passing said gas, together with weight 32% HCl, 0.00003% As and no free chlo Water vapor and carbon monoxide, in contact rine. To this material, there was added enough 50 with a catalyst of the group consisting of acti KClO3 to give a free chlorine content of 0.018%. wated carbon, coke, wood charcoal and silica, gel, The liquor was run continuously into a still and and maintaining temperature not less than 350 distilled at temperature of about 225 F. The F. during said contacting operation, the amount hydrochloric acid-Water Vapor mixture contain of Water vapor and reducing agent being in ex ing some free chlorine was passed upwardly thru 55 cess of that required to effect reduction of said a 6 inch deep bed of thru 8 on 14 mesh “Norite' free chlorine to HCl, carbon maintained by external heating at tem 4. The process of purifying hydrochloric acid perature of about 400 F. The mixture leaving gas containing free chlorine as an impurity which the catalyst chamber was cooled to condense hy comprises passing said gas, together with carbon drochloric acid and the slightly colored product monoxide and Water vapor, in contact with acti analyzed by Weight 35% HCl, 0.0002% free chlo 60 wated carbon while maintaining temperature not rine and 0.00000005% AS. This run covered a less than 350 F., the amount of carbon monoxide period of 3 hours. In a similar operation, sub and water vapor being at least Sufficient to effect stantially pure carbon monoxide was introduced reduction of Said free chlorine to HC1. into the hydrochloric acid gas-free chlorine-Wa 65 5. In the process of purifying aqueous hydro ter vapor mixture generated in the still, the chloric acid containing arsenical impurity by pro amount of CO being such that the total gas of cedure including distilling Said acid in the pres the condenser in which hydrochloric acid was ence of an oxidizing agent under conditions to liquefied contained some CO. The condensed form a gas-vapor mixture containing hydro product obtained was water white, and contained chloric acid gas, water vapor and free chlorine, by weight 35% HCl, and 0.00000005% As, and no 70 the improvement COmprising passing said mix free chlorine. On completion of this run, cover ture, together with carbon monoxide, in contact ing a period of 2 hours, the “Norite' showed no with a catalyst of the group consisting of acti Substantial Weight loss and contained no appre wated carbon, coke, Wood charcoal and silica, gel, ciable amount of chlorine. and maintaining temperature not less than 400 75 F. during said contacting operation, the amount 2,426,914. 7. of reducing agent being in excess of that required the improvement comprising catalytically react to effect reduction of said free chlorine to HCl, ing Said free chlorine with Carbon monoxide in 6. In the process of purifying hydrochloric acid the presence of said water vapor at temperature gas containing free chlorine as an impurity, the not less than 350 F. step comprising reacting said chlorine with car WALTER, S, ALLEN. bon monoxide: in the presence of water vapor and attemperature not less than 350 F., whereby said REFERENCES CTED chlorine is reduced to HCl, The following references are of record in the 7. In the process of purifying aqueous hydro 0. file of this patent: chloric acid containing arsenical impurity by pro UNITED STATES PATENTS cedure including distilling Said acid in the pres ence of an oxidizing agent under conditions to Number Name Date form a gas-vapor mixture containing hydro 2,213,544. Brown ------Sept. 3, 1940. chloric acid gas, water vapor and free chlorine, 1,220,411 Garner et al. ------Mar. 27, 1917