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Hydrochloric Acid & Sodium Sulphate Hydrochloric Acid and Sodium Sulphate Hydrochloric or muriatic acid is generally made by the action of sulphuric acid on common salt. It is a by-product of the Leblanc soda process, & in the early years of the industry was allowed lớn escape into the air, as the demand for it was small. But the nuisance caused by the acid fumes in the neighborhood of the alkali works became so great, that in England a very stringent law was enacted forbidding the soda makers khổng lồ allow more than 5 per cent of the gas to escape into the atmosphere. This made it necessary to absorb the acid fumes in water. The provisions of the present" Alkali Act" permit only 0.2 grain of hydrochloric acid per cubic foot of chimney gas lớn be discharged into the atmosphere. The Leblanc industry has declined in recent years, but there is an increased demand for hydrochloric acid, và at present this is one of the main products desired. Its chief use is for the generation of chlorine for the manufacture of bleaching powder; now nearly all soda makers also produce bleaching powder, and the profits derived from the latter have largely offset the decline in returns from soda-ash. Up lớn the present, no better method than the above has been devised for making this acid. The process may be represented by the equation: - 2 NaCI +H2S04 = Na2S04 +2 HCl. But as actually carried out it takes place in two stages, according khổng lồ the following reactions: - 1) NaCl +H2S04 = NaHS04 +HCl. 2) NaHS04 +NaCl = Na2S04 + HCl These reactions may be carried out by heating the mixture of salt and sulphuric acid either in an "open roaster," or in a muffle or "close roaster." These are both called "salt-cake furnaces." The mở cửa roaster (Fig. 34) consists of two parts, the cast-iron pan (A) và the reverberatory hearth (C). The salt và sulphuric acid (GO°Be., sp. Gr. 1.72) are put into the pan (A), & are moderately heated by a coke fire 011 the grate (E). The First reaction takes place at a comparatively low heat, và the hydrochloric acid vapors escape through the earthenware pipe (B). Then the fuse mass of sodium acid sulphate and undecomposed salt is raked up o the reverberatory hearth (C), where it is exposed khổng lồ the high temperature of the flame from (D). This completes the second reaction và a pasty mass of normal sodium sulphate is formed. The hydrochloric acid vapors, set free during the reaction, phối with the furnace gases from (D), and escape through the pipe (F) khổng lồ the absorbing apparatus. The furnace gases dilute the acid vapor so much, that a very concentrated solution of hydrochloric acid cannot be made with the open roaster; however, it yields acid strong enough for use in "Weldon"s chlorine process. Moreover the soot and dust from the furnace at (D) contaminate the acid, & may cause clogging in the passages & pipes of th absorption apparatus. The open roaster has the advantage over the close roaster, that it yields more sodium sulphate with smaller consumption of fuel. The crude sodium sulphate, called "saltcake," usually contains a little undecomposed salt và a slight excess of sulphuric acid The muffle or "close roaster" is used very generally on the continent of Europe, và yields a stronger và purer acid than (A) is built very much as in the open roaster, but is heated by the fUl"1lace gases from the grate (D). The acid vapors set free in the pan escape by the pipe (C) lớn the absorption apparatus. The muffle (B) is made of fire-clay or brick, and is heated by the flames from the grate (D). The mixture of acid sulphate và salt is raked from the pan (A) into the muffle (B), where it is heated lớn a reel heat, & the acid vapor liberated passes through the pipe (E) to the absorption apparatus. In this form of roaster, the soot and dust from the grate are kept away from the acid vapor. Also, a very concentrated acid vapor is obtained, which favors the formation of a concentrated solution of hydrochloric acid in the absorbers. But the muffles are expensive khổng lồ build, yield a smaller output of saltcake, and require more fuel than the xuất hiện roaster. Moreover, they very often crack, thus permitting acid vapors lớn escape into the flues & chimney, causing loss và creating a nuisance. It is customary khổng lồ maintain a slight pressure in the flues & chimney, so that if the muffle cracks, the flue gases force their way into it. This may cause a slight contamination of the acid, but no nuisance is created. Cheaper fuel may be used with these furnaces, but repairs are apt to lớn be expensive. The pan (A) in both furnaces is about 10 feet in diameter, 7 inches thick at the centre & 3 inches thick at the sides. After a charge is drawn, the pan is cooled somewhat before introducing another, for cold salt, coming in contact with the hot pan, might crack it. The sulphuric acid is generally heated lớn 1000 or 1300 C. For the same reason. During the second reaction, the charge is constantly stirred with a "rabble," a large hoe-shaped tool, khổng lồ prevent" crusting" or burning on khổng lồ the hearth or retort. The stirring is done by workmen, and as the work is very heavy, they are sometimes careless, and allow a crust to lớn form, which may crack the muffle. Consequently, many attempts have been made to lớn construct mechanical stirrers. "The Mactear furnace (Fig. 36) is the only one of these that has met with much success. This is a reverberatory furnace with a rotating hearth (A); in the centre of the hearth is a shallow pan (B) into which the mixture of salt và sulphuric acid is run in a slow, continuous stream. The mass overflows on lớn the hearth, where it is subjected to lớn the high heat of the flames from the grate (G); at the same time, it is mixed & pushed towards the edge of the hearth by stirrers (C), against which the charge strikes as the hearth revolves. The tốc độ is so regulated that the salt is all converted lớn sulphate by the time it reaches the edge of the hearth. There it falls into an annular trough (0, D), which carries the pasty mass out of the furnace. An apron attached khổng lồ the edge of the hearth (lips into this trough, so that the salt-cake forms a lute, và prevents the escape of the acid vapor into the space beneath the hearth. This only imperfectly protects the driving mechanism from the flame và acid vapors, và serious difficulties are consequently incurred in running .the furnace. Moreover, the acid vapors are much diluted with the fire gases, which renders their absorption difficult. Because of these disadvantages, some manufacturers who have tried mechanical appliances have abandoned them, và returned khổng lồ the hand-worked furnace. If a salt-cake không tính phí from iron is desired, lead pans instead of cast-iron ones are used. But these are easily overheated or injured. The hydrochloric acid vapor is absorbed in water, either by passing through tall towers (Fig. 37) filled with coke, over which water trickles; or in large earthenware Woulff bottles (bombonnes), provided with safety-tubes for back pressure, & with a coke tower at the over of the series. The purpose of the towel", which is fed by a spray of water, is lớn absorb any acid vapors which may pass uncondensed through the bombonnes. These are placed en cascade, t & joined by the side tubulatures, so that a stream of water or dilute acid from the tower will flow through them in a direction opposite to that in which the gas is moving. The Lunge-Rohrmann plate tower has been tried with some success as a substitute for the coke towel" and bombonnes, for hydrochloric acid absorption. The condensation of hydrochloric acid vapors is not so simple a process as it at first appears. The gases coming from the roasters are very hot, and must be cooled before they can be absorbed to form a strong acid. Moreover, with open roasters, there is a very large amount of inert gas present (nitrogen and carbon dioxide from the fire) which dilutes the acid vapors. Then, too, the vapors are not set không tính tiền regularly in any roaster, there being a rapid evolution during the progress of the first reaction, và a much slower liberation during the second. This may cause a temporary rush of vapors through the apparatus, so that they cannot be properly taken up by the water. The ordinary muriatic acid of trade is an aqueous solution of the acid vapor, having a specific gravity of about 1.20 & containing about 40 per cent by weight of dry hydrochloric acid vapor. It is impure, containing sulphuric acid, chlorine, iron chloride, arsenic, and, generally, lead and calcium chlorides. Its yellow color is partly due lớn organic matter, & sometimes to iron and miễn phí chlorine. To lớn remove arsenic và sulphuric acid, the acid is diluted to lớn 1.12 sp. Gr., and barium sulphide is added; a pure hydrochloric acid vapor is then driven out by distillation & absorbed in pure water. 01" a solution of stannous chloride in concentrated hydrochloric acid is added khổng lồ the crude acid, which latter must have a strength of at least 1.15 sp. Gr. A brown precipitate of arsenic with some tin separates and is removed by decantation. Sulphuric acid alone is removed by adding barium chloride and redistilling. To remove chlorine, the crude acid is digested with strips of copper for some hours. This precipitates arsenic, and the chlorine combines with the copper. The acid is then redistilled. Attempts to lớn recover hydrochloric acid from the waste liquors of the ammonia soda process have not proved very successful. The magnesium chloride mother-liquors from the potash salts of Stassfurt may be decomposed by distillation with steam, and a dilute hydrochloric acid obtained. MgCl2 +H20 = 2 HCl +MgO. But this has not proved a commercial success. The Hargreaves and Robinson process for the direct production of hydrochloric acid and sodium sulphate from salt, sulphur dioxide, water, & oxygen, is of some importance. The damp salt is pressed into blocks và dried; it is then charged into vertical cast iron retorts, a number of which are connected in a series. These are heated from without; the temperature of the reaction is from 4000 to 5500 C. The sulphur dioxide, steam, & ail" are made to lớn pass through all the retorts in succession, the hydrochloric acid being carried along with them. A slight excess of sulphur dioxide and steam is used lớn prevent the mutual reaction between the hydrochloric acid vapor và the oxygen, by which chlorine is phối free. The decomposition being slow, the gases must be kept in contact with the salt for a considerable length of time; a cylinder containing 40 tons of material requiring from 15 to đôi mươi days continuous action to secure complete conversion. "fhe process is an uninterrupted one; for as soon as no more sulphur dioxide is absorbed in a given cylinder, it is cut out from the series, the sodium sulphate removed, a new charge of salt blocks introduced, and the cylinder made the final one of the series; so that newly charged" salt is exposed lớn the most nearly exhausted sulphur fumes. The reaction representing the process appears quite simple: - 2 NaCl + 802 +H20 + 0 = Na2804 +2 HC!. But the mechanical difficulties encountered in working it were very great, and only within a very short time has the process met with any marked success. Sodium sulphate or salt-cake is most largely used in the production of soda by the Leblanc process. Large quantities are used for glass making, for ultramarine, in dyeing and coloring, & to some extent in medicine. For some kinds of glass the salt-cake must b free from iron, & consequently it is made in lead pans. Or the sulphate may be purified from iron và excess of acid by dissolving it in hot water, adding" milk of lime," and then stirring into it solution of bleaching powder. The iron is precipitated as hydroxid and settles on standing. By evaporation, crystals of Glauber"s salt (Na2S04 .10 H20) are obtained. But generally the purified solution is rapidly evaporated khổng lồ dryness, & the hàng hóa is calcined t remove all the water. Organic Chemistry for the industry Inorganic Chemistry for the industry Lixiviation Levigation Evaporation Distillation Sublimation Filtration Crystallization Calcination Refrigeration density Fuels Liquid fuels Gaseous fuels Water Sulphur Sulphur Derivatives Sulphuric Acid Sulphuric acid burners Fuming Sulphuric acid Salt Hydrochloric Acid Soda Industry Caustic Soda Treatment of tank Ammonia Soda Cryolite Soda process Chlorine Industry Electrolytic Chlorine Hypochlorites Chlorates Nitric Acid Nitrates Ammonia Potash Industry Fertilizers Lime, Cement Cement Glass Ceramic Industries Pigments Bromine Iodine Phosphorus Boric Acid Arsenic Compounds Peroxides Oxygen Sulphates Alum