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, and in the early years of the industry was allowed to 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 to 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 to be discharged into the atmosphere.
The Leblanc industry has declined in recent years, but there is an increased demand for hydrochloric acid, and 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 to 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 to 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 open roaster (Fig. 34) consists of two parts, the cast-iron
pan (A) and the reverberatory hearth (C). The salt and sulphuric acid (GO°Be., sp. gr. 1.72) are put into the pan (A), and are moderately heated by a coke fire 011 the grate (E). The First reaction takes place at a comparatively low heat, and 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 to the high temperature of the flame from (D). This completes the second reaction and a pasty mass of normal sodium sulphate is formed. The hydrochloric acid vapors, set free during the reaction, mix with the furnace gases from (D), and escape through the pipe (F) to 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, and may cause clogging in the passages and 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 and a slight excess of sulphuric acid The muffle or "close roaster" is used very generally on the continent of Europe, and yields a stronger and 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) to 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 and salt is raked from the pan (A) into the muffle (B), where it is heated to a reel heat, and 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 to build, yield a smaller output of saltcake, and require more fuel than the open roaster. Moreover, they very often crack, thus permitting acid vapors to escape into the flues and chimney, causing loss and creating a nuisance. It is customary to maintain a slight pressure in the flues and 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 be expensive. The pan (A) in both furnaces is about 10 feet in diameter, 7 inches thick at the centre and 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 to 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, to prevent" crusting" or burning on to 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 form, which may crack the muffle. Consequently, many attempts have been made to 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 and sulphuric acid is run in a slow, continuous stream. The mass overflows on to the hearth, where it is subjected to the high heat of the flames from the grate (G); at the same time, it is mixed and pushed towards the edge of the hearth by stirrers (C), against which the charge strikes as the hearth revolves. The speed is so regulated that the salt is all converted to 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 to the edge of the hearth (lips into this trough, so that the salt-cake forms a lute, and prevents the escape of the acid vapor into the space beneath the hearth. This only imperfectly protects the driving mechanism from the flame and acid vapors, and 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, and returned to the hand-worked furnace.
If a salt-cake free 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, and with a coke tower at the end of the series. The purpose of the towel', which is fed
by a spray of water, is to absorb any acid vapors which may pass uncondensed through the bombonnes. These are placed en cascade, t and 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 free regularly in any roaster, there being a rapid evolution during the progress of the first reaction, and 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 and 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 to organic matter, and sometimes to iron and free chlorine. To remove arsenic and sulphuric acid, the acid is diluted to 1.12 sp. gr., and barium sulphide is added; a pure hydrochloric acid vapor is then driven out by distillation and absorbed in pure water. 01' a solution of stannous chloride in concentrated hydrochloric acid is added to 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 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, and oxygen, is of some importance. The damp salt is pressed into blocks and 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, and ail' are made to 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 to prevent the mutual reaction between the hydrochloric acid vapor and the oxygen, by which chlorine is set 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 20 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 to 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, and to some extent in medicine. For some kinds of glass the salt-cake must b free from iron, and consequently it is made in lead pans. Or the sulphate may be purified from iron and 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 to dryness, and the product is calcined t remove all the water.
Organic Chemistry for the industry
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