The Ammonia Soda Process

The Ammonia Soda Process

The reactions involved in the ammonia soda process were discovered by H. G. Dyar and J. Hemming, about 1838, but owing to the mechanical difficulties, its practical success was not thoroughly established until 1873. In 1863, Ernest Solvay, a Belgian, constructed an apparatus which has led to an enormous development of the industry, by which one-half of the world's supply of soda is now made. Its advantages lie in the strength and purity of its products and the absence of troublesome by-products, such as "tank waste." But it does not yield chlorine nor hydrochloric acid, all the former going to waste as calcium chloride.
The ammonia soda process depends upon the fact that sodium bicarbonate is but slightly soluble in a cold ammoniacal solution of common salt. The technical success of the process depends chiefly on the proper regulation of the temperature during the precipitation, and on the capacity of the works to handle large quantities of gases and liquids. As far as possible, manual labor must be avoided, and the products moved and treated in solution or in suspension. The reactions are as follows: -

1) NaCl +NH3 +H20 +CO2= NH4Cl +NaHC03•
2) 2 NHlJl +Ca(OH)2 = CaC12+2 H20 +2 NH3'
 
The first equation is the chief one; the second represents the recovery of the ammonia, and is essential to the commercial success of the. process.
The salt is used as a very concentrated brine, which has been purified from iron, silica, magnesia, etc.; it is then saturated with ammonia gas, obtained from gas liquors, or by the recovery process according to equation (2). The carbon dioxide is obtained partly from lime kilns and partly from the calcination of the bicarbonate to form the normal carbonate. It must contain at least 10per cent of CO2, and is prepared in special forms of continuous limekilns. The lime resulting is used in the recovery of the ammonia (reaction 2), and for making caustic soda; the limekiln gases are cooled, and the sulphur dioxide removed, by washing in water before they pass into the carbonating towers. (See below.) The brine is contained in a tank, under the perforated bottom of which the ammonia gas is introduced, and rising through the liquor, is rapidly absorbed.
The heat evolved by the absorption is taken up by cold water circulating in coils. When saturated, the ammoniacal brine is pumped into a receiving and settling tank, from which it is delivered to the" carbonating tower" (Fig. 42).* This is from 50 to 65 feet high, built of cast-iron rings or segments (A, A), each about 3.5 feet high and 6 feet in diameter. At the bottom of each segment is a flat plate having a large hole in the centre. Above each plate is a dome-shaped diaphragm (D) perforated with a great number of small holes. In modern works a system of pipes passes through each segment, as shown at (B, B); in these, cold water is kept flowing, thus counteracting the heat generated by the chemical action. The ammoniacal brine is forced under pressure through the pipe (P), entering a little above the middle of the tower, which is nearly filled with brine. By this arrangement, any free ammonia in the brine, which would be swept away by the stream of gases passing up through the tower, is taken up by the carbon dioxide in the upper part of the tower. The carbon dioxide, having been previously well cooled, is forced through the pipe (C), entering under the lowest dome, and rising in small bubbles through the perforations in each  dome, comes into intimate contact with the ammoniacal brine. The bicarbonate of sodium thus precipitated gradually works its way down through the tower. A thick, milley liquid, containing the bicarbonate in suspension, and ammonium chloride and comlllon salt in solution, is drawn off through (H) at the bottom.
After a tower has been in use for some days, the holes in the domes become clogged with a deposit of bicarbonate crystals, which prevent the free passage of the gases. Consequently, every ten days or two weeks the liquitl lllUSt be drawn out and the crystals dissolved by filling the towel' with hot water or steam. The tower must be cooled before starting the process anew. As a rule, several towers are employed, so that one may be cleaned and coolec1without interrupting the operation.
The gases escaping from the top of the tower, consisting principally of nitrogen, carbon dioxide, and some ammonia, are passed through scrubbers , one of which contains brine, which afterwards goes to the ammonia saturating tank; in the other is dilute sulphuric acid, to absorb the small amount of ammonia which would otherwise be lost. The carbon dioxide and nitrogen are allowed to escape. The towers are run with the view to the utilization of all the ammonia possible, even though there is considerable loss of salt and carbon dioxide; usually about one-fourth of the salt remains undecomposed.
It is now customary to place a smaller carbonating tower in connection with the large one; in the former the brine is first treated with carbon dioxide and the ammonia converted to neutral carbonate (NH4)2C03; then the brine is pumped into the large carbonating tower, where it meets more carbon dioxide, and the bicarbonate is formed, causing the precipitation of the sodium bicarbonate. 1\1ore heat is liberated in the formation of the neutral carbonate of ammonia than in its conversion to the bicarbonate, hence the temperature of the precipitation is more easily controlled when two towers are used, and less free ammonia escapes with the waste gases. A temperature of about 35° C. is most favorable to the formation of a granular or crystalline precipitate of bicarbonate, and also to the most complete utilization of the ammonia. At higher temperatures, too much bicarbonate remains dissolved in the liquor; at lower temperatures there is a tendency to the crystallization of ammonium acid carbonate and ammonium chloride, while the bicarbonate separates as a very fine precipitate, which is difficult to filter from the liquor.
The milky liquor from the bottom of the tower, containing the sodium bicarbonate in suspension, is filtered on sand filters  connected with a vacuum pump; or better, it is run into centrifugal machines, which afford 1IIore rapid and complete separation of the mother-liquor. '£he bicarbonate is then washed with water, to remove as much of the sodium and ammonium chlorides as possible. The mother-liquors and wash waters go to the ammonia recovery process.
The sodium bicarbonate is then calcined in large covered cast-iron pans or ovens; this converts the acid salt into soda-ash, and drives out any ammonia or moisture still in the mass. The following is the reaction:-

2 N aHC03 = Nal~03 + CO~+ H~O.

The fumes are passed through coolers and scrubbers to remove ammonia; the concentrated carbon dioxide remaining is pumped into the carbonating towers. The ammonia liquors go to the ammonia stills.
A modification of the Thelen pan (Fig. 40) is sometimes used for this calcining. A gas-tight cover is placed over the pan, and the scrapers pass back and forth over the pan bottom, being moved by a connecting rod and crank. The gases and steam pass off through a pipe set in the cover. In practice, it has been found best to leave the mass in this pan only until all the ammonia and about 75 per cent of the carbon dioxide of the bicarbonate have been expelled; the calcination is completed in a reverberatory furnace.
The product of the calcination is called soda-ash; it is often very pure, containing only a trace of salt and a little bicarbonate, and is free from caustic soda, sulphide, and sulphate. But its density is only 0.8, while that of the Leblanc product is 1.2. This is disadvantageous, owing to the larger packages needed for a given weight and to the mechanical loss incurred in operations where the soda-ash is exposed to a strong draught of air. In order to increase the density, it is sometimes subjected to a second heating in a reverberatory (revolving) furnace.
The second reaction, on p. 8G, is that on which the recovery of the ammonia depends. The liquid in which the bicarbonate of soda was suspended contains undecomposed salt, ammonium chloride, and ammonium carbonate. It is passed through an ammonia still, usually a tall colu1lln or dephlegmator. Steam is admitted at the bottom of the apparatus, and bubbling up through the liquid, decomposes the ammonium carbonate into ammonia, carbon dioxide, part of the towel', or the still proper, where it is decomposed by "milk of lime." The ammonia set free is cooled and used to saturate the brine. The calcium chloride formed remains in solution, and together with the excess of salt, goes to waste. (For the various proposals to utilize the waste calcium chloride for the production of hydrochloric acid and chlorine) The damp bicarbonate is dried in an atmosphere of carbon dioxide, at a temperature of about 90° C.; this prevents decomposition of the sodium bicarbonate, while the ammonium bicarbonate is decomposed, the vapors passing to the scrubbers, where the ammonia is recovered. A considerable quantity of the bicarbonate of soda is sold directly to the manufacturers of ,baking powder and the poorer grades to the soda-water makers.
Caustic soda can be made stronger and purer from ammonia soda· ash than from Leblanc ash, and the process is not essentially different, except that no treatment to remove sulphur is necessary; but it cannot be made so cheaply as from the "red liquors" or the "tank liquors" of the Leblanc process. If pure lime is used for causticizing ammonia soda-ash, the product is better than in the case of the Leblanc ash, as it is free from sulphur, alumina, etc. Loewig's process (p. 81) appears especially suited for causticizing ammonia soda-ash, since it requires an ash free from silica. The Parnell and Simpson process * was expected to solve the problem of the Leblanc" alkali waste"; but while it is interesting, it has not justified the hopes of its promoters. It was proposed to combine to a considerable extent the two leading soda processes. The reactions involved are as follows: -

1) (NH4)2S+ CO2+ H20 = NH4HC03 + NH4HS.
2) NH4HS + CO2 + H~O=NH4HC03 + H2S.
3) NH4HC03 + NaCl =NaHC03 +NH4Cl.
4) CaS + 2 NH4Cl = (NH4)~S+ CaC12t

A solution containing a mixture of ammonium sulphide and salt is treated with carbon dioxide, as in the ammonia process. Sodium bicarbonate is precipitated and hydrogen sulphide set free; this is burned with air, and the sulphur dioxide sent to the lead chambers of the sulphuric acid process. Or the sulphur may also be recovered in a Claus kiln (p. 85). The ammonium sulphide is obtained by chloride liquors of the ammonia process, or those formed in this (Parnell-Simpson) process. Thus the ammonia is recovered and at the same time the troublesome Leblanc waste is disposed of. When the waste is boiled in the ammonium chloride solution, ammonia gas, together with vapors of ammonium sulphide, is liberated. These are led directly into the brine solution in the saturating tank. The ammoniacal brine is then pumped into the carbonating tower, which is very similar to that described earlier Here the first three reactions take place; * the hydrogen sulphide generated goes to the sulphur recovery, while the ammonium chloride solution, carrying the sodium bicarbonate in suspension, is drawn out and filtered.
The conversion of salt into sodium carbonate by any method involves an endothermic reaction in some part of the process. Thus energy must be expended, necessitating the use of fuel. In the ease of the Leblanc process, this expenditure of fuel is large, and is chiefly used in carrying out the reactions in the salt-cake and the black-ash furnaces. But much of the expended energy of this process reappears in the hydrochloric acid, the principal by-product. In the ammonia process the principal reactions are exothermic, but some fuel is consumed by the calcination of the precipitated bicarbonate and in the preparation of the quicklime used in the ammonia recovery and for generating carbon dioxide. Although less fuel is used than in the Leblanc process, the practical economy of the ammonia process is not so great as would at first appear; for all the chlorine is lost, together with a large part of the original salt used. As a method of producing soda-ash it is far superior to the Leblanc, but until a practical process for the cheap production of chlorine is discovered, the latter will continue to be an extensive industry.