In outline the process is as follows: We will suppose that a quantity of bicarbonate of sodium has been just precipitated from a brine solution, and we have the residual ammonium chloride to deal with. This is decomposed by "alkali waste," giving a final liquor of calcium chloride, which is run to waste, and a quantity of ammonium sulphide gas. This latter is led at once into a solution of salt in water, till saturation takes place. Into this liquor of brine and ammonium sulphide pure carbonic acid gas is now passed. The ammonium sulphide is decomposed, pure sulphureted hydrogen gas is given off, which is conducted to a gas holder and stored, while ammonium bicarbonate is formed in the liquor, which brings about the conversion of the salt into bicarbonate of sodium, ready for removal and preparation for the market.
It will be observed that we printed the word pure in italics in speaking of the carbonic acid used. This is one of the great points in the process, as in order that the sulphureted hydrogen gas obtained shall be concentrated and pure, only pure carbonic acid can be used in liberating it. The apparatus employed in its preparation is perhaps the most ingenious part of the works, and well worthy of attention by others besides alkali makers. The method is based on the fact that if dilute impure carbonic acid is passed into a solution of carbonate of sodium, the carbonic acid is absorbed, bicarbonate of sodium being formed, and the diluting gases passing away.
The bicarbonate of sodium on heating gives up the extra carbonic acid, which can be collected and stored pure, while the liquor passes back to simple carbonate of sodium, to be used over again as an absorbent. This is not at all new in theory, of course, nor is this the first proposal to use it commercially; but it is claimed that this is the first successful working of it on a large scale.
The gases from a large limekiln supply the dilute carbonic acid gas, which contains 25 per cent. to 30 per cent. of pure gas, the principal diluting gas being, of course, nitrogen. This kiln gas is drawn from the kiln by a blowing engine, and is first cooled in two large receivers. It is then forced into the solution of sodium carbonate in the absorption tower, 65 ft. high by 6 ft. diameter, filled with the liquor. The tower has many diaphragms and perforated "mushrooms," to cause a proper dispersion of the gases as they ascend through the liquor. The strength of liquor found best adapted for the work is equal to a density of about 30° Twaddell. After saturation the mud of bicarbonate of sodium is drawn off and passed into the "decomposer," a tower 35 ft. high by 6 ft. 6 in. in diameter, with perforated shelves, into which steam is blown from below, the liquor passing downward. The bicarbonate is decomposed, pure carbonic acid being given off. This is passed through a scrubber and into a gas holder ready for use. The liquor, which has now returned to the state of simple carbonate of sodium, only requires cooling to be ready to absorb a fresh lot of carbonic acid gas.
The cooling is effected in a tower packed loosely with bricks, the hot liquor trickling down against a powerful current of air blown in from below. Liquor has been cooled in this way, in once passing through the tower, from 220° Fahr. to 58° Fahr., but of course the exact cooling obtained depends more or less on the temperature of the atmosphere.
The next stage of the process, if we follow on after the preparation of the pure carbonic acid, is the employment of the gas for the decomposition of the ammonium sulphide absorbed in a brine liquor as above explained. The brine and ammonium sulphide are contained in what is known as a "Solvay tower," provided with proper means for dispersion and absorption of the carbonic acid gas. The precipitated bicarbonate of sodium is removed and washed, and prepared for the market in whatever form is required, the sulphureted hydrogen gas being led to a holder and stored, as before stated.
The decomposition of the ammonium chloride by means of "alkali waste" is carried out in a specially designed still. This is a tower 45 ft. high by 8 ft. diameter, divided by horizontal plates into compartments of about 3 ft. 8 in. in height. These compartments communicate with one another by means of pockets, or recesses, in the shell of the tower. A vertical shaft, with arms, revolves in the tower. The "waste" is fed in at the top by means of hopper and screw feed. The liquor is heated by steam blown in to over 212° Fahr. The ammonium sulphide is led direct into an absorbing vessel full of brine.
It now only remains to see how it is proposed to deal with the sulphureted hydrogen gas which represents the sulphur recovered from the waste. It can be burnt direct to sulphurous acid and utilized for the production of vitriol perfectly pure and free from arsenic, commanding a special price. But Messrs. Parnell & Simpson state that by a method of restricted combustion they are able to obtain nearly all the sulphur as such, and put it on the market on equal terms with the best Sicilian sulphur. We did not gather that this has yet been done on the working scale, however.
It will be seen that it is proposed that a Leblanc alkali maker shall continue to produce a portion of his make by the old process, but shall erect plant to enable him to make another portion by the Parnell & Simpson method, using his Leblanc "waste" in place of the caustic lime now employed by the ammonia soda people. He is thus to have the benefit of the cheaper process for, say, half his make, while he further cheapens the ammonia method by saving the cost of lime and by recovering the sulphur otherwise lost in his waste.
The saving in lime is stated to be one ton for each ton of sodium carbonate produced, or in cash value about 10s. per ton at Widnes, while the sulphur saved is estimated to be 6 cwt. per ton of sodium carbonate. We reproduce these figures with all reserve, not being ourselves sufficiently specialists to judge of them. But we were assured that they represent the minimum expected, and reasons were given to us to show that they would probably be exceeded.
Another gain for the Leblanc maker would be that he will escape the cost of removal and disposal of a portion of his refuse or waste.
The plant now erected was calculated for a yield of one hundred tons carbonate of sodium and about thirty-five tons of sulphur per week, but it now appears likely that this will be exceeded; while the carbonic acid plant was supposed to be equal to a yield of 6 tons of pure gas per day, and is now found capable of doing twice as much.
A few weeks will now bring this new combination process into the active and crucial test of the markets. Chemists and chemical engineers have all along taken a keen interest in the ingenious ideas of Parnell & Simpson. Commercial men are no less interested in the financial result of the experiment about to be tried at the expense of a few gentlemen of Liverpool and district. So far as we can learn, opinions are to some extent divided, though many good judges are very hopefully inclined. For our own part, speaking with diffidence, as being a little off our regular track of work, we will only say that we were favorably impressed with what we saw and heard; and we certainly wish the venture that full success which its cleverness and its pluck, as well as its great importance at this crisis, deserve for it. - Engineering.