The scope of this article is confined to simple distillation of fluids, such as the distillation of water to free it from non-volatile impurities, and the distillation of spirit to remove combined water. These processes and the apparatus pertaining thereto are capable of wide application and utility.
The total absence of potable water in many parts of the world, to which the existence of valuable mineral deposits attracts a considerable population, has called for the invention of some artificial means of supplying this, the greatest of all the necessaries of life. Perhaps in no part of the world has more attention been given to the subject than in the northern part of Chile, "the desert of Atacama." This region was traversed by Indian posts in the time of the Incas, the runners being supplied with water at various points on the road, with an immense expenditure of labour. The water was carried long distances, in large earthenware jars; and the inconvenience was reduced to a minimum by the care bestowed in laying out the roads, so as to take the greatest advantage of the fresh-water springs at the foot of the Andes.
About 30 years since, the method of procuring fresh-water from the sea by distillation was commenced. The original form of apparatus, and one that is even now largely in use, consisted merely of a Cornish boiler, the steam from which passed through a coil of wrought-iron pipes in an open tank, often of wood. Various improvements have been made from time to time, principally in the direction of enclosing the coils and conducting the steam given off to secondary, and thence to tertiary, condensers, similar to the apparatus designed by the late Dr. Normandy. The ordinary consumption of coal per unit of water in the original open condensers is about 1/6, but with several condensers and an air-pump on the last, a ratio of 1/16 is regularly obtained in daily work.
A serious inconvenience attending this method of production arises from the excessively bad quality of the water for use in a steam boiler, as it contains about 14 per cent. of salts - the principal of which are sodium chloride, and lime, soda, and magnesia sulphates. The freight on coals from the port, too, is enormous, amounting, at the time when the apparatus was designed, to 3-3 1/2 dollars per 100 lb. Taking into consideration the loss on the road, and the expense of repairs to boilers, condensers, etc, the cost of the water was about 4 cents per gallon.
With a view to overcoming this difficulty, an apparatus for the distillation of water in Las Salinas, by the action of the sun's rays, was designed by Charles Wilson in 1872. Las Salinas is situated about 70 miles inland from the port of Antofogasta, and is about half-way on the road to Caracoles, a great silver district, requiring, when in full work, the employment of about 800 carts and 4000 mules, which passed through Salinas, on an average, about once a week. The site selected for the establishment was a smooth plain, with an inclination of about 1 in 100 towards the old watercourse, in which are wells for salt water. The apparatus consists essentially of a number of long shallow troughs, filled with water, and covered by a sloping glass roof. The water is evaporated by the sun's rays passing through the glass; the vapour is condensed on the under surface of the glass, runs down to grooves cut in the wooden frame, and thence, by a system of pipes, to the fresh-water tank. There are in the establishment at Salinas 64 frames, each 200 ft. long by 4 ft. broad, giving a total area of 51,200 sq. ft. of glass. Each frame is composed of 2 principal parts, the water-trough and the roof.
The trough is constructed of 3 longitudinal sleepers, 4 in. by 4 in., on which the planking (1 1/2 in. thick) is laid. The sides are composed of timbers, bolted to the sleepers at every 6 ft., the whole being carefully jointed inside with putty, to render it perfectly water-tight, and having an inclination of about 1 in. in the total length in the direction of the wash-out plug. The roof is constructed in 10 lengths of 20 ft. each. The sides are of pine, with the upper edge properly cut to receive the glass, and a groove for conveying the condensed water to the outlet-pipes, which are placed at the lower end of each section, the grooves having an inclination of 2 in. in 20 ft., in addition to the inclination of the trough. The end frames of the 20-ft. sections of the roof, excepting those which coincide with the ends of the troughs, are carried down to a little below the water-level, to prevent the escape of vapour in the joint, there being, in fact, no outlet for the vapour, excepting by the small leaden pipes which carry off the condensed water. The ridge is supported by the end frames and intermediate uprights, resting on the bottom of the trough.
The sash-bars are movable, so as to suit varying widths of glass.
The salt water is admitted by a 1-in. brass cock at the higher end of the trough, and a wooden plug for washing out is provided at the lower end. There is also, at the lower end, an overflow pipe, the point of which is turned down below the water, to prevent the escape of vapour. The salt water is pumped from the wells by a windmill into a tank at the upper end of the grounds, sufficiently large to contain about 4 days' supply. The water from the tank is distributed to the various troughs by a 2-in. wronght-iron pipe, with the necessary connections. The fresh water is collected from the small leaden pipes into a l 1/2-in. wronght-iron pipe running between the troughs, and connecting with a 2-in. main-pipe at the end, which leads to the storage tanks. To increase the evaporation, the bottoms of the troughs ore blackened with logwood and alum, and are washed out every second day, by running salt water through.
When first set to work, the establishment produced daily, in summer, upwards of 5000 gal. of fresh water, about 3nal to 1 lb. of water per sq. ft. of glass; but after the opening of the railway, the owners grew careless, and allowed the troughs to get out of repair, so that, through leakages and insufficient cleansing, the production gradually fell off to about 1/2 of the above. When not properly attended to, crystals of soda and lime sulphate (Glauberite) form in the troughs, directly diminishing the production, and indirectly leading to loss by leakage when the crystallisation takes place between the planks, and so forcing open the joints. When properly maintained, the cost of water, including interest on capital, renewals of glass, etc, amounted to less than I cent per gallon. The principal item of expense is the renewal of glass broken by whirlwinds, which are very frequent in the locality. The staff consists of a clerk, who keeps the accounts, sells the water, and manages the business generally; and of a glazier, and 2 labourers for cleaning and repairs, and at intervals a carpenter to restore the woodwork.