Pohl states that In a subsequent trial, after lifting the top pan at the end nearest the fires to a height of 6 in. and lowering the other end to within 3 in. of the surface of the brine in the bottom pan, he obtained, as an average result of a series of boilings, 3 tons of salt for 1 ton of slack, the gases passing off at a still lower temperature; while in the top pan 200°-208°F. (93°-98° C.) was the temperature attained in front, 180° F. (82° C.) in the middle, and 160° F. (71° C.) at the far end.
The various descriptions of soda pan and setting are shown in Figs. 131 to 136. The apparatus is usually termed a "boat" pan from its shape, it will be noticed that the pan is so set in brickwork that the fire only plays upon the sides about half-way up. Consequently the soda salt, as it crystallises out, accumulates at the bottom of the pan and is then "fished" out up the sloping sides, being protected by the solid brickwork from being burned. An ingenious form of pan has been occasionally tried. It consists of 2 compartments, the one heated and the other kept cool, connected by a large tube. The liquors are kept in constant circulation between the 2 compartments, crystallising out in the cold one, and the mother-liquors being pumped back. It has also been proposed to fish salts of different value from the boiling-down pan at different stages of concentration leaving the mother-liquors to be finally worked up into a caustio ash. Upon the whole, the method of boiling down by the waste heat passing over the surface of the liquors is the most economical, aroper care in the subsequent finishing process rendering it perfectly easy to produce a satisfactory carbonate.
Two forms of evaporating pan for 137, 138. The former is the better, as the salt raked up on the shelving end of the pan shown in Fig. 138 is apt to burn, and the drainings are returned to the pan cold. In some works, the acetate liquor, instead of being allowed to crystallise out in the manner described, is boiled down to dryness in a pot of the form shown in Fig. 139. In this way only an inferior article is obtained: but by evaporating to dryness in a shallow sheet-iron pan, similar to that shown in Fig. H7, a tine product may result. Every manufacturer of potash chlorate is aware that his boiling-down pans are acted upon by the liquors, even when they are free from chlorine or hypochlorite. The clear liquid becomes turbid during the evaluation, and in the case of iron pans deposits a red muddy precipitate. In the case of lead, the formation of a mud is not so conspicuous; evidently the lead oxide originally formed decomposes with potassium chloand lead chloride, at least partially. Usually it is assumed that lead is less acted upon than iron, but the latter is often preferred as being more durable.
It seems to be an open question whether cast iron or wrought iron is more suitable for sueh boiling-down pans; the large sizes and more easily repaired than cast-iron pane, but are more quickly acted upon by the liquors. Some experiments made by Dr. Lunge induced him to arrive at the following conclusions:
1. All metals employed are acted upon by the boiling liquors treated therein, more so by concentrated than by dilute solutions of potassium chlorate, and most of all by the mixed solution of calcium chlorate and chloride formed in manufacturing.
2. The weight of metal dissolved is always smallest in the case of cast iron, by far the greatest in the case of lead, wrought iron holding a middle place, but being not much worse than cast iron. If we consider that the calculations from the chloride farmed are made from pure iron, but that cast iron only contains 90 to 93 per cent. of such, the difference between cost and wrought iron is still further reduced.
3. The weight of chlorate destroyed does not differ very materially whether cast iron or lead is employed.
4. Since in any case the quantity of chlorate destroyed is not essentially less in the case of lead pans than in that of iron pans, but the loss of metal dissolved (as well as the cost of firing and repairs) is much greater with lead than with iron, boiling-down pans made of iron are preferable to leaden ones. According to the practically most important aeries of experiments, there is no essential difference in respect of action between cut and wrought iron.