Patent GB-256294-A (GB)
Title: Improvements in the extraction of salts from aqueous solutions and apparatus therefor
IPC: C01D3/06
Filed: 1925-04-01
Abstract: 256,294. Stephens, A. J., (Salt Production Syndicate, Ltd.). April 1, 1925. Bulk evaporators; progressive evaporators ; evaporating plant.-In the evaporation of saline solution for the extraction of salts, in order to prevent the precipitation of any salts as scale on the heating surfaces, such salts are precipitated after a preliminary concentration of the solution, in an evaporator the calandria of which is heated to the critical temperature of the salts so that the precipitation takes place in the body of the liquor, separation of more soluble parts taking place in later stages. The application referred to is the extraction of salt from sea-water with the previous removal of calcium sulphate in an evaporator of which the heating surfaces are at or near a temperature of 38‹ C. One form of apparatus is shown in Fig. 1 and comprises three evaporators 10, 11, 12 in series. In the first the feed water, preheated by the condensate from the steam heated calandria 15, is reduced to about one third of its bulk. The vapour from this evaporator is compressed by a pump 28 and sent into the calandria along with the service steam from the pipe 25. The concentrated liquor is then passed by way of the pipe 34 to the second evaporator 11 and is maintained at such a level as to just cover the calandria 16 by balancing the hydrostatic pressure due to the height above the level in the precipitating sump against the suction in the head of the evaporator produced by the compression pump 38 which forces the vapour into the calandria 16. The temperature of the calandria in this evaporator is maintained about the critical temperature of the salt, e.g. calcium sulphate which is thus precipitated in the body of the liquid and collects in the sump 23. As the temperature of the liquor leaving the evaporator 10 is ;n general higher than that required in the evaporator 11, a shunt cooling device 58 is associated with the vapour eduction pipe 37. An air pump 33 connected to the calandria drip pipe 39 controls the vacuum in the evaporator head. The third evaporator 12 is fed by a pipe 44 from the precipitating sump of the previous evaporator and is in every respect similar thereto except that the temperature of evaporation may be any required. Steam service pipes 56, 57 may be connected to the pump circuits to make good any loss of heat in the system, and cooling coils 80, 81 may surround the pipes leading to the air pump 33. The calandria in the evaporators after the first stage are made of small depth so that the vaporizing temperature at the bottom and top is substantially the same. In a modification, shown in Fig. 3, the second and later stage evaporators may comprise a number of shallow pans 302, 303, 304 with calandria 321, 322, 323 arranged in tier fashion. The upper pan is fed from the preceding evaporator and the succeeding pans from the sumps 306, 307 of those above by feed pipes 332, 333. All the feed pipes have valves controlled by floats. The compression pump 316 is branched to all the calandria and the drip pipes are all connected to the air pump 331. In a further modification, the concentrated liquors from the pans are circulated by pumps to precipitating tanks.