GB456287A - Improvements to processes for the purification of lime aluminates - Google Patents

Abstract

<PICT:0456287/III/1> <PICT:0456287/III/2> <PICT:0456287/III/3> In a process for purifying calcium aluminates by gradual or systematic extraction with water, decantation is improved by effecting flocculaton of the particles of raw aluminate, either by feeding to the lixiviating - apparatus a thick mixture of water and crushed raw aluminate which has been stirred until flocculation occurs, or by introducing the raw material into the apparatus in the presence of a solution of calcium aluminate sufficiently concentrated for producing flocculation, say a solution containing more than one gram of aluminate per litre. In a modification, the raw material is extracted with water in one or more stages until flocculation occurs. The treatment may be effected in a series of vessels, in which case the time of treatment may be increased in each succeeding vessel and/or the amount of water used in each succeeding vessel be decreased; or a single vessel may be used which is fed at intervals with aluminate and water and from which the solution obtained is removed at intervals, whilst the residues are allowed to remain; or a counter-current process may be used. The temperatures used are such as to minimize the formation of insoluble hydrated aluminates, and in the event of temporary stoppage, the temperature is adjusted to the temperature at which formation of such aluminates is a minimum for the probable duration of the stoppage. Generally such temperatures lie between 20 and 30 DEG C. Extraction is preferably effected out of contact with the carbon dioxide of the atmosphere. In operating on the counter-current principle, the raw material is fed into a decantation chamber at the top of a column and water is introduced at the foot of the column. In order to accelerate the downward movement of the particles, the cross section of the column may increase from the top towards the bottom, or the water may be supplied intermittently or with a variable velocity, or as shown in Fig. 4, by pass conduits h<1>, h<2>, h<3> may be provided to collect solid particles from the sections a<1>, a<2>, a<3> and introduce them into the next lower section, or sections of enlarged cross section may be interposed between the sections a<1>, a<2> &c. If on the other hand, it is desired, for instance near the bottom, to retard the movement of the particles, the cross section of the column may be locally reduced, or screens or grids may be provided therein, or solid or hollow bodies of streamline section be inserted. When, as shown in Fig. 8, hollow bodies m are employed, the downward passage of the particles may be effected by means of a water ejector n. The sections a<1>, a<2>, a<3> may be arranged as separate columns either side by side as shown in Fig. 9, or vertically disposed. The Specification as open to inspection under Sect. 91 comprises also the following subject matter. Examples of operation showing the results of repeated extractions of a given mass of aluminate with equal amounts of water; and devices, illustrated by drawings, (cancelled), for effecting extraction in the absence of carbon dioxide. Such devices include bell-shaped covers for the extraction tank filled with an indifferent gas, or a floating piston, or a layer of fatty material, or the location of the whole apparatus in a room from which carbon dioxide can be excluded. This subject-matter does not appear in the Specification as accepted.