GB440691A - Improvements in or relating to the production of pentaerythrite - Google Patents

Abstract

Pentaerythrite which may be obtained by alkaline condensation of acetaldehyde and formaldehyde and precipitation of the condensation agent such as lime, is purified by treating the crystalline mass with a washing agent such as saturated pentaerythrite solution or an organic liquid in which pentaerythrite is insoluble, the liquid being caused to effect a scouring action so as to bring into suspension the adhering impurities, particularly di-pentaerythrite, which impurities are separated from the crystals by a process depending on the smaller size of the particles of impurities. The scouring action may be effected by strong agitation, kneading or pressing in the presence of the washing liquor which may be the mother liquor, and the impurities may be removed by a levigating and depositing process or by sieving. The washing may be effected several times in counter-current, the washing liquor being if desired, freed from suspended impurities by filtration between successive operations. The crystal mass of pentaerythrite is preferably freed from mother-liquor and washing liquor by pressing, and the washing liquors are added to the crude solution of a succeeding charge and concentrated therewith. Calcium hydroxide is preferably used for the initial condensation, and may be removed by sulphuric acid, the formic acid set free being neutralized with ammonia, or the calcium may be removed by ammonium sulphate; and the crude solution is preferably concentrated sufficiently to crystallize the greater part of the pentaerythrite. The crystallization is preferably effected with slow cooling while the liquor is at rest or in slow motion so as to obtain a coarse grain. In an example, the crystal mass is strongly agitated with saturated pentaerythrite solution and then pressed out on a silk gauze sieve while being strongly agitated, the process being repeated one or more times with filtration of the washing-liquor between successive operations. The Specification as open to inspection under Sect. 91 states that the crude crystals may be strongly agitated with washing liquor on a sieve which retains the pentaerythrite but not the impurities; and mentions alcohol as a suitable washing - liquor. In an example, acetaldehyde, formaldehyde, water and calcium hydroxide are reacted at 30--50 DEG C., the calcium is precipitated by ammonium sulphate, and the solution is concentrated and cooled slowly with gentle agitation, the crystals being separated by pressing and purified as described above. In another example, the crude crystals are washed in a tall cylinder having in its lower part a rapidly-running agitator causing strong eddies; washing liquor is continuously supplied at the bottom and withdrawn at the top. In another example, the impure filtering residue from the treatment of the washing liquor is dissolved in as little boiling water as possible and allowed to crystallize very slowly, whereby pentaerythrite separates in relatively large crystals mixed with impurities in smaller crystals, this mixture being then treated as above described. This subject-matter does not appear in the Specification as accepted.