Waschverfahren fuer auskristallisierten Zucker

摘要

In affining raw sugar crystals, molasses adhering to the crystals is removed by washing them with a solvent comprising methanol and an agent, which forms solvent-soluble compounds or complexes with methanol-insoluble calcium or magnesium compounds present, as impurities, in the molasses. The agent may be an acid or an acid anhydride, preferably sulphur dioxide, which is soluble in the solvent. The acid, e.g. corresponding to the anhydride, may have a dissociation constant greater than 0.01, and may react with the aforesaid impurities to yield solvent-soluble calcium salts. Suitable acids in this category are: hydrochloric, sulphurous, monochloracetic, trichloracetic and lactic acid. Other acids specified are: sulphuric, phosphoric, dichloracetic, formic and oxalic acid. Suitable complex-forming agents are: formamide, dimethyl formamide, urea and a salt of an aminodiacetic acid derivative, e.g. diaminoethane tetracetic acid, with an organic base such as diethylamine. The solvent may also comprise a liquid, which depresses the solubility of sucrose and precipitates recoverable sucrose from the solvent. Suitable solubility-depressing liquids are: acetone, methylene chloride, ethanol, diethyl ether, ethyl acetate, 2, 2-dimethoxy propane, tetrahydrofuran, tetrahydrofurfuryl alcohol, dioxan and acetonitrile. The methanol solvent may contain between 20 and 30% of acetone and up to 10% of water, by volume. The raw sugar may be treated with solvent, then with the agent in the presence of solvent, and again with solvent. Excess solvent may be removed from the affined sugar crystals, which may then be dried. In the countercurrent affining apparatus shown in Fig. 1, raw sugar is fed, by a rotating scroll 2, through the outlet funnel 3 of a hopper 1, into the lower part of an inclined conveyer comprising joined, twin tubes 9 surrounding two intermeshing scrolls 10 of opposite hand, which are driven so that the scrolls rotate in opposite rotational directions, and thus convey the raw sugar upwardly to mix with an agent preferably SO2, supplied through a pipe 11, and a solvent, preferably methanol-acetone, pumped into the conveyer through a pipe 12. The affined raw sugar is discharged from the upper part of the conveyer, through a pipe 13, into a receptacle 14. The solvent-agent mixture, containing molasses and impurities leached from the raw sugar, is discharged, through a pipe 16, into a receptacle 17. A baffle 23 prevents incoming raw sugar from becoming entrained in the discharging solvent-agent mixture. A draining valve 15 is provided at the lower end of the conveyer.