Use of waste whey in tanning industry
The name of the deliming process suggests that the aim of the process is to remove the lime from the white hide. However, this is not complex enough. The primary purpose of the deliming procedure is to prepare the hide for bating and pickling. In order to do that, it is necessary to reach a suitable pH level to permit the bating enzymes to be active, and partially remove the excess lime. It is not, however, necessary to remove any or all of the lime at this stage. The calcium will be almost completely removed upon completion of the pickling stage.
The neutralisation and removal of excess hydroxide during deliming is achieved by adding one of a number of organic or inorganic acids because water washing, even over long periods of time, is not effective enough (Chiarini at al, 1992). The excess alkalinity of the limed hide is eliminated by converting the base to a neutral salt which is soluble and thereby readily removed from the hide. The most used acid in practice is ammonium ions from ammonium sulphate. However, testing of tannery production wastes indicates that waste waters from deliming cause considerable problems due to the concentration of ammonium ions. Up to 70 % of the ammonia ions discharged in tannery effluent is the result of the use of ammonium sulphate in the deliming and bating preparations. This is why the alternative method of neutralising calcium hydroxide and calcium collagenate with lactic acid is being investigated.
Taylor at al. (1988) paid considerable attention to the replacement of ammonium salts with magnesium salts; Koopman (1982, 1975, 1979) concentrated on magnesium sulphate or magnesium chloride either as the salts alone or in combination with acid.
Magnesium lactate was chosen as the deliming agent because when applied to a limed hide, insoluble magnesium hydroxide and soluble calcium lactate are produced. The solution resulting from this process has a pH of approximately 10. It is then possible, and necessary, to use common bating preparations containing alkaline proteases which are active at this pH. Thus digestion of undesirable proteins is assured to prepare the hide for tanning. In the context of the entire technology of deliming, bating and pickling, it is possible to bate the hide complete ly at this higher pH level. Nevertheless, additional pickling acid must be added in the next step to ensure that the proper pH level is reached for addition of the tanning solution.