Process for bleaching oils and fats
In the industrial production of oils and fats, bleaching earths are used to remove clouding, discolorations or also to remove oxidation accelerators. The taste, colour and storage stability of the oils and fats can be substantially improved by adsorptive purification. Different classes of bleaching earths are used for purification. The class of highly-active, mostly montmorillonite-based bleaching earths (HPBE=High Performance Bleaching Earth) forms a first group. This groups includes in particular acid-activated montmorillonites, wherein the acid activation is carried out in a costly process by dealuminizing the crude clays with concentrated acids at high temperatures, mostly at boiling heat. In this process a bleaching earth product with a very large specific surface and large pore volume is obtained. The use of only small quantities of this highly-active bleaching earth leads to the perceptible purification of the crude oils. Attempts to use small quantities in the bleaching process are worthwhile because, firstly, the bleaching earth used binds to residual quantities of oil, whereby the yield is decreased and, secondly, the bleaching earth used must be disposed of according to the regulations in force.
A disadvantage with these highly-active bleaching earths is that, because of the dealuminization with acid during production, large quantities of acidic salt-rich waste waters form which can be treated or disposed of only in costly processes. The high costs for the disposal of waste and the costly production process are the reason for the comparatively high prices of such highly-active bleaching earths.
The class of naturally active clays (NABE=Natural Active Bleaching Earth) forms a further group. These naturally occurring bleaching earths already have been used for hundreds of years for the purification of fats and oils. These naturally active systems (also called Fuller's earths) can be made available at very favourable cost. However, they possess only a small bleaching power, with the result that they are mostly not suitable for the purification of oils and fats which are difficult to bleach. Furthermore, compared with highly active bleaching earths, substantially larger quantities of the adsorbent must be used in order to achieve the desired bleaching result. Large losses of oil or fat must thereby be accepted, as the bleaching earths cannot be separated in pure form and some quantities of oil or fat remain in the bleaching earth.
A compromise between low production costs and acceptable activity is represented by the third class of bleaching earth, the so-called surface activated systems (SMBE=Surface Modified Bleaching Earth). Here, a naturally active crude clay is subjected to small quantities of acid and an “in situ” activation is thus achieved. Attapulgite- and hormite-containing crude clays in particular have proved themselves successful for this process. These have a very high specific surface area for natural crude clays of approx. 100 to 180 m2/g and a pore volume of approx. 0.2 to 0.35 ml/g. However, as salts formed during the acid activation or non-reacted portions of the acid are not washed out, these remain on the product and are also stored at least in part in the pores. As a result, these acid-activated bleaching earths generally do not achieve the same efficiency as is achieved by highly-active bleaching earths (HPBE) which are produced by dealuminization with acid. However, the simple production process makes possible a comparably cost-favourable production, wherein, as a particular advantage, no acid waste waters form.
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