From the above bleaching earth grade selector it can be seen that different edible oils and fats require different grades of bleaching earth, which will again vary in their dosage. The true cost performance when using AMC bleaching earths in the refining of oils and fats are made up of the following 4 groups of costs.
- cost for the bleaching earth used per tonne of oil
- costs incurred due to oil loss in the bleaching process
- disposal costs for spent bleaching earths
- Yield from the use of spent bleaching earths in extracted meal as animal feed to be included where this step is utilised.
The bleaching earth costs for processing oils and fats are shown in the table below. Whilst this table is a useful guide, definitive costs are determined by a detailed plant trial in the refinery.
Oil loss is incurred from the oil residue absorbed by the bleaching earth which may be determined from analysis of the filter cake after the bleaching process. The spent bleaching earth cake is normally blown down at the end of the filtration cycle, leaving approximately 4% residue oil in cake.
The disposal of the spent bleaching earth is frequently associated with additional costs which depend on local government controls and transport costs for the disposal of the spent bleaching earth.
Most crude oils of vegetable, animal or marine origin destined for edible purposes need refining before use. The complete process involves removal of phosphatides, gums, trace metals, fatty acids and other impurities, following by decolourisation and, finally, deodorisation to give a bland stable oil of acceptable colour (see animation of the process). AMC clays have been developed over many years to give maximum efficiency in the bleaching stage, not only giving required colour removal but also effectively removing impurities such as traces of phosphatides, soap, metallic ions and hydro-peroxides from the oil. It is essential that the spent bleaching earth is readily removed from the oil by filtration after use. AMC provides bleaching earths that are specially controlled to give rapid filtration under all conditions and with minimum retention of oil in the spent filter cake.