Organics in the Bayer process
1:Origins and Chemistry
1.1 Origins: Where do Bayer organics come from?
This is answered in detail in our review (Power & Loh 2010). The following is a brief summary including a selection of diagrams and tables from the review.
Most of the organics in Bayer liquor (usually expressed as g/L total organic carbon, 'TOC') comes from the bauxite. That's no accident; it is inevitable because organics are an essential part of bauxite genesis. For bauxite to form, alkaline components (Na, K, Si etc) must be leached from parent rock to leave a residue of aluminium and iron minerals rich in aluminuim hydroxides.
The best leachant is water from rain forest undergrowth because:
There is an abundance of it
The forest cover prevents decrease of soil moisture, ensures high soil humidity, and facilitates rainwater percolation through the profile
The root systems of trees facilitate drainage
The organic matter produced by plants creates local reducing conditions (low Eh) resulting in partial leaching of iron minerals
Micro-organisms assist in the decomposition of alumino-silicates by acting as complexing agents and catalysts
Forest trees and grasses contribute to desilication by extracting silica from the soil into the leaves, and
The forest protects the deposits from erosion.
This is why the richest bauxite provinces are of lateritic origin and are distributed in tropical and sub-tropical regions as per the grey-shaded areas in the map below (Adapted from 'Lateritic Bauxites', Bardossy and Aleva 1990). Dark grey indicates the bauxite 'provinces' within the broader regions of laterite. The table shows the organic carbon content (expressed as dry wt%TOC) for a number of bauxite sources.
The variation of TOC between bauxites is visualised in this chart, which also shows the variation in TOC/Available Alumina ratio.
TOC generally varies with depth. It is usually highest near the top of the deposit and lowest at the bottom as illustrated here for a Darling Range bauxite (Power 1991).