The presence of Sodium Oxalate in the Bayer Process Circuit leads to control strategies which adversely affect the liquor productivity and/or the product quality. For example, sodium oxalate crystals increase the carry-over of fines in the secondary and tertiary hydrate classifiers, leading to poor fines control. Also, sodium oxalate crystals reduce the effectiveness of the hydrate agglomerators.
An effective Oxalate Removal System would allow the Bayer Plant to be operated more flexibly, and so would be able to respond to market conditions.
The paper describes the development of a successful Oxalate Removal System, from bench-scale, to pilot-scale and finally to industrial scale.
Spent Liquor is evaporated and passed to a continuous stirred tank reactor (CSTR), where it is contacted with oxalate seed. The partially de-oxalated liquor is passed to a Dense Bed Reactor, where the liquor is contacted with an elutriated bed of oxalate crystals. The clarifying zone of the Dense Bed Reactor removes the fine oxalate crystals from the liquor, and the de-oxalated liquor is passed to the digestion section. The underfiow from the Dense Bed Reactor is used to back-seed the CSTR; or is passed to a vacuum belt filter and the sodium oxalate disposed of.
Start-up and operational experiences are also described.