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Rogers, M.G.

Quality control of the chemical and physical properties of alumina is one of the principal demands of smelters. One of the major chemical quality requirements is a low soda content with minimisation the responsibility of the alumina refinery.

The total soda in alumina is a combination of bound and soluble soda in the filtered and washed hydrate. Any variation in soluble soda is controlled at the calcination filters, while the bound soda is determined by the hydrate precipitation conditions. A high soluble soda in the calcination filter cake discharge can make a significant contribution to the total soda in the calcined alumina.

This paper is a digest of the observations and conclusions from recent studies into filter table operations.

While more than 20 contributing factors were identified as direct or indirect causes of high soluble soda, the major factors were:-

  • Selective sampling.
  • Media blinding.
  • Holes in the media,
  • Uneven cake thickness and poor cake washing.
  • Wash water flow rate and distribution.
  • Low concentration filter feed.
  • Low vacuum level.

Extreme variations in soluble soda concentration were common over long and short term periods with high soda concentrations in patches within individual filter sectors. The soluble soda in the liquor-rich patches was 10-20 times higher than the background soluble soda, so that a small portion contaminated the remaining hydrate and degraded the whole calciner feed.

Identifying the cause and effect relationships, using plant filtration data and concurrent bench scale test work, to predict the level as well as possible causes of variations, has produced a diagnostic guide to maintain control of the soluble soda in the hydrate filter cake.