SCALE INHIBITION AND CONTROL: RESULTS OF A FIVE YEAR FIELD STUDY INTO UNIQUE ELECTROCHEMICAL TECHNIQUES
In the alumina industry, hundreds of millions of dollars are lost annually due to the effects of scale formation in process vessels and pipework. These operating losses are associated with:
- Unrecoverable product in scale
- Production losses with vessels off-line for cleaning
- Production losses when vessels are on-line because of reduction in available volume
- Steam costs for cleaning liquor
- High pressure water and/or physical cleaning costs
- Production opportunity cost with recoverable product in scale
- Management of safety issues
- Area operations personnel diverting their focus from maximising production onto scale issues.
An electrochemical technique developed by Savcor in Finland and with a (20+ year) track record of scale control in caustic liquors in the pulp and paper industry has been adapted to Bayer process liquors and the technique further tested and refined through a series of site based trials in Australia. The trials were conducted over a five year period and focused on the control of whiteside scale in seed thickeners and precipitator circuits.
The trials were conducted in three consecutive stages:
- The first trial phase established the correct operating parameters in a Bayer liquor environment and the effectiveness of these parameters by using test plates in operating seed thickeners
- The second trial phase was a 180 day campaign conducted on an overflow pipe between a first row final precipitator and second row final precipitator to confirm similar scale control could be achieved in precipitation circuits
- The third trial phase was based on the success of the first and second trial phases with a large pilot installation to a 60m diameter seed thickener completed and then operated for a period of 114 days (a normal thickener maintenance turnaround period).
The third trial phase pilot test period demonstrated reductions in scale thickness in the order of 90–95% when compared to the previous seed thickener turnaround period.
Also, without any electrochemical treatment, the seed thickener wall does not clean to bare metal and is normally returned to service with a residual 3–5mm of uniform hard scale. The residual scale film that was formed under the test conditions was able to be cleaned at lower than normal water blasting pressures and an estimated 80–90% of the tank wall exhibited complete shelling of “old” scale back to the bare metal surface.