Scale formation in Bayer precipitation tanks constrains slurry flow, reduces tank volume and adds mechanical stress to tanks and associated process equipment. Combined, these factors decrease precipitation productivity and increase the levels of process and safety risk. The Australian alumina industry therefore considers scale management generally, and precipitation scale formation specifically, to be critical areas of research and development over the next 15 -20 years.
In plants that practice sodium oxalate co-precipitation as a means of organics removal, there are often dramatic differences in scaling rate, scale structure and scale distribution between adjacent tanks. The objectives of this project were to explain such differences and to provide detailed mechanistic models that will allow existing processes to be optimised and/or other scale reduction technologies to be successfully applied.
In the Bayer precipitation and classification circuit, crystallisation fouling, particulate fouling and erosion processes were found to compete to determine the type and extent of scale formation. By conducting detailed analyses of scale samples obtained from an alumina refinery, and of those prepared under controlled conditions in the laboratory, the key factors in each of these processes were determined.
This paper describes the simulation of fouling in a laboratory crystalliser, provides supporting measurements of local slurry velocity and flow patterns in an equivalent physical model, and introduces the concept of the scaling predominance diagram. The scaling predominance diagram, when accurately mapped, provides an important means of quantifying the opportunities to change the scaling regimes in Bayer precipitation tanks and other key parts of the process plant.