Friday, March 22, 2019

Critical Process Steps to Improve Bauxite Residue Settling Rate in Alumina Refinery

Hi Friends,

In continuation to earlier technical papers, I would like to present this paper of utmost importance covering critical problems and their remedial actions pertaining to bauxite residue settling in Alumina refinery. The paper is purely based on personal experience of the author in operative Alumina Plants in India and abroad. Trust, you will like the content of this paper.

Bauxite residue is the waste generated in Alumina refinery. The residue generation rate is dependent of Bauxite quality and alumina extraction efficiency. In Bayer process, Bauxite with around 40% Al2O3 and less than 4% total silica (Module > 10) is considered as metallurgical grade bauxite for production of calcined alumina. The alumina digestion process results in dissolution of bauxite in caustic soda and causes generation of about 1.15 to 1.35 tonnes of dry residue as waste per tonne of alumina.

It is a known fact that storage and disposal of Bauxite residue is a big problem for all Alumina producers due to environmental problems in the vicinity because of dust generation and percolation of caustic to natural water streams. Thus stress and emphasis of concerned Governments is always given to store it in well protected area so as to combat probable environmental problems and health hazard for the community around because of the passage of alkaline water to natural streams and acute dusting in the area during summer. Though attempts are being made by researchers and scientists to find commercial utilization of residue but very limited utilization has been made possible so far because of economic disadvantages. The laboratory scale development of various processes were done till 1980s for its utilization in manufacture of red mud bricks, cement, red mud PVC plastic, pigment and many other products but only a few has been commercialized till date. 

In present post, we will discuss the key contributing parameters to improve the settling characteristics of Bauxite residue in Alumina refinery. As such, there are unlimited variables which affect the settling rate of residue but we will like to highlight the major parameters to improve the settling rate of residue particles in settlers and washers of Alumina refinery thereby improving the production rate due to steady operation, improved product quality due to less carry over of suspended solids in filtrate during polishing filtration and lower soda loss with residue due to increased residue washing efficiency in residue washers. The major parameters are briefly described in subsequent paragraphs-

1.       Mineralogical composition of Bauxite: 

The mineralogical composition of Bauxite clearly reflects that oxide of iron is present in the form of hematite, goethite and siderite whereas Oxide of titanium in the form of anatase and rutile in addition to oxides of other minor metals. The presence of goethite badly affects the settling rate of residue. Thus the ratio of hematite to goethite is preferably maintained more than 3 by adopting suitable blending techniques before processing in the Bayer circuit. 

2.       Granulometry of Bauxite in ground slurry: 

The specific gravity of Bauxite ranges from 2.50 to 2.75 depending on the chemical and mineralogical composition of Bauxite. As such, the bauxite particle of minus 60 mesh has been found to be okay for satisfactory extraction as well as recovery of alumina but to ensure the same the granulometry in ground slurry is controlled over 85% of minus 60 mesh fraction. The slurry contains about 1 to 2% of plus 10 mesh fractions. With this particle size distribution, the specific surface area of dry bauxite is observed between 12 to 15 m2 per gram. This figure is also used as a guideline for required granulometry in ground slurry. Over grinding of bauxite is avoided as it badly affects the settling rate of residue because of further disintegration of particles during digestion and slurry flashing circuit of Alumina refinery. 

3.       Selection of suitable flocculents: 

It has been seen that mixed natural and synthetic flocculent gives better result than any individual flocculent for desired level of overflow liquor clarity as well as compactness of underflow residue. However, the ratio of mixture varies widely depending on mineralogical composition as well as settling characteristics of Bauxite. Thus, the ratio of mixing is finalized by carrying out settling rate test in Laboratory with digested blow-off slurry under the plant operating conditions. The dosing rate of natural flocculent generally varies from 0.25% to 1% of dry bauxite residue entering the settlers and washers.

4.       Temperature profile of Settling and Washing circuit: 

Though, there may not be wide fluctuations in temperature profile of thickeners and washers but at times even the minor deviations in temperature profile either due to ignorance of operating personnel or malfunctioning of instrumentation and control system may cause serious problem in the settling rate as well as overflow liquor clarity due to increase in viscosity of aluminate liquor at lower temperature. Thus close watch on steady temperature profile is considered as one of the major factors.

5.       Monitoring and control on P2O5 content in process liquor: 

This is one of the very important parameters which requires constant vigil for steady control within limits by taking required preventive measures in the upstream of the process. For the purpose, suitable chemical is added continuously to the process circuit.

It has been experienced that close watch on these critical parameters results in trouble-free as well as steady and efficient operation of Alumina refinery. We would appreciate your valued comments.
Best regards.

Rajendra Kunwar
Principal Consultant-Engineering

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