Control of Organic Impurities in Bayer Liquor of Alumina Refinery

Hi Friends,

In present post, we will discuss in brief the major sources of organic impurity build up, their negative impacts on process efficiency and methods to control the organic impurity level in plant liquor as outlined here under-

Major sources of organic build up:

In Bayer process of Alumina refinery, Bauxite is the basic input raw material used for production of alumina. The majority of Bauxite deposits in the World has moderate to high level of organic carbon ranging from 0.10 to 0.40% on weight basis. The carbon goes into Bayer liquor on dissolution of Bauxite with caustic soda. The organic carbons keep on increasing with circulating plant liquor and hence build up of organic impurities in Bayer liquor. In addition to Bauxite, use of additives like flocculants, defoaming agent and crystal growth modifiers in various process streams further add to the organic impurity level of Bayer liquor.

Negative impacts of organic impurities:
Organic impurities have detrimental effects in process because of their adverse impacts on plant performance like excessive foaming, lower liquor productivity, product purity, physical appearance of hydrate due to coloration, acute residue settling problems and faster rate of scale formation in lines and heat exchangers. Excessive foaming badly affects the volume utilization of plant, high scaling rate impacts heater performance and lower liquor productivity causes high consumption of thermal as well as electrical energy. Thus, it becomes essential to control the impurity level in Bayer liquor by adopting techno-economically viable process in the Alumina refinery.

Methods to control organic impurities:
There are many physical and chemical methods to control impurities in Bayer liquor of Alumina refinery as listed below-

1. Sorting of stems of trees from ROM Bauxite, 
2. Salting out and Causticization,
3. Seed hydrate washing,
4.  Wet oxidation,
5.  Crystallization of sodium oxalate,
6.  Liquor calcination and
7. Plasma treatment.

Methods stated at serial number 1 and 2 are used by almost all alumina producers in the World where seed hydrate washing system has been incorporated in selective plants. Wet oxidation technique has been successfully implemented in AOS Alumina refinery in Germany. Crystallization and separation of Sodium oxalate is done by Hindalco Belgaum in India. Liquor calcination (or Liquor burning) system is the efficient method which has been implemented in one of the Alumina refineries in Western Australia. This method is very effective but capital intensive and it causes generation of unpleasant odour as well as excessive carbon dioxide. Plasma treatment is the recent development which has not been commercialized so far.

Conclusions:

All out efforts are made to control Total Organic Carbon (TOC) in plant liquor below 5.0 gpl by selecting most appropriate technique depending on process conditions of the plant and the economics as well. Therefore, process experts should take final decision about adoption of most appropriate environment friendly and economically viable process route for controlling organic impuritie in Bayer liquor of Alumina refinery.

I invite you to put your feedback / suggestions / views / remarks / comments, if any.
Regards.


Kunwar Rajendra