Key Process Control Techniques to Produce Improved Quality of Alumina

It’s proven fact that improved quality of products / services have the edge over normal quality. Quality always plays vital role in value addition and there by attributes in sustained business attracting confidence of customer. This paper describes key operational and process control measures to produce consistent quality of hydrate as well as calcined alumina in any Alumina plant. Major reasons affecting the quality of product are highlighted along with key control measures for improvement.

There are two main factors which badly affects the quality of product as listed below-

1.       High soda content in product hydrate and alumina

2.       High suspended solids in Thickener overflow pregnant liquor.

Step-I: In different Alumina refineries across the globe, we have experienced high soda content in product hydrate as one of the basic reasons for deterioration of product quality. The identification of this issue is confirmed through simple analytical technique by analyzing the Na₂O content in product hydrate ready for dispatch to respective customers. If total Na₂O content in Hydrate is found more than 0.30% then it confirms that Hydrate generated in decomposers has not been washed properly on product filter wherein the maximum component of leachable soda is removed. After observing this high soda content, vigorous washing of hydrate using hot water at temperature around 95-98^oC helps in achieving total Na₂O content below 0.30%. For further improvement on washing efficiency of hydrate, steam-hood at the discharge side of product filter has been found to be more advantageous.

Step-II: High Fe₂O₃ content in product hydrate above 0.010% is the indication of high suspended solids in filtered aluminate liquor feeding to Decomposers (Crystallizers / Precipitators). The suspended solids must be controlled below 10 mgpl in pregnant liquor in feed to decomposers. Higher solids in pregnant liquor happens because of inadequate settling of suspended red mud particles in Liquor thickener for the major reasons listed below-

(a)    High generation of fines during grinding of Bauxite:  

Control fineness of Bauxite at optimum level to ensure higher settling rate of red mud particles in thickener. Generally, minus 60 mesh fraction in ground bauxite slurry is maintained more than 85%. Check and control it accordingly.

(b)    Higher viscosity of thickener overflow liquor because of its lower temperature: 

(c)     Temperature of Thickener overflow liquor must be controlled at around 100-102^oC if the concentration of liquor in thickener overflow is at more than 145 gpl Na₂O. High concentration profile is the need of modern Alumina refinery in order to have improved productivity so as to minimize thermal and electrical energy consumption.

(d)    Selection of right settling agent: 

Settling agent used for faster settling of residue particles play vital role in achieving desired quality of product. There are different types of natural and synthetic high density polymers are available for different applications. It has been observed that mixed natural starch with particular type of synthetic flocculent are cost effective for Alumina refinery. 

(e)    Selection of proper filter cloth for polishing filtration equipment: 

Selection of proper filter cloth is equally essential for controlling suspended solids in filtered aluminate liquor fed to crystallizers. Polypropylene filter cloth of more than 30 EPI and 32 PPI have been found to be the most suitable cloth for security filtration.

(f)     Controlling P₂O₅ concentration in aluminate liquor: 

At times, high P₂O₅ content in Bauxite affects the settling rate of bauxite residue (Red mud) in decanters. To eliminate this issue, small quantity of Burnt lime (CaO) may be added with bauxite being fed to Grinding mills. This results in reducing the Phosphorus content in aluminate liquor forming Calcium Phosphate.

(g)    Control Filter feed pressure below 2.0 kg/cm².g: 

Lower filtration pressure results in better filtrate quality. Pressure around 2.0 kg/cm².g at inlet of security filtration system has been found to be optimum for security filtration.

(h)    Control solids in Thickener overflow Liquor well within 100 mgpl: 

Suspended solids in feed liquor to polishing filter should be controlled well within 100 mgpl so as to avoid extra load of residue on polishing filtration unit thereby improving the filtrate quality to decomposers,

(i)      Optimize dosing of TCA: 

TCA (Tri calcium aluminate) addition to filter feed liquor helps in getting improved filtrate quality. It has been established in various plants across the globe that mass flow of TCA equal to 10 times of suspended solids in decanter overflow liquor is optimum. This is considered as main controlling parameters for reducing the impurities in filtered aluminate liquor and subsequently the quality of product hydrate and calcined alumina. TCA dosing has been found to be the most effective step in improving the filtrate clarity.

Step-III: Replacement of Filtration Equipment:

Even after checking and monitoring all the above mentioned parameters stated under Step-II, if there is no appreciable improvement in filtrate clarity, then it concludes that something wrong with filtration equipment itself. In such situation, replacement of existing Polishing filters with suitably sized and designed security filters is the ultimate solution to this problem. Since this step requires additional capital expenditure for the plant, thus decision needs to be taken only after thorough investigations of results obtained under the guidance of process expert.