Advancements in Polishing Filtration System in Alumina Refinery

Hi Friends,

In previous posts, we have already discussed about the salient features of technological equipment installed in Alumina refinery. In present post, we will specifically discuss the past and present design features of equipment used for polishing filtration.

Polishing filtration unit of Alumina refinery has very crucial role in quality of product hydrate and alumina, thus this section is given enormous importance for trouble-free operation of associated equipment with allowable down time. In 1940s, Vertical leaf pressure filters as well as Horizontal leaf pressure filters were installed and operated with manual controls on pressure and filtration rate including the dislodging of residue cake from the leaf. The operational efficiency of Horizontal leaf pressure filters were found better compared to those of Vertical leaf pressure filters. Thus, most of the Alumina refinery constructed till 1980s in the World preferred to install Horizontal leaf pressure filters. The sequence of operation in such filters involve precoating of leaves for about 4 to 6 minutes, liquor filtration for 6 to 8 hours, draining of aluminate liquor from shell for about 10 to 15 minutes and manual dumping of residue cake for about 30 minutes to 45 minutes. That means, every filtration cycle has the downtime of about 1.25 hours. High down time for such a critical equipment affects the overall availability of filters for 75 to 80% only. The sequence of operation clearly reveals that manual operation of the equipment is the basic reason for high downtime, thus the development of Auto-dumping vertical leaf pressure filters came into existence sometime in late 80s as replacement to manually operated vertical and horizontal leaf pressure filters. These filters have auto dumping facilities for dislodging of residue cake from filter leaves by back flushing arrangements with aluminate liquor. Such modifications have  improved the availability of equipment to around 90% with steady controls and sophisticated automation system. The equipment designers and and manufacturers are  marketing the Auto dumping pressure filters under different brand names. With regard to operational efficiency parameters, the modified equipment has following main advantages over earlier equipment-

1. Specific filtration rate with manually operated vertical and horizontal leaf pressure filters varies from 0.60 to 0.75 m3/m2.hour as against 1.50 to 2.0 m3/m2.hour with auto dumping vertical pressure filters. A few operative plants have achieved the same at around 2.50 m3/m2.hour. 
2. Earlier precoating system was not very effective because of which the suspended solids in filtrate was averaging at around 20 mgpl. However, the continuous dosing of TCA with feed liquor has resulted in improving the filtrate clarity by reduction in suspended solids to the level of 6 to 8 mgpl. Thus, direct positive impact in improvement on product quality with respect to oxides of Silica and Iron impurities.
3. Auto dumping system for dislodging the accumulated residue cake has improved the working environment and drastic reduction in man power requirement as well.
4. The reduction in filtration pressure in auto dumping pressure filters at around 1.5 kg/cm2.g compared to 2.5 kg/cm2.g during manual operation has resulted in drastic reduction in filter cloth consumption because of increase in leaf life. 

These are the main technical features of Auto dumping filters over manual filters. Hence, most of the new Alumina refineries under execution and planning stages in the World  are going ahead with Auto dumping vertical leaf pressure filters. It would be prudent to mention here that in spite of all advantages, Auto dumping vertical leaf pressure filters may not be cost effective for smaller capacity Alumina  refinery because of higher capital investment. Thus, it is advised to have a critical look at the techno-economics before taking the decision to go ahead.

Trust, the comparative data / information have been elaborated to the extent possible. Please put your views / remarks / suggestions / comments, if any.

Regards.

Kunwar Rajendra