In the early 1990’s, QAL started looking for an alternate agitation technology for precipitation tanks that was high efficiency, low cost and scale growth inhibiting, seeking to replace the existing draft tube agitators. A partnership between QAL and CSIRO resulted in the joint development of SWIRLFLOW®, a slurry mixing technology that reduces sedimentation and scale formation in alumina precipitation tanks, thereby improving tank productivity.

QAL plant trials commenced in 1997 and were supported by modelling tasks undertaken by CSIRO. The initial version of SWIRLFLOW® was only suitable for precipitation tanks with conical bottoms. However, ongoing R&D that included numerous plant trials at QAL has established that running a backswept impeller is significantly more efficient than the original design. Furthermore, laboratory and plant trial results showed that a larger impeller-to-tank diameter ratio could reduce power consumption, allowing SWIRLFLOW® to operate in fillet-flat bottom tanks at QAL.

Compared to draft tubes, QAL has found that precipitator tanks utilizing SWIRLFLOW® technology typically have thinner scale deposits on tank walls, achieve increased operating factor, and are much easier to restart. All these factors lead to operational and maintenance cost savings. Based on laboratory studies and other plant implementations, it is possible to further optimize SWIRLFLOW® to consume lower power and to operate with higher solids concentrations than presently achievable.

At the time of writing, SWIRLFLOW® has been retrofitted to 30 of 106 precipitation tanks at the QAL refinery, with more conversions planned.

Scale management in Precipitation circuits is of vital importance in an alumina refinery to ensure adequate availability of equipment for meeting the production targets. A common problem in alumina refineries is scale formation in precipitation tanks which, if not managed properly, could lead to heavy scaling in the tanks, requiring lot of time and efforts to resolve the problem and a significant production loss. At Al Taweelah alumina refinery, which is set up with an oxalate co-precipitation circuit, enhanced gibbsite and oxalate scaling was observed in the precipitation tanks during the first 2 years of operation with an impact to the precipitation yield and a potential loss of control on availability of tanks. Enhanced scaling led to longer outage of tanks, as well as frequent flow drops and availability issues of Interstage Coolers (ISCs), requiring multiple Caustic Cleaning Liquor (CCL) washes to clear scales. This paper presents how Al Taweelah alumina employed modern thermal imaging techniques, combined with heat transfer concepts, to develop an in-house estimation of the scale quantity in tanks. Through better estimation of scale formation, tank turnaround schedules were optimised and tanks exhibiting increased scaling could be identified in advance. Additionally, the paper discusses in-house solutions aimed at reducing ‘gravel’ scale impacts on ISCs, including the concept of ‘milking’ tanks (small quantity of slurry taken out from tank and passed through a trommel screen to remove some of the suspended scales from the tank), modifications to ISC scale trap design and operation. These have resulted in a significant reduction in the scale quantity in precipitators, increased availability and utilisation rate of equipment, hence leading to an improvement in yield and reduction in caustic consumption.

By August 2021, ALTEO Gardanne was fully engaged to transform the alumina refinery from a bauxite feed to an alumina trihydrate (ATH) feed by January 2022. As part of the transition from bauxite to ATH feed, the 20 year old 80 m² Hassler belt filter, that was being used as an oxalate removal unit by washing the fine classified seed, would become the product deliquoring and washing filter: taking coarse classified hydrate slurry in the spent liquor to obtain, in a single filtration stage, the finished product hydrate with low leachable soda and moisture content. In less than 3 months, the modifications required to ensure the target operation condition of this new filtration mode were implemented, knowing that no back-up would be available from now on. This paper describes how ALTEO together with Sahl Regen modified the belt filter in two steps to achieve target quality in the product hydrate while minimising specific water consumption. The modified 80 m² belt filter has since achieved product specifications without any issues.

Vacuum disc filters are running successfully in seed filtration applications in all alumina refineries worldwide. The paper will discuss the latest developments of vacuum disc filters operated in alumina refineries. In addition to the latest process data in fine seed and coarse seed applications, the paper will provide designs and process data of two special applications.

First, the BoVac Disc Filters of BOKELA allow for seed re-slurry in an extension of the discharge chute. This is a very cost efficient and maintenance friendly opportunity especially if the seed filters are installed on top of the precipitation tanks. With a re-slurry in the filter discharge chute extension, the re-slurried seed can be guided not just in the precipitation tank under the filter, but through the connected pipe in any of the neighbour tanks. This increases availability and flexibility of the operation in combination with a minimum of extra equipment and extra cost.

Second, the BoVac Disc Filters of BOKELA have an optional design that allows for cake wash. In alumina refineries this design is used for fine seed wash. The current standard is still to wash the fine seed on drum filters that have two to three times the footprint compared to disc filters. This requires more space, bigger and thus higher CAPEX. Therefore, the ability of using disc filters for fine seed wash applications reduces both CAPEX and OPEX which is in line with reduction of energy consumption and associated reduction of the plant’s CO2 footprint.