Alumina refinery residue storage: from lagooning to dry stacking
Tailings and byproducts in the mineral processing, mining and refining industries have been the greatest challenge since the beginning. In the case of alumina refineries, bauxite residue has relevant public consideration, due to the spillage occurred in the last years. For this reason, bauxite residue is one of the fundamental subjects in every conference, covering from disposal methods to its utilization.
But why after more than 100 years of Bayer process the derived residue and their respective storages are still being an issue? The first cause is economics, massive amount of residue without strong economically feasible use – alumina refinery residue exceeds 2.7 billion tons excluding the amount of rain water remaining in the dams worldwide. Easy and inexpensive storage methods were used in the beginning of the mining industry, when we lacked the knowledge of the environmental and economic impact of these solutions in the future.
In the recent years, environmental rules have been reinforced with compulsory registration, licensing for operations and penalty fees, and technologies have been developed to custom the necessities of this industry in a more economically viable form. Old dams are lasting, and finally, all the costs related to remediation after failures and the non-economically estimated loss of life of many are the drivers for moving from elderly technological paradigms to innovative and advanced technical solutions.
The bauxite residue storages that have been implemented in alumina refineries are summarized in this article, which also provides an overview of key factors for the success of dry stacking storage over other technologies and the role of filter press in the development of this type of disposal.
The article Bauxite Residue Management published by the bauxite.world-aluminium.org and many other papers named in the hereto articles provide a short overview of the different types of residue storage.
Wet disposal: residue slurry containing high content of liquid (Cooling, D. J., 2007) is disposed in lagooning ponds, barriered by dam walls. The consolidation of solids is very slow. The water management is very complex due to the enormous surface, and the collection of rainfall run-off and water return to be processed back in the refinery is negligible (Davies, 2011). This method was implemented in all alumina plants until 1975 (Suchita Rai, 2012), and after that, few new alumina refineries adopted other technologies.
Bauxilum bauxite residue storage in Venezuela, source Google Maps (Bauxilum, 2019). Black area is the initial lagooning disposal for thickened residue up to 30% of solid. After modernization of the mud washing train, paste slurry residue with 55% was pumped to the red area, expanded section implemented to switching from lagooning to sloped deposition including a sand wall filtering the liquid (Adrian, 1996)
Dry disposal: filtered residue slurry with a solid content between 45 to 60 wt% is disposed in a pond, barriered by dam walls. Normally, mud is filtered at the alumina refinery using drums vacuum filters and trucked to the embarkment ponds. In a few refineries by the application of hyperbaric filters, the solid content has been increased up to 75 wt%. Consolidation of solids and water management are similar to wet disposal (Suchita Rai, 2012).
Sloped deposition: residue slurry with approximately 48-55% is pumped to the disposal and distributed forming layers to facilitate dry out and consolidation; allowing also for liquid collection.
Farming: residue slurry is pumped to certain sections of the deposit area forming farm galleries, allowing for evaporation of the liquid; plough machines are passed through (Cooling, D. J., 2007) moving the residue for additional evaporation and densification of the solids. At the edge of the farmed section, the solid content is up to 70% and towards the middle, it is close to 60% (QAL, www.qal.com.au, kein Datum).
Sea discharge: residue slurry is piped into deep sea water. This practice has been discontinued for newly installed refineries.
Queenland Alumina Limited bauxite residue storage in Australia, source Google Maps (QAL, Google Maps, 2019), (Benoit Cristol, 2018) for the paper. The left image shows the initial lagooning pond including seawater to neutralize the liquids before disposal to the surrounding. The right image shows the expanded storage area including also thickening and farming
Dry stacking: residue containing >70% of solids obtained via filtration in filter press, enables the deposition in compacted layers. Retention dam walls are not necessary. Proper design strategy must be considered to avoid re-slurry of the tailing during heavy rain periods and by action of the run-off water.
As shown in the pictures from Google Maps, many alumina refineries during their operational lifetime have switched to other storage modes, adapting or expanding existing installations (Suchita Rai, 2012). Nowadays, numerous operating alumina refineries have a combination of more than one storage method. The reason of it is that long-operating plants which have been expanded generate greater amount of mud than initially, suffering limitations of space, cities proximity, etc.
A summary of the design of dry storage is outlined in the paper Filtered Dry Stacked Tailing – The Fundamentals (Davies, 2011). Main considerations among others are: site conditions, topography and climate, water management for collection and for preventing contact of water with the stacked residue, transportation and logistics for distribution in certain disposal section, reclaim and closure; operational, including the sprinkler system to avoid fugitive dust, and, one of the most important, the geo-mechanical characteristic of the residue including the optimum moisture. The optimum moisture can be achieved by filtering the mud using filter press upstream of the disposal. The use of filter press has been forestalled due to lack of strategy for processing the massive amount of mud in a batch-wise process making this technology unaffordable for operators.
Simplified representation of the mud filtration using FFP filters for dry staking storage
The Outotec Larox® FFP membrane filter press has been developed to process bauxite residue, with the high throughput capacity, lower technical time for opening and closing for each filtration cycle which decreases the number of filters to be installed, operated and maintained, being the greatest differences of our filters over other filter presses available in the market. From process perspective, the FFP filters are able to reduce further the caustic concentration of the processed residue, via the efficient cake washing by displacing the mother liquid contained in the cake. Very low moisture – below 22 wt% – has been consistently obtained, due to efficient squeezing technology and drying out system. The excellent process performance of the FFP filters has been demonstrated in operation facilities validating Outotec-Larox approach. Pilot scale testing of the specific bauxite residue has established the basis for an optimized filter design and operation. Generally, the filter press performance depends on the bauxite residue characteristics inherited from the bauxite properties and the particular process of the alumina refinery. One very important property is the particle size, showing a reduction of filterability of the residue caused by the too fine particle size.
Cake moisture trend and leachable soda content with and without cake washing, Eti Aluminium (Sedat Arslan, 2018)
Particularly in the case of Eti Aluminium in Turkey, the bauxite residue has d50 of 2.86 µm and d90 is finer than 12.8 µm representing a challenge for the filterability. This was successfully solved with a strategical testing program carried out in cooperation with Outotec. The operating plant meet the expectations in terms of process parameters and the derived advantages reducing storage footprint, environmental impacts, valuable raw materials utilization and loss reduction (Sedat Arslan, 2018).
Eti Aluminium Residue Storage, Seydisehir, Turkey. Source Google Maps (Eti, Google, 2017), (Eti, Google Maps, 2019). The left image shows the initial lagooning pond, the right image shows the storage area 2019 after the installation of press filter
In conclusion, residue storage has its origins in the limitations of the technologies applied for the reduction of the liquid holds within the residue slurry, which is the reason of the residue storages implemented in the past. A key factor for implementing dry stacking technology to storage the residue, is the moisture content in the mud which it is important for assuring optimum compaction. Optimum moisture can be achieved overcoming the critical pressure of the mud fines particles by applying edge filter press technologies (Oeberg, 1996). Outotec-Larox have been designing FFP filters to produce optimum mud cake in terms of moisture and caustic concentration customized to any alumina refinery name capacity.
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Sonia Noack | Senior Product Engineer, Refinery
Sonia Noack is Senior Product Engineer for alumina refinery, where she is responsible for the process calculations and process design of integrated alumina refinery. Sonia has been working on product development for the red side, in particularly for mud separation, washing and filtration. In the white side, she is very experienced on hydrate filtration and calcination design. Aluminium hydroxide and alumina conveying within the calcination plant are the major subject of her investigation improving subsystem of the calciner. In the last years, her research plan is more focused on improvement of bauxite quality.