Quality pumps extract the best from hazardous by-products
22 Oct 2019
In the aluminium refining process, scrap is melted in rotary or reverberating furnaces under a bath of molten salt which floats on the metal surface. Molten aluminium and its salt cover are then tapped from the rotary drum surface.
The final salt mix tapped from the furnace contains residual aluminium metal (around 5%) and various metal oxides, mainly aluminium oxide. This mixture solidifies in pans to become salt slag, a hazardous waste that can be produced as a by-product of aluminium smelting, which must be disposed of under controlled conditions
Historically, in Europe, slag produced as an aluminium smelting by-product was landfilled but tighter regulations and high costs ended this practice, so now slag is recycled in dedicated plants.
RVA, located between Reims and Metz in northeast France, uses reprocessing technology that converts the slag from a waste stream to a source of essential raw materials. The plant is the only one of its type in the country, processing 110,000 tonnes of salt slag residue annually.
The process comprises four stages, two of which employ the Bredel pumps in the transfer of abrasive fluids
The operation relies on a closed-loop process that makes minimal demands on the environment: there is no solid waste; water used for washing is recirculated; gaseous emissions are incinerated to harmless residues; and ammonia gas is neutralised by dedicated scrubbers. A proprietary computerised control system monitors the production process to ensure that key variables remain within pre-defined limits, while outputs meet stringent specifications.
A number of its pumps are used to transport an abrasive oxide slurry, comprising a mix of liquid and solids which is also high in temperature. For this the company relies upon Bredel hose pumps supplied by Watson-Marlow Fluid Technology Group (WMFTG).
The process comprises four stages, two of which employ the Bredel pumps in the transfer of abrasive fluids. Firstly, salt slag is milled with optional recirculation to liberate aluminium using an eddy current separator, and iron via a magnet. Fine particulate from the mill plant is then removed by a de-duster device.
Next, the remaining salty material is introduced to a dissolution section where it is mixed with water (recovered later in the process). This brine-laden water is transferred by two high-flow Bredel 100 pumps into pressurised reaction vessels, before Bredel 65 and 40 pumps transfer aluminium oxide as a slurry to the reactors.
This aluminium oxide is very abrasive and, at this stage, high in temperature. Gaseous reactants are produced, including hydrogen, methane and ammonia, which are incinerated and exhausted from the stack. Energy from the waste gases is recovered for use in other parts of the process.
Residue from the reaction phase is then conveyed to a belt filter. Brine and water are pumped out under vacuum using a Bredel 80, leaving a solid residue known as Valoxy. Clean water – along with water removed at the dissolution stage – is used to wash the solids.
In the final stage, the effluent brine continues to the crystallisation section. Sodium chloride and potassium chloride are crystallised out of solution, initially separately and then in combination, to bring the salt mixture to the required specification. An in-line decanter increases the concentration of solids in the slurry, facilitating higher salt recovery. The final salt mixture is conveyed to storage bays for onward shipment.
Recovered aluminium granules/powders and salt are returned to the customer’s production process, while a third product, Valoxy, is a proprietary alumina-rich substance that can be used as an alternative to alumina and bauxite in applications such as cements, binders and bricks.