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case study: Queensland alumina ltd

Queensland Alumina Limited (QAL) commenced operation in March 1967 with an annual production rate of 600,000 tonnes of alumina. Today QAL is one of the world’s largest alumina refineries, producing 3.95 mtpa of the world’s best smelter grade alumina. With an average workforce of 1,050 employees, QAL is the region’s largest employer and

annually contributes approximately $400 million directly to the local economy.

Processing

The QAL refinery at Gladstone can produce up to 800m3/h of red mud slurry, derived from digesting the aluminium containing ore, bauxite, with sodium hydroxide. After washing, the red mud residue is pumped to the Residue Disposal Area (RDA) on Boyne Island to be stored pending neutralisation of any remaining caustic soda.

To avoid expansion of their existing dam facilities, yet address high slurry volume and increasing environmental and availability demands,QAL implemented a complete refurbishment of the original disposal area as part of their plan to increase the available dam capacity for a further fifty years of operation.

QAL seawater neutralisation Facility

Organisation:Queensland Alumina Ltd (QAL)

Site:QAL Seawater Neutralisation Facility, QLD, Australia

Year:2008

Application:Seawater neutralisation of bauxite residue (red mud)

Challenge:Lump sum turnkey installation of seawater neutralisation plant

Solution:Vane Feedwell™ technology; strong partnership approach to optimise customers design concept

Key benefits:Red mud neutralised and �concentrated for semi-dry-stacking

Small footprint clarifier �

Consistently clear water overflow �meeting specific chemistry

Zero harm to the environment �

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separation plant, with the process itself designed by QAL. This scope included civils and all remote site facilities (ie office and maintenance space). Being remote from the main refinery, the entire infrastructure, had to be delivered to the RDA site and also designed to be as self-managing as possible, requiring minimum operator activity and controlled from the refinery.

With regards to the clarifying process, the customer initially considered two clarifiers, however, Outotec’s testwork and design modelling indicated that a single clarifier would meet requirements. Further design input from Outotec included the use of gravity systems for all slurry forwarding.

chALLenges

The neutralisation step produces a low specific gravity precipitate (hydrotalcite), which can cause adverse solids compaction and reduce the bulk density of the neutralised, thickened mud. To effectively thicken this mixture

and produce a clear overflow, the clarifier required a feedwell capable of maximising the entire volume. The feedwell also had to ensure minimum breakage during the mixing of flocculant and mud/aggregate particles.

Outotec had just recently patented its Vane Feedwell™ design, which was the first major feedwell innovation since the 1990s. This design has seven global patents and incorporates unique innovations to maximise performance.

VAne FeedweLL™

To accommodate the high energy of the incoming feed whilst maintaining adequate mixing of dilution liquor and flocculant, the new concept of axially sloped vanes, coupled with a radially sloped feedwell, was proposed. This design also provided a low energy zone and low shear necessary for aggregate growth.

CFD modelling indicated an increased hold-up of the high density

Seawater

seawater

Neutralised mudCLARIFIER

NEUTRALISATIONREACTOR

Red mud Flocculent

Magnesium deficient

Neutralisation with seawater, combined with semi-dry stacking was elected as the safest method to allow long term storage of the red mud residue. However, as the process can use up to 5000m3/h of seawater, a dedicated seawater neutralisation plant was required to provide continuous supply.

The neutralisation process utilises the magnesium content in the seawater to react with the caustic content in the red mud to form insoluble salts, thus reducing liquor phase alkalinity and soluble aluminium. The neutralised red mud and liquor are then separated into a slurry that is sufficiently concentrated to allow semi-dry stacking of the solids and a clean water stream, sufficiently low in suspended solids, dissolved aluminium and at the correct pH for discharge to the environment.

The scoPe

Outotec’s scope was the delivery of a lump sum turnkey (LSTK) seawater neutralisation and

Seawater neutralisation process

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to uniformly discharge under low shear conditions.

CFD modelling of the flow pattern demonstrated a far improved retention of solids concentration with the modified feedwell. A new Outotec Vane Feedwell™ was thus retrofitted to the clarifier and had the immediate desired impact with consistently improved underflow density and overflow clarity.

FooTPrinT

Industry demands for lower capital and maintenance costs have resulted in the trend towards smaller footprint clarifiers capable of handlinglarge slurry flowrates. The VaneFeedwell™ clarifier at the seawater neutralisation facility is a single unit with a relatively small footprint. The red mud from Queensland Alumina, however, is produced via three parallel banks of counter current washers.

adsorption, reducing coarse/ fines segregation, ensuring all particles are aggregated together by the flocculant. The lower zone promotes gentle mixing for continued aggregate growth, with the option for secondary flocculant dosing. This zone also enables aggregates

feed while removing energy from the feed stream and effectively splitting the feedwell into upper and lower zones. The upper zone into which feed, dilution water and flocculant are added provides enhanced mixing and energy dissipation. This promotes maximised flocculant

How the Vane Feedwell™ works

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susTAinAbiLiTy & enVironmenT

QAL’s long term residue disposal plan involves achieving and maintaining a sustainable operation with zero harm to the environment and the semi-dry stacking process used at the facility was chosen for this reason.

The alkaline liquor phase of the red mud, (originally at a level of pH 14), is neutralised and lowered to pH 9 and is therefore harmless and safe for release into the surrounding estuary. Water quality is monitored routinely, not only by QAL, but also by the Port Curtis Integrated Monitoring Program (PCIMP) group, who carry out independent testing of the surrounding estuarine waters.

If you would like more information click here

to contactroger.bligh@outotec.com

resuLTs

Outotec interpreted the customer flowsheet to deliver a robust design to meet the customer’s stringent performance and availability requirements and also capable of complete remote control from the refinery itself.

Outotec’s focus on cost minimisation was achieved by using gravity forwarding systems where possible and via a high availability single clarifier which, through efficient flocculant techniques, negated the need for an additional unit, so reducing the footprint. With Vane Feedwell™ technology, this single clarifier is capable of separating 5800m3/h of neutralised red mud

whilst also achieving a consistent and desirable underflow density of 340g/l.

The LSTK project has successfully demonstrated that large scale seawater neutralisation of red mud, with an environmentally acceptable discharge, is achievable. The new plant has enabled QAL to continue utilising their existing containment areas and improve the vegetation and environmental conditions in the area.

Meet Outotec at Stand 21 & 23, Grand Chancellor Hotel,

Hobart, TasmaniaOctober 29-31st, 2012

11th AusIMM Mill OperatOrs’ COnferenCe 2012

Quick tipdesigning mill foundationsSome factors the engineer should consider include:

Mill supplier data �Loadings and directions of loadings are easy to misinterpret.Local conditions �Small inaccuracies in strata data can dramatically compromise mill performance.Outside influences �Vibrations from other equipment, and adjacent mills in particular, can accumulate to detrimental effect.