Acid Sulphate Soil Treatment
Acid sulphate soil (ASS) is the common name given to soils and sediments containing iron sulfides, the most common being pyrite. When exposed to air due to drainage or disturbance, these soils produce sulphuric acid from reacting with oxygen, often releasing toxic quantities of heavy metals to the wider environment.
Excavation of potential acid sulphate soils (PASS) can lead to the creation of actual acid sulphate soils (AASS). To reduce the possibility of potential acid sulphate soils becoming actual acid sulphate soils, there are four main management techniques used to minimise this possibility and its effects.
- Minimising disturbance
- Reclaiming land
- Water cover
In many cases minimising disturbance is not possible especially in relation to construction projects. Reclaiming land and water cover are based on removing the possibility of oxygen coming into contact with the sulphate within the soil, but in many circumstances is not possible. Therefore neutralisation is the only management technique available in many situations where acid sulphate soil is present.
The way in which acid sulphate soil is neutralised is relatively simple in that a highly basic material, such as lime, is mixed with the acid sulphate soil. The amount of lime required to be mixed with the acid sulphate soil is determined by analysing a sample of soil to calculate actual acidity and relating the amount of lime required to be added to effectively neutralise the soil.
In many cases of acid sulphate soil treatment, agricultural lime is used to neutralise the soil. However, there are other reagents available, which are effective such as hydrated lime, mixtures of hydrated lime and agricultural lime, and also lime slurries, which are added as a liquid.
The McMahon Services Solution
McMahon Services as a specialist treatment contractor provides the equipment and know-how in treating acid sulphate soil using a range of products and reagents. We are able to do this by having an understanding of the technical requirements of treating acid sulphate soil, as well as using state-of-the-art treatment equipment.
Treatment of ASS centres on the Komatsu Reterra BZ210-1 G-Mode, a specialised soil recycler. The Reterra is specifically designed to handle heavy marine sediments and accurately dose with a range of reagents. Reagents can range from dry hydrated lime, to wet agricultural lime as well as slurried reagents. Dosing hoppers and liquid manifolds are used to add the reagents with the ASS, all designed for maximum efficiency.
The G-Mode model offers a dramatically improved and increased workload capability and soil improvement efficiency, and most importantly has set a new benchmark for treating acid sulphate soils.
The ASS and reagents are mixed in a chamber consisting of a soil cutter and three flailing hammers, then discharged via another soil cutter. This ensures there is effective mixing between the neutralising reagent and the sulphur compounds containing within the ASS. The whole process is controlled with an on board computer system, which monitors throughput of reagents and soil to ensure volumes of material processed are accurately known.
McMahon Services has showcased its ability to treat ASS with numerous trials undertaken for a range of clients. This has led to the engagement by the Fulton Hogan McCosker Joint Venture for the treatment of approximately 20,000 cubic meters of ASS on Curtis Island (QLD) as part of a LNG development.