Gypsum

Gypsum is a key ingredient in the wallboard, plaster, cement and agriculture markets. It appears as a white or colourless mineral and is composed of calcium sulfate dihydrate.

How gypsum is mined
Gypsum is mined at Lake MacLeod using a floating dredge. The dredge pumps a slurry of gypsum and brine to a cluster of classifying cyclones that remove ultra fine material from the slurry and returns it to the lake.

The remaining slurry is pumped to a leaching stockpile near the shore of the lake. The stockpile is leveled to create a flat surface on which is placed a sprinkler system. Water from the sprinkler then permeates through the heap, leaching out unwanted salts. Potable water created on site using a reverse osmosis plant is used in the final stage of the leaching process.

Dampier Salt undertakes intensive quality control, using its on-site laboratory, to closely monitor the levels of chloride and other chemicals of the gypsum. At the end of the leaching process, the gypsum will have reached a very low level of chloride content, making it suitable for shipment to the customer.

The gypsum is then loaded into 240 tonne capacity haul trucks and transported to the dedicated gypsum stockpile at the port of Cape Cuvier, a deep-water port capable of accommodating vessels of up to 100,000 tonne.

Uses for gypsum
Natural gypsum has the following key uses:

• Wallboard: Wallboard is used as a substitute for plastering and is a large sheet of material made of paper or similar material with a hardened gypsum core. Quality requirements of gypsum by wallboard manufacturers are typically very high and Dampier Salt's Lake MacLeod gypsum operation was established using the wallboard industries' quality requirements as the fundamental benchmark. This enables Lake MacLeod's Gypsum to be competitive, quality wise, on a worldwide basis. 
• Plaster: Natural gypsum is also the main component in the manufacture of plaster compounds, including plaster mouldings for internal decorations in buildings and certain wall coatings. Plaster is generally made by mixing sand, water and a binding agent such as gypsum. "Plaster of Paris" is the common name for calcined gypsum. 
• Cement: The cement industry also requires large quantities of natural gypsum as a setting retardant. The quality specifications for natural gypsum in cement manufacture is less demanding than in wallboard manufacture and Lake MacLeod gypsum readily meets the quality specifications in this market. 
• Agriculture: Lake MacLeod's natural gypsum is useful as an additive in heavy soils to improve soil structure and water penetration. The high quality, low moisture content and very low sodium chloride levels of Lake MacLeod's gypsum offers enormous benefits to farming/soil treatment and associated applications. The lack of impurities allows extensive soil conditioning with little or no negative effect on the actual soil structure. In comparison to some chemical fertilisers, natural gypsum is regarded as an effective low cost source of sulphate sulphur for crops such as canola.

Gypsum is also used in tennis court construction, beer production, pottery plaster, toothpaste, food additives, paint manufacture, casts and sculpture.

Gypsum from Lake MacLeod is of a high purity level, offering distinct advantages for Rio Tinto customers' production processes, whether in wallboard, cement, plasters or agriculture. The finer particle sizing of the gypsum from Lake MacLeod offers significant capital and operating cost saving potential, as expensive primary stage crushing facilities may not be required. The Lake MacLeod gypsum also exhibits minimal dust generation problems.

For more information on Rio Tinto's gypsum operations, please visit the Rio Tinto Minerals website. For more information on our other products, see our corporate fact sheets.