The Pidgeon Process
The existing Magnesium Plant (and its proposed expansions) use (will use) the Pidgeon process. The basic raw material supply and magnesium production process is as follows:
1. Mining and crushing of the dolomite: The dolomite is mined and crushed by the supplier at the quarry, and delivered to the Magnesium Plant in 15-25mm granules.
2. Calcination of the dolomite: The dolomite is calcinated by roasting it at 1,100-1,200ºC to remove the carbon from the magnesium carbonate compound in the form of carbon dioxide. The heat energy required is supplied by gas generated from on-site coal-to-gas converters. The chemical formula of the calcination is as follows:
MgCO3.CaCO3 (solid) → MgO.CaO (solid) +2CO2 (gas)[+Q]
The calcined dolomite is then delivered to a store house for cooling.
3. Making briquettes: The reducing agent, ferrosilicon, is crushed to below 10mm and is mixed with the calcined dolomite and fluorite powder to certain ratios controlled by a microcomputer. The mixed material is then milled by a ball miller to form powder of granules below 120[mm] and is delivered to airproof storage by conveyer belt. The mixed powder is compressed into a briquette and is delivered to reduction workshop for deoxidisation.
4. Deoxidisation: The briquettes are loaded into retorts (46 for each reduction chamber) and deoxidization takes place (under a vacuum) at temperatures of between 1,200-1,250°C. The process is performed under a vacuum to reduce the necessary reaction temperature and to avoid re-oxidisation of the magnesium. The heat energy required is supplied by gas generated from on-site coal-to-gas converters. The chemical formula of the deoxidisation is as follows:
2(MgO.CaO) (solid) +Si(Fe) (solid) → 2Mg (gas) + CaO.SiO2 (solid) + (Fe)(solid)
The magnesium gas, once cooled, forms solid “crowns” of crude magnesium crystals at the top of the retort. The deoxidization process takes about 10 hours. When finished, the raw magnesium is removed and is delivered to the refinery workshop for purification.
5. Refining and casting: After cleaning, the magnesium crowns are melted at a temperature controlled at between 670-700ºC, burning coal gas as the heat source. Using an Axial-Flow mixer, melted magnesium and flux is carefully churned for 5-10 minutes. Once purified, the temperature is briefly raised to 720ºC and then allowed to drop to 710ºC, at which time casting commences, using a transfusion pump, pouring machine and continuous casting machines to form solid magnesium ingots. The magnesium ingot is cleaned, finished, inspected, measured and packed while it is hot, and then delivered to the finished product storeroom for sale.