The Mannheim process is used to produce sodium sulfate. Sulfuric acid is reacted with sodium chloride within the Mannheim furnace. The furnace is a large cast iron kiln where the sodium chloride and sulfuric acid are first fed onto a stationary reaction plate where an initial reaction takes place. The stationary plate is up to 6 m in diameter. Rotating rabble arms constantly turn over the mixture and move the intermediate product to a lower plate. The kiln portion of the furnace is constructed with bricks that have high resistance to direct flame, temperature, and acid. The other parts of the furnace are heat and acid resistant. Hot flue gas passes up over the plates carrying out liberated Hydrogen Chloride gas. The intermediate product reacts with more sodium chloride in the lower, hotter section of the kiln producing sodium sulfate. This exits the furnace and passes through cooling drums before being milled, screened and sent to product storage facilities. The process involves two chemical reactions. In the first step sodium chloride and sulfuric acid are combined to produce sodium bisulfate and hydrochloric acid, an exothermic reaction that can occur at room temperature. The second step of the process involves an endothermic reaction, requiring energy input. A temperature of 600° to 700°C is required and maintained within the furnace to convert additional sodium chloride and the intermediate product to produce sodium sulfate and more hydrochloric acid. Based on public information about Mannheim Furnace production, about 80% of the production cost results from purchasing chemicals and 10% of the cost is for fuel (energy) and sulfuric acid.