Gypsum from phosphoric acid production

Among the naturally occurring sulfate ores, only gypsum has ever been of any importance as a raw material for sulfur production, and only in the direct production of sulfur dioxide for sulfuric acid manufacture. Byproduct calcium sulfate from phosphoric acid production (phosphogypsum) has been used to produce sulfuric acid. Elemental sulfur has never been extracted from sulfates on any significant scale because of high energy costs. Experimental work has been done on both chemical [721... 

Guillini A process for making gypsum from the waste product from the Wet Process for making phosphoric acid. The waste is heated with water in an autoclave this removes impurities and converts the calcium sulfate dihydrate to the hemi-hydrate. 

Koopman C, Witkamp GJ (2000) Extraction of lanthanides from the phosphoric acid production process to gain a purified gypsum and a valuable lanthanide by-product. Hydrometallurgy 58 51-60... 

Fihration - The function of the filtration step is to separate the gypsum (and any insoluble materials derived from phosphate rock or formed in the reaction) from the phosphoric acid product as completely, efficiently, and economically as possible. All modem plants use only continuous horizontal vacuum filters. 

Nitric Phosphate. About 15% of worldwide phosphate fertilizer production is by processes that are based on solubilization of phosphate rock with nitric acid iastead of sulfuric or phosphoric acids (64). These processes, known collectively as nitric phosphate or nitrophosphate processes are important, mainly because of the iadependence from sulfur as a raw material and because of the freedom from the environmental problem of gypsum disposal that accompanies phosphoric acid-based processes. These two characteristics are expected to promote eventual iacrease ia the use of nitric phosphate processes, as sulfur resources diminish and/or environmental restrictions are tightened. 

Gypsum is also obtained as a by product of various chemical processes. The main sources are from processes involving scrubbing gases evolved in burning fuels that contain sulfur, and the chemical synthesis of chemicals, such as sulfuric acid, phosphoric acid, titanium dioxide, citric acid, and organic polymers. 

The raw material should contain at least 50 per cent, of Ca3P208 and be as free as possible from sesquioxides. It may be ignited if high in organic matter, reduced to a fine powder, and fed continuously into tanks lined with wood or hard lead alloy, where it meets on the counter current principle hot sulphuric acid of about 5 per cent, concentration. The reaction is quickly completed and the precipitated calcium sulphate is allowed to settle and filtered off continuously through filter presses. This sulphate is phosphatic gypsum and contains 3 to 4 per cent, of phosphoric acid of which 1 per cent, is soluble in water. The solution is evaporated in wrought-iron pans up to a concentration of 50 per cent, phosphoric acid, which may be further refined for use in pharmaceutical products or foods. 

Coupled with the energy consumption is the environmental effect from production of these binders. For every ton of phosphoric acid, 5 ton of phosphogypsum (calcium sulfate) is produced [15]. This waste can be recycled into value-added products such as gypsum board, but often there is a radioactivity issue. Some phosphogypsum contains radium and emanates radon gas. At the present time, such waste can only be disposed in a landfill. 

This wash sequence is repeated from first once to three or four times, each time using the filtrate from the subsequent wash stage as the wash liquor, until the last stage where pure water is used. In this way, the gypsum is virtually freed of adhering residual acid and in the process the countercurrent reuse of the wash water also minimizes the amount of dilute phosphoric acid to be incorporated into the product. Thus, the product acid concentration is kept as high as is feasible. 

A typical downstream process includes (1) removal of production microbes (biomass) and solids (e.g., gypsum) from the broth, (2) recovery of crude lactic acid, and (3) purification of lactic acid. The biomass and solid waste can be separated from the liquid streams by various means, such as filtration, centrifugation, and decantation. If calcium alkali is used to control the fermentation pH, it produces calcium lactate precipitates which must be dissolved by acids such as phosphoric or sulfuric acid to extract lactic acid back into solution. After sulfuric acid has been added, calcium sulfate (CaS0 -2H20, known as gypsum) is formed and must be removed from the liquid stream as a major solid waste. 

Reaction System - There are so many types of reaction systems in use throughout the world that no attempt will be made to identify all of them, The objective in designing the reaction system is to carry out the reaction between phosphate rock and sulfuric acid so as to recover a maximum percentage of the P2O5 from the rock as product phosphoric acid in the simplest and least expensive manner. Since the filtration step is the most critical and expensive step in the process, a primary objective in the reaction step is to form gypsum crystals of such size and shape that the filtration and washing can be carried out rapidly and efficiently. 

Frank and Hirano (1990) survey the potential for the production and consumption of alternative, usable, commercial byproducts in conjunction with a major reduction in national emissions of SO2 and NO,. Hiey conclude that the potential byproduct yields from the U.S. acid rain control program greatly exceed available markets for the chemical products. Byproducts evaluated in the study include gypsum, sulfuric acid, ammonium sulfate, ammonium sulfate/nitrate, and nitrogen/phosphorous fertilizer. Henzel and Ellison (1990) present a review of past, present, and potential future disposal practices and commercial FGD byproduct utilization. Hiey indicate that the only discemable trend is the production of usable gypsum by wet FGD systems. The 1990 Clean Air Act Amendments may create a need for disposal sites, which tend to be expensive and scarce and which could in themselves be environmental problems. Systems that produce usable byproducts are expected to become more important in the future as the disposal option becomes less viable. 

---