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Phosphorus

Phosphorus was discovered by German alchemist Hennig Brand in 1669 through a preparation from urine, which contains considerable quantities of dissolved phosphates from normal metabolism. Working in Hamburg, Brand attempted to create the infamous Philosopher's stone through the distillation of some salts by evaporating urine, and in the process produced a white material that glowed in the dark and burned brilliantly. Since that time, phosphorescence has been used to describe substances that shine in the dark without burning.

Phosphorus was recognized as a chemical element at the emergence of the atomic theory that gradually occurred in the late part of the 18th century and the early 19th century, and was formulated by John Dalton.

Phosphorus was first made commercially, for the match industry, in the 19th century, by distilling off phosphorus vapor from precipitated phosphates heated in a retort. The precipitated phosphates were made from ground-up bones that had been de-greased and treated with strong acids. This process became obsolete in the late 1890s when the electric arc furnace was adapted to reduce phosphate rock.

Phosphorous
Phosphorous

Phosphorus occurs in nature only in the form of the salts of phosphoric acid. From these it is obtained by reduction. There are three allotropic forms of elementary phosphorus: white, red and black. Of these, black phosphorus is the form that is most stable at room temperature; it is obtained from the white form by the application of high pressures.

Black phosphorus is mainly of scientific interest, whereas the other two allotropic forms are technically important. White phosphorus melts at 44.1°C, when finely divided, reacts with atmospheric oxygen even at room temperature. Red phosphorus is obtained from white phosphorus by heating the latter in a closed container.

Phosphorus is prepared by heating calcium phosphate with carbon and silica (SiO2) in an electric furnace. The reaction is represented by the following equation:
2Ca3(PO4)2 + 6SiO2 +10 C -> 6Ca SiO4+P4+10 CO

The phosphate is fed to the furnace in lump form. To make them suitable for processing in this way, finely granular phosphates first have to be agglomerated by palletizing. Agents used for binding the particles together into pellets are soda water glass (sodium silicate), Cottrell dust obtained from electrostatic precipitation processes and other admixtures, which are intimately mixed with the phosphate grains in a screw mixer and then palletized in a revolving pan. The pellets are transformed to firm, hard balls (nodules) by sintering at high temperatures in rotary kilns or on special sintering grates. The nodules, mixed with coke and silica pebbles, are fed to the furnace.

The three-phase electric furnace consists of a steel tank of which the bottom part is lined with hard-burned carbon blocks and the top part with fireclay bricks. At the bottom are two tapholes for tapping the ferrophosphorus and the slag respectively. The furnace cover is provided with openings for the three electrodes, the feed pipe and the gas outlet. The electrodes are made of carbon and are fed from above at a rate sufficient to compensate for loss by burning.

The gas discharged from the furnace, consisting of phosphorus and CO, is passed through Cottrell-type electro-static dust precipitators (dust filters) in which the dust in the gas is trapped and collected. These precipitators are heated to prevent condensation of phosphorus inside them. The dust is returned to the sintering plant.

The exit gases, which have a temperature of 250° –350° C, are passed to Stroder washers in which the phosphorus is condensed. The white phosphorus obtained in this process is stored under water to prevent the spontaneous combustion that results from contact with air. The CO gas that is discharged from the washers is utilized for heating the sintering plant and steam boilers or is burned at the top of high flare stacks.

Phosphorus is used in the manufacture of detergents. The plastics industry uses phosphorus-based plasticizers. Red phosphorus is employed in the friction striking surfaces on match boxes.

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