The present invention relates to an improved process for recovering elemental phosphorus from phosphorus-containing waste material, and more particularly from the phosphorus-containing waste material formed during the production of elemental phosphorus by the smelting of phosphate rock.
The basic method for producing elemental phosphorus is accomplished by the reduction of phosphate rock with coke or other carbonaceous reducing agents in the presence of silica. This is referred to as the furnace "charge" or "burden". The phosphorus production is generally carried out in an electric furnace at a reaction temperature of about 1,400.degree. to about 1,500.degree. C.
The mechanism of the reduction of phosphate rock to elemental phosphorus is quite complex and the exact path of the reaction sequence has not been conclusively defined. The overall reaction is generally represented by the following simplified equation: EQU 2Ca.sub.3 (PO.sub.4).sub.2 +6SiO.sub.2 +10C.fwdarw.6CaSiO.sub.3 +10CO+P.sub.4
During the course of the reaction, the phosphorus produced vaporizes, rises, and is cooled, condensed, and collected under water. The phosphorus vapor is generally accompanied by carbon monoxide and appreciable quantities of entrained dust comprising phosphate burden, fluorine, calcium oxides, potassium oxides and the like. The dust and vapor mixture can be passed through an electrostatic precipitator where most of the dust is removed prior to cooling the phosphorus vapor. The carbon monoxide gas can be recovered for use as fuel or properly disposed of in accordance with pollution requirements. Solid furnace residue comprising calcium silicate is drawn off from the bottom of the furnace as a molten liquid. Iron phosphide or "ferrophosphorus" formed from the iron impurities present in the phosphate ore is also drawn off as a melt from the bottom of the furnace.
The condenser drains into a sump wherein the phosphorus product is collected. Three separate layers generally form in the condenser sump.
A layer of relatively high grade phosphorus is obtained at the bottom. The intermediate layer is a mixture referred to as "sludge", which consists of phosphorus droplets or globules, solid impurities, and water. Above the phosphorus-containing waste material layer is a water layer. The boundary between the phosphorus-containing waste material layer and the water layer is not clearly defined.
The amount of phosphorus-containing waste material produced will vary, depending upon factors such as the initial composition of phosphate rock charged, the operating conditions and design of the furnace. Phosphorus-containing waste material can contain from about 5% to about 90% by weight elemental phosphorus. The phosphorus content of the phosphorus-containing waste material produced can vary from about 10 to about 60 weight percent or more, of the furnace output of elemental phosphorus.
The phosphorus-containing waste material, or "sludge" as the term is used in the art and herein, appears to be a poorly defined emulsion containing solid impurities, water and phosphorus in widely varying proportions and having a density between the density of phosphorus and the density of water. The phosphorus-containing waste material can have the characteristics of a "phosphorus in water" type emulsion, that is, the phosphorus being the discontinuous phase and the water being the continuous phase, or a "water in phosphorus" type emulsion, that is, the water being the discontinuous phase and the phosphorus being the continuous phase. Microscopic examination of the phosphorus in water type phosphorus-containing waste material shows that the phosphorus is present in small globular particles which will not coalesce. The size of the particles are generally in the range of sub-micron to millimeter, and larger dimensions.
Various methods for recovering elemental phosphorus from phosphorus-containing waste material have been suggested in the prior art. Among these are physical methods for separating the phosphorus, such as filtration, distillation, stirring and settling, vibration, centrifuging, extractions, electrolysis, and the like.
Presently, phosphorus produced in electric arc furnaces can be further processed by batch filtration techniques. The batch filtration method consists of performing several filter runs on production mud, transferral of remaining material through various tanks (allowing additional settling of phosphorus), then refiltering again. Prior to each of the three runs, diatomaceous earth is used to precoat the filters.
The resulting unfilterable phosphorus-containing waste material is then further processed usually by thermal means such as a "mud" furnace, roaster, or recycled back to the electric furnace to evaporate the phosphorus contained in the sludge as well as reversion of red phosphorus to yellow phosphorus and its subsequent evaporation. The resulting evaporated/condensed phosphorus will then be recycled to the batch filters. The following analogy will incorporate the use of a roaster to thermally treat the sludge although the following disadvantages listed will be analogous to all the before mentioned methods of thermally treating sludge.
The roasting operation has become more expensive due to rising energy costs. It is also hazardous due to pressures generated inside the roaster. In addition, roasting leads to high P.sub.2 O.sub.5 emissions, a pollution problem. Other disadvantages are: the average batch filtration efficiency is 60% by volume; the present system involves considerable transfer of phosphorus bearing material among various tanks and its inherent associated risks; and the relatively low filtration efficiency results in an undue load to the roaster operations. Other methods for treating the phosphorus-containing waste material include burning it and making low grade phosphoric acid.
U.S. Pat. No. 3,684,461 discloses a process for treating the waste water containing phosphorus-containing waste material obtained in the electrochemical production of elemental phosphorus. The disclosed process comprises filtering the water in a filtration zone and drying the filter cake a plurality of times. Gaseous and vaporous matter recovered from the drying zones are conveyed to a condensation zone where the phosphorus and water contained therein is condensed and recovered.
The present invention achieves the recovery of phosphorus values from phosphorus-containing waste material by transporting the phosphorus-containing waste material from a contamination site, preconditioning the transported phosphorus-containing waste material, and sizing the solid constituents of the phosphorus-containing waste material as well as homogenizing the stream and filtering to obtain the phosphorus values therefrom.