4.6. Waste products of phosphorus production. The characteristic of departing gases, dust, sludge and slag. Circuits of their clearing, neutralization and recycling.
As raw material for phosphorus production apatite and phosphorite ores serve. Apatitys are named the minerals of general formula Me10X6O24Z2, where Me - Ca, Mg, Sr, Ba, Mn, Na; X - P, Si, C; Z - F, Cl, OH. It is most distributed fluoroapatite Ca10P6O24F2 or Ca5P3O12F. A continuous number of isomorphic replacements in system fluoroapatite - hydroxylapatite – carbonateapatite is known: Ca10P6O24F2 – Ca10P6O24(OH)2 – Ca10P5CO23(OH)3.
Phosphorites represent fine-crystal sedimentary products of these replacements. Except for apatite minerals phosphorites contain carbonates, quartz, aluminates and silicoaluminates, pyrite, plaster and organic substances.
Phosphorites are basically applied to phosphorus production, apatites serve as the basic raw material for manufacture of wet-process phosphoric acid. The largest deposit of phosphorites is Karatau field - the basic raw-material base of the phosphoric industry of our country.
Total reaction of threecalciumphosphate reduction by carbon and phosphorus sublimation is expressed by the equation:
Ca3(PO4)2 + 5C = P2 + 5CO + 3CaO - Q (16)
During sublimation concurrent reactions are taken place as a result of which are formed: CO2, H2, H2S, PH3, SiF4 and other substances. Gas leaving the furnace contains 4-10 % (volumetric) Р4, 65-85 % CO, 0,5-1 % SiF4 and other products. Gas is cleared from a dust (burden) in electro-filters. Dust particles contain adsorbed phosphorus. Phosphorus-containing dust is removed from bunkers of electro-filters by water, as a result waste water, so-called «kottrel milk», is formed. From electro-filters gas is given in condensers of phosphorus where it is washed out by water and is cooled thus up to 600 C. Condensed liquid phosphorus gathers under water in collections. In some cases before disposal of waste gases in an atmosphere the additional gas clearing by water adsorption is provided.
In baths of condensers the constant water level is supported. In process of phosphorus accumulation the polluted water is drained in the sewage collection. The basic impurity of sewage of phosphorus manufacture: the suspended substances, Р4, Р2О5, F-, SiO32-, cyanides, sulfides, phenols phosphine.
Phosphorus-containing sewage is formed also at washing of phosphorus in storehouses and sediment bowls, washing of tanks, etc.
For 1 t phosphorus manufacture is required about 500 m3 of waters, from them 475-480 m3 of waters is spent for cooling of products. 20-25 m3 of water are spent for the technological purposes, and the basic amount of this water is in circulating system. The quantity of the polluted sewage, containing elementary phosphorus, makes 5-8 m3 on 1 ton of phosphorus. Elementary phosphorus is been, basically, as the suspended particles as solubility of phosphorus is small.
Manufacture of dry-process phosphoric acid includes the following processes: 1) burning of liquid phosphorus; 2) cooling of gases; 3) hydration and absorption of phosphorus oxides; 4) condensation of dry-process phosphoric acid and catching of phosphoric acid fog. Sewage of dry-process phosphoric acid manufacture is usually cleared together with sewage of phosphorus manufacture.
Sewage treatment provides: 1) sewage neutralization; 2) phosphorus extraction with the aim of reduction of industrial losses. As phosphorus is in water as suspended particles together with sludge particles the mechanical clearing is applied. Sediment bowls and hydro-cyclones are used for separation of phosphorus-containing sludge particles. The circuit of clearing includes a sediment bowl (time of settling is 1 hour) and two hydro-cyclones in diameter of 75 mm. This circuit of clearing provides 80-85 % clarification.
To increase the efficiency of sedimentation process coagulates are applied (aluminium sulfate, iron (II) chloride) and flocculants (polyakrilamide). Processing of waste water by aluminium sulfate (25 mg/l) raises efficiency of waste water clearing up to 98 %. Application of polyakrilamide (20 mg/l) raises productivity of neutralization stations approximately twice.
As a result of mechanical sewage treatment phosphoric sludge is formed, it contains 10-30 % of phosphorus. Sludge is given on burning or on installation of distillation.
For sewage treatment from phosphorus the methods based on oxidation of suspended and dissolved particles of phosphorus by oxygen of air, chlorine and other oxidizers are developed.
At oxidation by oxygen of air:
P4 + 5O2 + 6H2O = 4H3PO4 (17)
At oxidation by chlorine:
P4 + 10Cl2 + 16H2O = 4H3PO4 + 20HCl (18)
H3PO3 + Cl2 + H2O = H3PO4 + 2HCl (19)
The researches have shown, that the maximal rate of oxidation is received at рН = 7-8. It is necessary to note, that in the alkaline environment the reaction of phosphine formation from elementary phosphorus intensively proceeds. Phosphine is the toxic and explosive gas. Therefore neutralization of sour phosphorus-containing sewage should be carried out cautiously, not supposing the formation of the alkaline environment (рН no more than 7,5-8).
Now the circuits of phosphorus-containing sewage clearing are developed and introduced into manufacture. These circuits allow reuse the cleared sewage in manufacture of phosphorus and phosphoric acid.
One of these circuits is shown in figure 4.10. In neutral or low-alkali waste water coagulate (aluminium sulfate) is added in quantity equal 25 mg/l. Then water is given in sediment bowls where there is separation of phosphorus-containing sludge. After that in waste water limy milk is entered, and the suspension formed in a reactor is separated from water in the following sediment bowl. Before the feed of water in the sediment bowl in water polyakrilamide is dosed. The clarified waste water is drained in the collection from which the part of water is given on a reuse, and other part - in aeration column for removal of toxic gases (phosphine, etc.). The cleared waste water also is reused in manufacture. The carried out check has shown, that impurity of the cleared waste water do not render negative influence on technological processes and products of manufacture. Accumulation of soluble impurity in the closed cycle of water use does not occur, as a plenty of water is spent on slag granulation, wash-out of a dust from electro-filters, and also is deduced with sludge.
On the basis of the received experience the basic circuit of "non-sewage" phosphorus production is developed. This circuit provides use of fresh water (from a reservoir) only for replenishment of circulating water supply system (for cooling of devices), and also for water supply of boiler-house, laboratory and for household needs.
For replenishment of closed water supply systems, for wet gas-dust cleaning and technological processes blowing water of circulating water supply system and sewage of laboratories is used.
The given circuit is introduced into the industry and allows to exclude completely the disposal of toxic phosphorus-containing sewage in reservoirs.
Firm waste products of phosphorus production are slag and ferrophosphorus. The calcium silicates forming slag are applied in the building materials industry: in manufacture of a silicate brick, slag pumice, slag gravel, cement. Ways of slag glass ceramics production are developed.
Ferrophosphorus is applied in manufacture of individual kinds of steel, in manufacture of pig-iron castings. Many methods of ferrophosphorus processing on phosphates, phosphorus chlorides, etc. are developed.
Fig. 4.10 – The circuit of installation of sewage treatment of phosphorus production: 1, 3 - sediment bowls, 2 - a reactor, 4 - the collection of the cleared water, 5 – an aeration column