Control questions on the topic “Waste products formed at caustic soda manufacture and methods of their processing”
Application field of caustic soda.
Which chemical methods of caustic soda production you know?
Which electro-chemical methods of caustic soda production you know?
Call basic stages of the mercury cathode method.
Call advantages and disadvantages of this method.
Kinds and characteristic of the waste products formed at the caustic soda manufacture.
Way of gas purification from mercury-containing compounds.
What substances can be used as adsorbents for removal of mercury from gases?
Explain basic stages of the sulphide method of waste water treatment from mercury-containing compounds.
Which kinds of synthetic pitches can be used at ion exchange treatment of mercury-containing waste water? Regeneration methods.
4.4. Waste products of acid processing phosphate raw material. Removal of fluorine compounds. Waste water treatment. Processing solid waste products (phosphogypsum, limy sludge).
At manufacture of wet-process phosphoric acid and phosphoric fertilizers the waste water polluted with fluorine compounds are formed.
Conditions of waste water formation. The extractive sulphuric acid way of phosphoric acid production consists in the decomposition of natural phosphates with sulfuric acid and separation of the solid phase formed – calcium sulfate – from the phosphoric acid solution:
Ca5F(PO4)3 + 5H2SO4 = 5CaSO4 + 3H3PO4 + HF (7)
The phosphoric acid produced by this way has low concentration (25-32 % of Р2О5), therefore it is evaporated up to higher concentration. Depending on the process temperature and concentration of the phosphoric acid obtained the calcium sulfate formed can be separated as dihydrate CaSO4·2H2O (gypsum), half-hydrate CaSO4·0,5H2O and anhydrite CaSO4 (anhydrous salt).
Depending on it three ways of wet-process phosphoric acid production are distinguished: dihydrate (CaSO4·2H2O), half-hydrate (CaSO4·0,5H2O), anhydrite (CaSO4) methods. The dihydrate way is the most widespread; it provides obtaining the phosphoric acid at 65-800С contained 25-32 % of Р2О5.
Fig. 4.4 – Basic stages of wet-process phosphoric acid production
The dihydrate process of wet-process phosphoric acid production includes the following stages:
Decomposition of an apatite concentrate with sulfuric acid in an extractor.
Cooling a pulp in a vacuum-evaporator and removal of fluorine compounds from water steams and gases in a washing tower. The washing tower is irrigated with a circulating hot H2SiF6 solution. Then the water steam is condensed in a barometric condenser irrigated with water. The water from the barometric condenser is discharged in a sewer system.
Fig. 4.5 – A scheme of two-level absorption of fluorine-containing gases: 1 – a superphosphate chamber; 2, 3 – absorption chambers; 4 – a spray catcher; ______ gas; -------- H2SiF6 solution, _________ H2SiF6 recirculation
3. Filtering the pulp on a vacuum-filter. The formed calcium sulfate layer (phosphogypsum) is washed out with water three times. On 1t of Р2О5 it is formed 5,8-6,0 t of damp phosphogypsum.
4. Concentration of the wet-process phosphoric acid obtained to 50-55 % of Р2О5 by means of evaporation (usually under vacuum).
5. Absorption of fluorine-containing gases and obtaining fluosilicic acid.
For full removal of the waste water formed at the wet-process phosphoric acid and phosphoric fertilizer manufacture the most rational decision is their purification with the purpose of reuse.
The waste water of wet-process phosphoric acid and phosphoric salt enterprises contains phosphorus pentoxide Р2О5 and fluoric compounds. Р2О5 presents in a water-soluble form and as suspended particles of initial phosphatic raw material, and fluorine – mainly as fluosilicic acid H2SiF6.
Treatment of such the waters is carried out by means of neutralization by limy milk with the subsequent coagulation of the deposit formed with help of polyacrylamide. As a result of the waste water treatment the sludge is formed, which contains fluorine and phosphorus compounds. Besides, circulating water flows are not completely consumed; therefore unbalanced flows arise, which demand additional purification before discharge in reservoirs.
Fig. 4.6 – A schematic diagram of a neutralization installation of fluorine-containing waste water by chalk: 1, 6 – bunkers; 2 – a conveyer; 3 – a storage of chalk; 4 – a clamshell crane; 5 – a collector-balancing tank; 7 – a reactor; 8 – a fan
Thus, at the treatment of shop waste water of wet-process phosphoric acid and phosphoric fertilizer manufacture there are two problems: 1) additional purification of unbalanced waste water; 2) recycling the sludge of neutralization stations.
For additional purification of the waste water from fluorine compounds (20-30 mg/l) some methods are suggested. One of them is two-stage waste water treatment with addition of aluminium salts.
The first stage: the waste water is processed by lime with CaF2 sedimentation:
H2SiF6 + 3Са(OH)2 = 3СаF2 + SiO2 + 4H2O (8)
The second stage: addition of sodium phosphate and sedimentation of calcium fluorine apatite Ca5F(PO4)3:
3HPO42- + 5Ca2+ + 3OH- + F- → Ca5F(PO4)3↓ + 3H2O (9)
Recycling the sludge. The sludge contains (mass %): 10-15 % of Р2О5, 8-10 of F in terms of a dry substance and 60 % of Н2О. Recycling the sludges is expedient 1) from the point of view using valuable components containing in them; 2) with the purpose of environmental protection from toxic fluorine and phosphorus compounds. On the majority of factories produced mineral fertilizers the sludges of neutralization stations are stored together with phosphogypsum.
The most rational method of recycling these sludges is their acid decomposition with the subsequent use of valuable components for fertilizer manufacture. A production way of complex-mixed NPKS fertilizers with a ratio of nutritious elements 1:1:1:1 has been developed. The process includes the following stages:
1) Decomposition of the sludge by sulfuric acid excess (the sludge is mixed in a reactor with excess of concentrated H2SO4 within 30 minutes);
2) Decomposition of an apatite concentrate by the pulp formed with the purpose of use of excess sulfuric acid (an apatite concentrate is added to the pulp containing 48-52 % of free sulfuric acid, and a mixture is mixed within 2,5-4 hours. For the maintenance of an optimum ratio solid : liquid water is added to the pulp during the hashing.);
3) Neutralization of the pulp by ammonia;
4) Mixing potassium chloride and carbamide with the ammoniated pulp;
5) Granulation, drying and air-conditioning of a ready product.
The fertilizer produced is hygroscopic; for decrease of its hygroscopicity it is necessary to carry out additional processing, which prevents caking of the granules (powdering, oiling).
Thus, the waste water of wet process phosphoric acid and phosphoric fertilizer manufacture contains significant amounts of P2O5 and fluorine compounds and requires additional treatment before their dump in a sewer system or reservoirs.
The most rational way of waste water recycling is their processing by lime with the subsequent settling, separation and returning in a work cycle. A volume of the unbalanced waste water in this case is considerably reduced.
The additional purification of unbalanced waste water from fluorine compounds is possible by means of ion exchangers, coprecipitation on СаСО3, addition of aluminium salts, flotation.
The most efficient method of recycling the sludge is its acid decomposition with the subsequent use of the pulp formed for manufacture of finished goods (for example, a complex-mixed fertilizer).
Phosphogypsum. Main impurities, which prevent from phosphogypsum application instead of natural gypsum, are fluorine compounds and phosphorus pentoxide. However, in connection with increase in the phosphogypsum volume the decision of a problem of its recycling for manufacturing useful end-products is necessary.
Now a degree of phosphogypsum use in industry as initial raw material is very low. For example, in the USA only about 2 % of formed phosphogypsum is used. Its basic amount is removed in dumps, sludge settling tanks or dumped in seas and oceans.
Transportation of phosphogypsum in dumps, development of screens under the dumps, neutralization of the waste water formed at the phosphogypsum storage are connected with high capital expenses. Besides, the sludge settling tanks of ground or underground types occupy huge ground areas necessary for agriculture.