Treatment of the vapour condensate by ion exchange method
The condensate is purified in two stages: 1) NH4+ ions are removed from the waste water by means of a cationite (КУ-2); 2) NO3- ions are removed from the water with the help of a weak alkaline anionite (АН-2Ф). The choice of the cationite is caused by its high absorbing capacity on ammonia. In addition it keeps this ability to 90-1000С. The cationite is recycled with a 15-20 % НNO3 solution. The anionite is recycled by a 10 % ammonia solution. The reclaimed solutions are returned in the ammoniac saltpeter production. This purification method is used on a number of enterprises. A treatment unit consists of three cationite filters and three anionite filters.
Treatment of the vapours in gas washers
The vapours are purified in plate washers. The vapour is tangentially fed through a connecting pipe in a bottom part of a washer, passes through a perforated plate irrigated with nitric acid, then passes through second and third plates, on which cooling coils are located. A layer from ceramic Raschig rings in height of 200 mm is located in a top part of the washer. As a result in the washer solutions of 14-17 % of NH4NO3 and 2 % of HNO3 are formed. After neutralization these solutions are reused in the production. At the nitric acid irrigation the waste water is practically completely washed from ammonia, and NH4NO3 content in the condensate does not exceed 0,5 g/l.
For increase of effectiveness and profitability of the condensate treatment it is suggested a combined two-stage circuit:
- at the beginning the vapour is purified in washers;
- then the vapour condenses in surface condensers;
- and is purified from residual NH4NO3 and NH3 by means of ion exchange.
The given circuit allows to achieve high purification degree and to use all the treated water for manufacture of nitric acid and other products.
On a number of factories a part of the production waste water is used for irrigation in absorbers for purification of the air-out from a drum dryer or a granulation tower. As the ammoniac saltpeter concentration in the solution increases a part of the solution is taken away from a cycle and given on evaporation; the solution remained is diluted with the waste water.
Control questions on the topic “Ways of processing of sewage and industrial wastes by ammoniac saltpeter manufacture”
1. Where is ammoniac saltpeter used?
2. List the main stages of ammoniac saltpeter production.
3. Call kinds of industrial waste products formed at the ammoniac saltpeter production.
4. Which methods of waste water treatment are applied in this manufacture?
5. Explain the characteristics which influence on the choice of ion exchange reagents used for the condensate treatment.
6. Which solutions are used for the anionite and cationite regeneration?
7. Explain the main stages of condensate purification in plate washers.
4.3. Waste products formed at caustic soda manufacture and methods of their processing
Main consumers of caustic soda are the paper industry, manufactures of artificial fibres, alumina, various chemical compounds, soaps, etc.
There are chemical and electrochemical ways of caustic soda production. Chemical ways are: 1) caustification of a soda solution with lime; 2) calcination of a mixture of iron oxide and soda with preparation of sodium ferrite and its decomposition with water. The electrochemical way is electrolysis of a sodium chloride water solution (brine). Products of the electrolysis are sodium hydroxide NaОН, gaseous chlorine and hydrogen. Now owing to expansion of chlorine application field (manufacture of polyvinyl chloride, products of methane chlorination, synthetic rubber, and chlorine-containing poisonous chemicals) 95 % of caustic soda is made by the electrochemical way.
Two basic electrochemical production ways of caustic soda, chlorine and hydrogen are used: a diaphragm method and a mercury cathode method. In the first method a mixture of NaОН and NaCl in a mole ratio 1:1 is formed. The end-product – NaОН, separated from this mixture, is polluted with sodium chloride, sulfates and carbonates.
Use of the mercury cathode technique allows obtaining a concentrated high-purity caustic soda solution; it is a main advantage of the given method. A basic disadvantage of this method is formation of ample quantity of waste products including mercury-containing highly toxic emissions. Now on the global scale a ratio of these NaОН production methods is approximately equal 1:1.
Reduction of negative influence of the waste of caustic soda manufacture on the environment proceeds in accordance with two directions:
1) Improvement of the technology and purification methods of mercury-containing waste products;
2) Development of the diaphragm method, use of ion selective membranes, which allow produce more concentrated and pure sodium chloride solutions.
A process of obtaining NaОН with use of a mercury cathode (fig. 4.2) is based on the formation of sodium amalgam in an electrolyser at action of constant electric current on a table salt solution:
2NaCl + 2nHg = Cl2 + 2NaHg n (4)
Then the amalgam formed is decomposed with water in a special apparatus with formation of NaОН and Н2, and the mercury regenerated is pumped over in the electrolyser:
2NaHgn + 2H2O = 2NaOH + H2 + 2nHg (5)
As a result of the electrolysis three commodity products – caustic soda, chlorine and hydrogen – are produced.
Industrial waste products of the mercury-cathode sodium hydroxide manufacture:
- mercury-containing sludge formed at the purification of anode mass and final purification of the caustic soda produced;
- waste water after washing the equipment and shop territory, cooling hydrogen;
- waste gases formed at ventilation of production shops;
- СаСО3 and Mg(OH)2-containing sludge formed at purification of initial NaCl brine from Са2+ and Mg2+ ions.
Mercury-containing waste products are of special hazard to people and the environment (approximately 150 g/t of chlorine).
Cleaning the gases from mercury compounds. The hydrogen formed at the sodium hydroxide production contains mercury and water vapours and splashes; its temperature makes 95-1250С. The simplest way of cleaning the gases from mercury is multistage cooling of the gas up to 5-150С. In so doing mercury and water vapours are condensed, separated from the hydrogen and returned in the electrolyzer For improvement of the mercury separation the gas compression to 0,15-0,2 MPa can be used. At cooling the hydrogen to 15oC the mercury content in it makes 15 mg/m3; it corresponds mercury losses of 0,6 kg on 100 t of Cl2
The further purification of hydrogen or mercury-containing ventilation gases is fulfilled by means of adsorption or absorption methods. At absorption purification an absorbent is a returning brine containing about 250 g/l of NaCl and 0,6-0,9 g/l of Cl2, which is used for irrigation of a plate-type or packed column (pH=2-4). At the interaction of the brine with mercury vapours and drops soluble complexes are formed. Regeneration of mercury from the absorbent occurs in an electrolyzer; concentration of residual mercury in the waste gas increases to 0,1 mg/m3.
Other absorbents are also used: a diluted sodium hypochlorite solution in plenty of NaCl (in this case mercury extraction degree makes 95-99 %; residual mercury content increases from 15 to 0,75 mg/m3); solutions containing manganese ions (for example, КМnO4 solutions).
So, some Japan companies are processed a mercury-containing gas in a packed column with a 20 % sulphuric acid solution containing 1,6 g/l of KMnO4 with the subsequent washing the mercury by water in other a column. In the process mercury is separated in elemental state; the solution does not need in regeneration, and is substituted in 6-12 months. The final mercury content in the gas purified is 0,005 mg/m3. At absorption of mercury vapors by solutions containing Mn+3 ions the final mercury content in the waste gas reaches 0,01 mg/m3. Добавить с.170
Basic adsorbents of mercury from gases are activated coal and synthetic zeolites. Activated coal is preliminary processed with mineral acids, sulfur, iodine and some other substances. There is chemosorption of mercury
Fig. 4.2 – Principle circuit of NaOH, Cl2 and H2 production by method with mercury cathode and formed waste products
vapours with iodine, sulfur and other substances. Regeneration of the sorbent occurs by distillation of mercury in retorts at high temperature. The chemosorption way of cleaning gases from mercury emissions is introduced into a number of manufactures. It is based on the adsorption of mercury vapours by a layer of activated coal impregnated with hydrochloric acid.
Effective solid sorbents of mercury vapours are molecular sieves – the zeolites contained cations of Na2+, Ca2+, Pb2+, and Ag+. For improvement of adsorption properties the zeolites are impregnated with solutions of metal sulfides and iodides. The process proceeds as follows: the hydrogen is cooled and a major portion of mercury and water is separated, then the hydrogen gas is passed through 1-2 zeolite layers. Then 90 % of the hydrogen purified is dispatched to consumers, and the remaining hydrogen is heated up to 3200С and used for regeneration of the second zeolite layer. Then the hydrogen saturated with mercury vapours is cooled (in so doing the mercury vapours are separated) and added to the basic hydrogen stream, which is given on cleaning.
Fig. 4.3 – A schematic diagram of waste water treatment from mercury compounds (a sulphide method): 1 – a precipitating tank; 2 – a vat with peat; 3 – a furnace; 4 – a condenser; 5 – a scrubber; 6 – a mercury collector; 7 – an ozone generator; 8 – an ozonizer.
Waste water treatment from mercury compounds and recycling mercury-containing sludge. Among techniques of waste water treatment from mercury the most widespread and effective methods are sulphide, ion exchange, regenerative methods and their combination.
Sulphide method (fig. 4.3) is based on sedimentation of mercury ions as the almost insoluble mercury sulfide:
HgCl2 + Na2S = HgS + 2NaCl (6)
The solution with precipitated mercury sulfide is filtrated through a filtering layer of coal, peat or pumice impregnated also with sodium sulfide. Then the peat is wrung out and burnt for the mercury regeneration at its condensation from the roast gas.
At ion exchange purification synthetic pitches are used, which contain a SO3H-group or a SH-group. The pitches are regenerated with concentrated hydrochloric acid. Дописать