DE3724512A1 - Process for decreasing the calcium content of waste waters from gas scrubbers operated with lime or limestone for separating off sulphur oxides and other acidic pollutant gases - Google Patents

Abstract

Salt-containing waste waters (effluents) from exhaust gas scrubbers operated with lime or limestone for acidic pollutant gases frequently contain calcium chloride as the principal constituent. If discharge is still not possible after treatment, these waters must be evaporated. The salt product obtained is hardly utilisable and difficult to dispose of. The use of soda for a reprecipitation to give utilisable sodium chloride is frequently proposed. However, this solution only displaces the site of the disposal problems, since calcium chloride arises in turn in the production of soda. It is proposed to reduce the calcium content of the waste waters by interventions in the exhaust gas scrubber itself. This is possible by recycling the magnesium, precipitated in a waste water treatment in the form of Mg(OH)2, which is contained in virtually all waters of this type. As a further solution, the addition of sodium hydroxide solution or sodium sulphate directly to the scrubber is provided. Recycling or addition are carried out substoichiometrically with a stoichiometric factor of 0.5-0.99, based on the calcium content of the waste water with a conventional procedure. Fig. 1 shows a block diagram of the process according to the invention with recycling of Mg(OH)2 from the waste water treatment. Lime or limestone suspension (6), untreated gas (7) and make-up water (8) are fed to the exhaust gas scrubber (1). The purified gas (10), calcium sulphite and gypsum (9) and a waste water stream (11) leave the scrubber. The waste water is [lacuna] to a ... Original abstract incomplete. <IMAGE>

Description

Method for lowering the calcium content of wastewater from lime or limestone-operated gas scrubbers for the separation of sulfur oxides and other acidic pollutants.

The invention relates to a method for lowering the calcium content wastewater from lime or limestone gas scrubbers for separation of sulfur oxides and other acidic pollutants.

The deposition of SO₂, SO₃, HCl, HF, NO₂ and other acidic noxious gases from waste gases from combustion plants and other industrial processes occurs frequently with gas washes, in which the gases with lime or limestone suspensions be brought into contact. Here are in the Scrubber simplifies z. For example, you can do the following processes:

Ca (OH) ₂ + SO₂ CaSO₃ + H₂O Ca (OH) ₂ + 2 HCl CaCl₂ + 2 H₂O CaCO₃ + SO₂ CaSO₃ + CO₂ CaCO₃ + 2 HCl CaCl₂ + CO₂ + H₂O

The reactions with the other acidic noxious gases are similar in each case forming the corresponding calcium salts.

In many technical applications, the resulting sparingly soluble Calcium sulfite either directly or after oxidation also sparingly soluble calcium sulphate discharged in solid form. Among the usual The calcium sulphite precipitates as hemihydrate, CaSO.sub.3. ½ H₂O and the calcium sulfate as dihydrate, CaSO₄ · 2 H₂O (gypsum).

The more readily soluble constituents such as chlorides, bromides, iodides, nitrates etc., however, remain in solution. Depending on the quality of the laundry used Water, the composition of the lime or limestone and to be cleaned Gases can be found in the water u. a. also magnesium, sodium, potassium, Ammonium and numerous heavy metals and trace elements.  

Depending on the design of the laundry, the salt content in the wash water may be specific Do not exceed values such that a partial flow is generally continuous, in exceptional cases also discontinuous, is discharged.

This wastewater will in most cases after treatment Receiving water or sewer. All usual wastewater treatment methods however, can be practical in terms of the total salt load to be derived do not change anything.

In locations where this salt load the receiving water or sewers unreasonable loaded, the effluents are subjected to evaporation. The The end product of evaporation is a salt mixture whose regulated and environmentally friendly Disposal, however, is also problematic. In many Such residues from operated with lime or limestone gas washes Calcium chloride is the main component of this mixture.

Reuse is contrary to the contamination of the product, calcium chloride as a forced co-product of soda production already in non-usable quantities.

It has therefore been variously made the proposal, the wastewater after a pre-treatment with sodium carbonate (soda) to treat and the then from the subsequent evaporation to be recovered sodium chloride Use to be supplied.

For wastewater whose main constituent is calcium chloride, this is however an ecologically unsatisfactory solution. The proposed implementation

CaCl₂ + Na₂CO₃ CaCO₃ + 2 NaCl

represents a reversal of the Solvay process for soda production. That is with the need for soda at the place of soda production a corresponding Amount of calcium chloride free, the existing disposal problem will only relocated locally.

Only if it succeeds in changing the wastewater composition in the process, that the calcium content drops significantly and thus the soda requirement considerably can be lowered, this method is usefully applicable.  

The invention therefore an object of the gas washing process so too modify that the calcium content in the waste water is reduced so far can, that the above-described displacement problem only of minor Meaning is.

According to the invention this is achieved in that in virtually all Such wastewater present in the wastewater treatment as magnesium Magnesium hydroxide precipitated and the washing process depending on the respective Concentration ratios wholly or partially recycled becomes.

A complete return is especially when starting the system proposed. It may even be advantageous to use magnesium hydroxide or oxide of other origin. In the stationary state is However, in general, only a subset to be recirculated, so that the amount of recycled magnesium is mathematically sufficient between approx. 50 and 99% of the calcium originally dissolved in the wastewater.

This ensures that the calcium content in the wastewater drops while the magnesium content increases. As wastewater is then instead of a contaminated Calcium chloride solution also contains a magnesium chloride solution receive. The conversion to sodium chloride can then by adding sodium hydroxide solution, whereby magnesium hydroxide precipitates. Just soda may be required for the low residual calcium content.

Based on the Gesamtchloridfracht the wastewater can be the soda needs in this way safely down to about 20-30%. For cheap wastewater compositions even better values can be achieved.

Basically, the same effect is of course also with magnesium hydroxide or else magnesium oxide of other provenance achievable. In use However, the in-process product is generally a special one Advantage. The consumption should be based on the example of a typical wastewater from flue gas desulphurisation with limestone behind a coal fired furnace Steam generators are clarified.  

wastewater composition

The numbers mentioned are to be understood as indicative, depending on The feedstock and the operating conditions can vary significantly occur. Also, the allowable concentration, mostly on the Chloride content is - here 30 000 mg / l -, is at the plants differently.

For the example mentioned arise for the reprecipitation of the solutions Sodium chloride sols following soda consumption

Usual driving: 35 433 mg / l with Mg (OH) ₂ recycling: 8,844 mg / l (after precipitation with caustic soda)

If one sets the soda consumption for the total chloride content equal to 100%, then there would be an arithmetic requirement of 44 844 mg / l. The above Quantities are around 80% and 20%, respectively.

Fig. 1 shows a block diagram of the inventive method according to claims 1 to 9. The waste gas scrubbing ( 1 ) lime or limestone suspension ( 6 ), raw gas ( 7 ) and additional water ( 8 ) is supplied. The laundry leaves the purified gas ( 10 ), calcium sulfite or gypsum ( 9 ) and a waste water stream ( 11 ). The wastewater is subjected to a pre-treatment ( 2 ), the type, extent and necessity of which depends on the respective wastewater condition.

Clearing and heavy metal precipitation are usual here, whereby it can be meaningful to achieve a high cleaning performance, in order to achieve good precipitation results to raise the pH value into the incipient magnesium hydroxide precipitation according to claim 7. If the pH is raised further, almost complete precipitation of the magnesium hydroxide ( 3 ) takes place, which is optionally returned to the waste gas scrubber ( 1 ) via dehydration, purification and / or thermal treatment ( 5 ) ( 14 ). The magnesium hydroxide precipitation takes place with sodium hydroxide solution ( 12 ). After the subsequent rest calcium filling ( 4 ) with soda ( 13 ), the obtained, essentially sodium chloride-containing solution ( 15 ) of the evaporation crystallization is supplied.

Other inventive solutions of the task for lowering the Calcium content in wastewater consists of calcium directly in the wash substituted by sodium. This can be done by adding sodium sulfate as well as by adding sodium hydroxide in the wash done. The Dosage is expediently in an amount based on the calcium load discharged with the sewage without addition scarce substoichiometrically so that the sulphate solubility is not intolerable Values are rising.

Based on the without addition of sodium hydroxide solution and / or sodium sulfate in dissolved form with the waste water discharged calcium becomes a stoichiometric addition of these additives from 0.5 to 0.99 according to the invention prefers.

Which of the three variants is to be preferred in a particular case depends from the local conditions, such. B. cost price of the chemicals, Transport distances and options, composition of the Lime or limestone, the exhaust gas and thus the resulting wastewater.

Claims (14)

1. A method for lowering the calcium content of waste water from operated with lime or limestone gas scrubbing for the deposition of sulfur oxides and other acidic noxious gases, characterized in that in a treatment stage in the effluent from the wastewater discharged magnesium is precipitated in the form of magnesium hydroxide and magnesium hydroxide wholly or partly the gas scrubbing is returned. 2. The method according to claim 1, characterized in that the magnesium hydroxide is precipitated with sodium hydroxide solution and the precipitated products in the form be returned to a thin sludge. 3. The method according to claim 1, characterized in that the magnesium hydroxide with caustic soda and the laundry after a drainage in the form of a thick sludge or filter cake whole or partially fed again. 4. The method according to one or more of the preceding claims, characterized characterized in that the magnesium hydroxide before returning to the Gas scrubbing is subjected to a cleaning. 5. The method according to one or more of the preceding claims, characterized characterized in that the magnesium hydroxide before returning a Undergoes laundry. 6. The method according to one or more of the preceding claims, characterized characterized in that the magnesium hydroxide precipitation in the waste water a separate wastewater treatment is connected upstream. 7. The method according to claim 6, characterized in that in the pre-cleaning already a partial precipitation of the magnesium hydroxide takes place.   8. The method according to one or more of the preceding claims, characterized characterized in that the precipitated magnesium hydroxide before the Returned to the laundry subjected to a thermal treatment becomes. 9. The method according to claim 8, characterized in that in the thermal Treatment the product is completely or partially calcined, d. H. a conversion to magnesium oxide takes place. 10. The method according to one or more of the preceding claims, characterized characterized in that magnesium hydroxide or magnesium oxide other Provenance for the same purpose the exhaust gas scrubbing is supplied. 11. A method for lowering the calcium content of wastewater from with Lime or limestone operated gas scrubbers for the separation of sulfur oxides and other acidic noxious gases, characterized in that already in the gas scrubbing the calcium in solution largely is replaced by sodium. 12. The method according to claim 10, characterized in that the gas scrubbing Sodium hydroxide is added. 13. The method according to claim 10, characterized in that the gas scrubbing Sodium sulfate is added. 14. The method according to one or more of claims 10 to 12, characterized characterized in that the total in stationary operation of sodium sulfate and / or caustic soda added amount of sodium is mathematically sufficient, between 50 and 99% of the original dissolved in the wastewater To substitute calcium.