GB2212147A - Sodium chloride crystallisation - Google Patents

Description

2212 147 1 A method and apparatus for processing waste solutions from flue

qas cleansinq Dlants is The invention relates to a method and apparatus for processing waste solutions from flue gas cleansing plants, which solutions contain alkali metal salts and alkaline earth salts as chlorides, sulphates and nitrates. Particularly, it relates to such methods and apparatus in which the mother lye is subjected to multistage evaporation after neutralisation of the solution to precipitate mangesium ions therefrom.

Waste solutions as referred to above occur in particular in flue gas cleansing in coal-fired power stations and refuse incineration plants, and must be disposed of. Flue gases which occur e.g. in refuse incineration plants or in power stations fired by fossil fuels, particularly brown coal or hard coal, as well as the unconsumed constituents of the combustion air contain pollutants which reach the waste gas due to the combustion process. Such pollutants are e.g. sulphur dioxide, hydrogen chloride, hydrogen fluoride and other chlorine, sulphur and nitrogen compounds. If the legally prescribed limit values for the permissible pollutant contents are exceeded, cleansing of the flue gases is required before they may be discharged into the surrounding air.

A large number of cleansing processes for flue gases are known. In the socalled wet absorption process, which is preferred due to its greater degree of cleansing and the lower development of solids compared with the dry absorption process, liquid waste solutions occur. In order to absorb the pollutants, alkaline washing liquids such as milk of lime, limestone or 2 dolomite slurry, caustic soda or ammonia are used. The wet processes based on milk of lime or limestone are also preferred because the absorption agent is considerably cheaper than the other chemicals.

is In the line washing process, the sulphur dioxide is discharged from the washing liquid in the form of flue gas gypsum (CaS04 x 2H20) in the solid form with thickeners, filters or centrifuges, while the hydrogen chloride remains in the washing liquid in the dissolved form as calcium chloride. In order to avoid too high concentrations of calcium chloride, a partial flow of said calcium chloride solution must be continuously removed from the washing cycle (expulsion). If for environmental reasons this concentrated calcium chloride solution may not be discharged, processing of this waste solution is necessary.

For this purpose, it is known to process such solutions completely by single-stage or multi-stage evaporation in a vacuum, the calcium chloride being obtained in the form of its hydrates, preferably as calcium chloride dihydrate (CaC12 x 2H20), in the crystalline form. If the solution is treated even before evaporation with suitable precipitation agents to remove the heavy metals present, an extremely pure product can be produced.

The obtention of calcium chloride is however not always desired and may cause problems. For instance, calcium chloride is highly soluble, and saturated solutions have high boiling points, which makes energy-saving evaporation difficult. Furthermore, it is known from German Patent Publication OS 35 38 231 that the physical properties may change disadvantageously due to concentrating minor constituents of the washing 1 3 solution, in particular the viscosity may increase to such an extent that the use of circulating evaporators is very greatly restricted.

is An additional property of saturated calcium chloride solutions is that the solubility of calcium hydroxide is practically zero. An increase in pH value due to the addition of lyes or milk of lime is not possible due to the low solubility of calcium hydroxide.

Therefore it is not possible to prevent secondary reactions upon evaporation such as, for example, the formation of elementary iodine as a result of oxidation agents present, which leads to a high material stress and to the selection of costly materials. Furthermore, there may form double salts which are troublesome during evaporation due to the presence of magnesium chloride which prevent the production of a solidifiable concentrate; see German Patent Publication OS 36 34 666.

Depending on the washing process and the type of the fuel burned, waste solutions with approximately the following compositions are commonly obtained: Ca++ Mg++ Na+ K+ NH4 + ClS04--N03IBrF- 1500 15000 mg/1 5000 mg/1 1500 ng/1 800 mg/1 1 500 mg/1 2000 - 40000 mg/1 1000 - 1800 mg/1 - 5000 mg/1 - 150 ing/1 - 300 mg/1 2 - 20 mg/1 4 Moreover, the solutions still contain metal ions. A pH value of 5 - 7 should be regarded as typical.

When processing said waste solutions, the magnesium is precipitated as magnesium hydroxide in a known way by the addition of caustic soda and is separated off. If then in an additional precipitation stage sodium sulphate is supplied, the calcium can be precipitated and separated off in the form of gypsum until the limit of solubility, a solution then remaining which contains mainly sodium chloride.

If such a solution were to be evaporated, calcium sulphate as well as sodium chloride would precipitate on exceeding saturation. Moreover, sodium sulphate from the excess of the precipitation agent would still also contaminate the crystals, as the solubility of sodium sulphate is also very greatly affected by sodium chloride. Such a sodium chloride product is not usable and would have to be dumped. If a deficiency of the precipitation agent were supplied, there would then result at the very high evaporation grades a calcium chloride solution with the previously described disadvantages.

The present invention is therefore directed at a method and an apparatus for performing processing waste substances, broadly in the above manner, but which avoids the known disadvantages and provides the purest and most usable possible end product (table salt). To this end, the invention provides a method for processing waste solutions from flue gas cleansing plants, which solutions contain alkali metal salts and alkaline earth salts as chlorides, sulphates and nitrates, comprising 1 1 neutralising the waste solution to precipitate magnesium ions as hydroxides, separating the hydroxides, and subjecting the mother lye to multi-stage evaporation, wherein after separation of the magnesium hydroxide the calcium ions are precipitated in at least two stages with partial evaporation of the waste solution between the stages, the addition of precipitation agent being restricted to a level not greater than stoichiometric relative to the calcium ion content, wherein calcium sulphate is precipitated in at least one calcium precipitation stage and subsequently separated, and wherein sodium chloride crystallises in the pure form by further evaporation and the crystals are separated from the mother lye.

A device according to the invention for performing the above method comprises a first source of precipitation agent for precipitating calcium ions and first means for separating the precipitation product arranged before the first evaporation stage; a second source of precipitation agent arranged behind the first evaporation stage; means for controlling the supply of precipitation agent using sensors monitoring the content ot calcium ions from the respective precipitation solution, wherein second means for separating the precipitation product is disposed behind the second precipitation agent source and third means for crystallising sodium chloride by evaporation and separating said sodium chloride crystals succeed the separating means.

The invention will now be described by way of example and with reference to the accompanying schematic drawing which shows in block diagram form the process stages.

6 In precipitation stage 1 so much caustic soda b is added with pH regulation to the waste solution as partial flow a of the washing solution from a flue gas cleansing process (not shown) until the magnesium contained in the solution has been precipitated as a hydroxide according to its solubility. In this process a pH value of at least 9 must be set.

The magnesium hydroxide d in the suspension m is separated from the liquid n using a suitable separation device 2, for instance a filter, and either used further, for instance as an absorption agent in flue gas cleansing, or is processed. The clear solution n passes into an additional precipitation stage 3, in which calcium sulphate dihydrate is precipitated out by the addition of e.g. a sodium sulphate solution rll. The resulting gypsum suspension o is separated into the solid e (gypsum) and the clear solution using a separation device 4. A thickener in combination with a filter or a centrifuge are particularly suitable as separation devices. For the production of the sodium sulphate solution r, a partial flow q of the clear solution p is used expediently, in order to avoid the supply of added water which is unfavourable with respect to the later evaporation. In the dissolving stage 6, solid sodium sulphate c is dissolved in the clear solution q. The mass flows q and c can be controlled using the density of the solution r. The mixing of the flows n and rll (partial flow of r) takes place in such a way that a deficiency of sodium sulphate is added stoichiometrically in controlled quantities to the present content of dissolved calcium chloride. The saturation concentration of calcium sulphate in the precipitation stage 3 depends especially on the 7 composition of the solution n and cannot be influenced directly. In order to avoid at all events excess sodium sulphate, which would otherwise contaminate the end product during crystallisation of the sodium chloride, at this point a clear deficiency of sodium sulphate is set. which at the same time means the further presence of calcium chloride in the solution. The regulating variable for the supply of sodium sulphate rll is the calcium concentration of the solution p, which must be higher than the gypsum saturation. The Ca++ content may be measured by suitable measuring methods such as automatic titration or by the use of ion-selective electrodes.

The partial flow y of the solution p, which now contains mainly sodium chloride as well as calcium chloride and the other constituents of the solution, is concentrated in an evaporation stage 5. In this evaporation stage 5, as a result of concentration and due to the reduction of the saturation concentration of the calcium sulphate and due to the higher temperature, gypsum in the form of the calcium sulphate hemi-hydrate precipitates out, so that a suspension flow s is obtained. A residue supersaturation resulting from the precipitation in stage 4 is reduced completely in said stage 5. This object is achieved by recycling a partial gypsum flow z from the subsequent gypsum separation stage 7 (e.g. hydrocyclone), which makes it possible to set an optimum solid concentration in the evaporation 5. This also ensures incrustation-free operation of stage 5 as a positive side-effect. Expediently a circulating evaporator is used for processing stage 5. The nonrecycled part f of the gypsum separated off in stage 7 may be combined with the considerably larger gypsum flow e from the separation stage 4. The clear solution 8 t obtained after separating off gypsum from stage 7 is passed to a stage of crystallisation by evaporation 8 in order to crystallise out the table salt.

is Expediently, the evaporation is performed in such a way that the vapour flows i and k of the evaporator stages 5 and 8 contain almost the entire water fraction of the waste solution a to be processed. If for example half of the water fraction is evaporated in stage 5, there results e.g. for the chlorides from solution flow y to the solution flow t a concentrating up by a factor of 2. If furthermore in the crystallisation by evaporation 8 from solution flow t to solution flow w an additional concentrating up of the still dissolved materials (e.g. nitrates) by for instance a factor of 25 takes place, the total concentration factor is 2 x 25 = 50.

If instead of the sub-stoichionetric supply of precipitation agent described in precipitation stage 3 a final sodium sulphate dosing (in a stoichiometric ratio) were to be attempted, as a result of fluctuations of the saturation concentration for calcium sulphate, of control fluctuations, measuring tolerances and of basic desired/actual value deviations a total deviation of about 0.18% Ca++ from the saturation value would be expected. With direct concentrating up by a factor of 50, with an excess of sodium sulphate a Ca++ concentration of 0.18% Ca++ x 50 = 9% Ca++, therefore about a 25% CaC12 solution, would result therefrom, or with an excess of sodium sulphate an equivalent sodium sulphate concentration, which however would no longer be soluble in the system, but would crystallise with the table salt and contaminate it.

it 9 If, however, the addition of sodium sulphate is carried out in accordance with the present invention largely in stage 3 and with a smaller quantity r' not until in stage 5, which produces calcium sulphate anyway, a considerably more accurate setting of a defined minimum Ca++ excess can be achieved in accordance with the same principle (addition of precipitation agent corresponding to the Ca++ content of the clear solution t). Since as a result of the high temperature and the composition of the solution the saturation concentration for calcium sulphate in stage 5 is considerably lower and above all is to the greatest possible extent independent of the composition of the solution, there then results, for a smaller total deviation of only about 0.06% Ca++ with a concentration by a factor of 25 in stage 8, a maximum concentration of Ca++ of 0.06% Ca++ x 25 = 1.5% Ca++, which means a CaC12 concentration of 4.2% by weight.

The clear solution t is evaporated in a circulating evaporation crystalliser 8 as far as the concentration of the minor constituents such as Ca++ and N03- permits with respect to the product quality.

In this case almost the whole quantity of sodium chloride crystallises out. The suspension u is fed to a separating device 9 and the crystalline sodium chloride g is separated off. in order to improve the purity, it is advisable to wash the crystals with a partial flow v of the clear solution p from stage 4. The separating device 9 expediently contains a combination of thickener or hydrocyclone and centrifuge. The mother lye and washing solution v are recycled into stage 8 as flow x or removed as partial flow w, which balance-wise contains all the concentrated impurities, and passed to separate processing 10. The water 1 is drained from said solution flow w, e.g. with a roller dryer, so that a salt mixture h is obtained.

Due to the combination of process steps according to the invention a largely pure sodium chloride is obtained which can be used for commercial purposes, for instance for the regeneration of ion exchangers. Instead of waste products, a saleable product is therefore obtained. This is connected to the additional advantage that for the cleansing of flue gas in addition cheap absorption agents based on milk of lime, limestone or dolomite can be used. The separation of the calcium sulphate takes place in separate stages independently of the sodium chloride crystallisation, so that a sufficiently pure product is obtained. Compared with a direct evaporation of the waste solution, a pH value adjustment is possible in all evaporator stages, as only a small quantity of calcium chloride remains in the solution and thus the danger of corrosion is greatly reduced. Also the elevations of boiling point of saturated sodium chloride solutions are considerably lower than those of calcium chloride solutions, so that a process which is more favourable in terms of energy results. Advantageously, the minor constituents will only be concentrated up so far that no disruptions as.a result of changes in the physical properties of the solution or a reduction in the purity of the end product will result. The concentrate may be processed separately and does not disrupt the actual process of obtaining table salt.

Whereas in the example of embodiment shown only sodium sulphate was used as a precipitation agent, sodium carbonate may also be used for stage 3 and the calcium i; 11 ions be precipitated as calcium carbonate. However, there hereby occurs no reduction of the concentration of the sulphate ions, and sodium sulphate as well as sodium chloride was also crystallised. This undesired precipitation of sodium sulphate can be avoided by adding a stoichiometric deficiency of sodium carbonate in precipitation stage 3, so that a part of the quantity of the original calcium chloride present in the solution is still maintained.. The excess of calcium chloride is then restricted in the following evaporation stage in the manner already described by supplying sodium sulphate. There also result hereby the advantages of the two-stage addition of precipitation agent according to the invention, namely a specific, narrowly restricted adjustment of the remaining concentration of Ca++.

The process described herein, by including the addition in at least two stages, of preferably sodium sulphate as a precipitation agent and by the use of evaporation in at least two stages, has the known benefit of saving energy, and additionally permits separate separation of calcium sulphate and the obtention of a largely pure sodium chloride.

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Claims (17)

Claims

1. A method for processing waste solutions from flue gas cleansing plants, which solutions contain alkali metal salts and alkaline earth salts as chlorides, sulphates and nitrates, comprising neutralising the waste solution to precipitate magnesium ions as hydroxides, separating the hydroxides, and subjecting the mother lye to multi-stage evaporation, wherein after separation of the magnesium hydroxide the calcium ions are precipitated in at least two stages with partial evaporation of the waste solution between the stages, the addition of precipitation agent being restricted to a level not greater than stoichiometric relative to the calcium ion content, wherein calcium sulphate is precipitated in at least one calcium precipitation stage and subsequently separated, and wherein sodium chloride crystallises in the pure form by further evaporation and the crystals are separated from the mother lye.

2. A method according to Claim 1, wherein the supply of precipitation agent takes place in substoichiometric quantities, the addition of precipitation agent being controlled using the calcium ion concentration of the precipitation solution.

3. A method according to Claim 1 or Claim 2, wherein the calcium ions are precipitated in the first precipitation stage with sodium carbonate and in a subsequent precipitation stage with sodium sulphate.

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4. A method according to Claim 1 or claim 2, wherein the calcium ions are precipitated in each stage with sodium sulphate.

5. A method according to any preceding Claim wherein the precipitation takes place in the form of calcium sulphate by the addition of sodium sulphate to the waste solution during the partial evaporation.

is

6. A method according to Claim 5, wherein a solids concentration of 2 to 25% by weight is maintained during the partial evaporation.

7. A method according to Claim 6, wherein the solids concentration is set by recycling part of the calcium sulphate which has been separated.

8. A method according to Claim 6 or Claim 7, wherein the solids concentration is controlled by the size of the mass flow of clear solution removed from the partial evaporation.

9. A method according to any preceding Claim wherein the evaporation stages are carried out at different conditions of temperature and pressure.

10. A method according to any preceding Claim wherein the separated sodium chloride crystals separated are washed with a partial flow of the clear solution separated from the precipitated calcium sulphate.

11. A method according to any preceding Claim wherein a partial flow of the mother lye is removed from the sodium chloride crystallisation and is subsequently supplied to an additional separate processing or to 14 complete evaporation in order to produce a depositable mixed salt.

12. A method for processing waste solutions from flue gas cleansing plants substantially as described herein with reference to the accompanying drawing.

13. A device for carrying out a method according to any preceding Claim with apparatus for neutralising the waste solutions, for precipitating and separating off magnesium hydroxides, and for multi-stage evaporation of the mother lye, which device comprises a first source of precipitation agent for precipitating calcium ions and first means for separating the precipitation product arranged before the first evaporation stage; a second source of precipitation agent arranged behind the first evaporation stage; means for controlling the supply of precipitation agent using sensors monitoring the content of calcium ions from the respective precipitation solution, wherein second means for separating the precipitation product is disposed behind the second precipitation agent source and third means for crystallising sodium chloride by evaporation and separating said sodium chloride crystals succeed the separating means.

14. A device according to Claim 13, wherein the first evaporation stage has a pipe for supplying part of the separated calcium sulphate precipitation product.

15. A device according to Claim 13 or Claim 14, wherein the third means for separating the sodium chloride crystals has a washing device with a pipe for supplying part of the clear solution coming from the iz first means for separating the calcium precipitation product.

16. A device according to any of Claims 13 to 15, wherein a final evaporation stage is disposed behind the means for separating the sodium chloride crystals for complete evaporation of the mother lye.

17. A device for processing waste solutions substantially as described herein with reference to the accompanying drawing.

Ptiblished 1989 a' ThePatent Office. State House.66 71 High Ho-,::-r.. Lond-,n WC1R4TP- Further copies mkv be obtained:l-or,-, 1'ne PaTentoftee Sales Branch- St MaTY' CraY. Orpingior- Kenz BR.5 3RL Printed by M-Lil-plex techmz,.les i ' S1 M,,,, Gray. Kent. Gor. 1 87