DE1517441C - Method for concentrating salt water - Google Patents

Description

The invention relates to a method for concentrating salt water, in particular sea water or brines, by vacuum evaporation in an evaporator under supervision of the salt concentrate the liquid. - .'.-. · -

In general, when seawater or brine is heated and concentrated, calcium carbonate CaCO ₃ appears first; when concentrated to a quarter of the initial volume, calcium sulfate CaSO _{4 is} deposited. These "deposited salts" settle on the heating surface of an evaporator to form scale, but scale inhibits heat conduction. Many methods and devices have been proposed to limit the adherence of scale to such a heating surface. One common method of removing scale is is to knock off in the preparation of the brine to scale the walls of the processing vessels and pipes used by means of pneumatic hammers or other mechanical devices. It is also known, prior to the deposition of _CaSO4 make a "blow-by" / Obviously such methods to Elimination of the scale is expensive, and a fundamental solution to the problem of preventing the scale from adhering to the damper walls is therefore absolutely desirable.

It is known from German Patent 239 205 to separate the gypsum in flaky form from gypsum-containing feed waters by adding sodium sulfate or another alkali sulfate. However, there are no more precise details of how to proceed in order to achieve the desired success in German patent 239 205. It contains neither quantitative data for the alkali metal sulfates nor an exemplary embodiment. The teaching of German patent specification 239 205 is also limited to feed water that is as low in salt as possible solely to avoid the risk of corrosion. ^; . ■ V

Surprisingly, it has now been found that it. It is possible to largely eliminate the undesired scale formation from calcium carbonate and calcium sulphate by separating the calcium ions of the water as the double salt calcium sulphate · sodium sulphate. Investigations into the formation of separation products which are to be separated off during the salt extraction process had shown - .. that the usual analytical method is flawed. According to the analysis used in the investigations, Na ₂ SO _{4 is} always present in the separated product. The improved analysis

driving was presented in 1961 in the "Journal of the Japanese Salt Society", Vol. 16, pp. 23-26. It is based on the discovery that by washing a sample with alcohol and separately analyzing the washing liquid and the residue, the mother liquor adhering to the sample can be dissolved and returned to the washing liquid so that only that contained in the sample is in the residue Crystal remains. By analyzing this crystal, the presence of the double salt CaSO ₄ · Na ₂ S ^ O _{4 was} determined and confirmed by X-ray diffraction photographs. _r

The investigation of the above-mentioned double salt, which was synthetically produced from salt water, showed the following:

1. If the concentration with respect to Na ^{+ is} more than three times normal, the formation of the deposited double salt changes with its heating temperature. For example, at 115 ° C, Na ₂ SO ₄ -CaSO _{4 is formed} , at 10O ⁰ C Na ₂ SO ₄ -5 CaSO ₄ · 3 H ₂ O and at 75 ° C 2Na ₂ SO ₄ CaSO ₄ -2H ₂ O.

2. All three double salts arise directly from CaSO ₄ · 2H ₂ O without intermediate formation of

CaSO ₄ · 5H ₂ O or CaSO ₄ . Should be, however

If CaSO ₄ - ^ H ₂ O or CaSO ₄ form, the double salts cannot be produced from them.

_{3. CaSO 4} · 2H ₂ O can be obtained directly by hydrolysis of the double salts.
4. Each of these double salts forms a very large crystal, which arises from a first separated crystal nucleus. >

... ■ ■

The invention is a method for

- Concentration of salt water containing calcium ions, especially sea water, by heating the salty liquid in evaporator systems, fume cupboard, the steam and discharge of the highly concentrated Solution, which is characterized in that the sodium ion concentration in the liquid to be treated adjusts to a normality of 3 to 4, expediently by adding NaCl solution,

and maintains. . .

If the process is carried out continuously, the amount of sodium ions in the inflow of saline liquid into the evaporation stage is preferably set according to the amount of sodium ions in the discharged highly concentrated solution containing _{calcium ions of the salt water in the form of the double salt CaS O 4} · Na ₂ SO _{4 suspended.}

By monitoring the setting of the sodium ion concentration in this way, the in the Salt water contains calcium ions instantly

■ deposited as the double salt CaSO ₄ -Na ₂ SO ₄ inside the liquid. The formation of the separated double salt differs depending on the heating temperature. The double salt deposited at the first moment serves as a crystal nucleus for the calcium ions of the newly added liquid. Then the crystal nucleus grows and a very large crystal is created - but this prevents scale from attaching to the heating surface of the evaporator.

It goes without saying that the thought of adding a crystal seed is to prevent the formation of scale to delay the evaporator for salt extraction,

not new in itself. According to earlier methods of adding germs however, the intention is to return a salt precipitated in an evaporator to the evaporator, so that it acts as a crystal nucleus there. At a low concentration, for example 3 to 15 ° Be, but the deposited calcium salt goes according to the reaction '

CaSO ₄ '2H ₂ O> CaSO ₄ · - ^ - H ₂ O> CaSO ₄

in limescale over. It was confirmed by experiment that only when the concentration of the liquid is increased and reaches a constant value of 3η or more with respect to Na ⁺ , the above-mentioned double salts are deposited within the liquid and the initially deposited salt acts as a seed crystal to which the newly deposited salt sets in. Therefore, if the concentration of the liquid in the evaporator is adjusted in advance _{so that such a double salt is deposited, the formation of the single salt CaSO 4 is} suppressed from the start, so that only the double salt is deposited inside the liquid! Accordingly, a prerequisite must be created so that only the double salt is applied in the liquid. Only if this requirement is met, ie if the crystal nucleus is formed, the def nucleus of a very large double salt crystal, can the addition of nuclei be considered sensible. The decisive condition is that the heated liquid has a concentration of more than 3 η with respect to Na ⁺ . When this condition is met, the calcium ions contained in the incoming salt water are instantly blocked in the form of the above-mentioned double salt, so that the formation of scale is suppressed.

According to general opinion, sea water contains NaCl, CaSO ₄ , MgSO ₄ , MgCl ₂ , MgBr ₂ , KCl etc. According to a proposal by Lyman and Fleming (J. Marine Research 2, pp. 134 to 146, 1940), however, the components of the sea water in the dynamic process of the enrichment of sea water more sensibly indicated _{as NaCl, Na 2} SO ₄ , MgCl ₂ , KCl, MeBr _{2 etc.} The following experiment proves that this view is appropriate and correct. 5

A cooking salt solution (170 g / l; 102 ⁰ C) was subjected to constant Volumverdampfung and calcium sulfate (CaSO ₄ -) - solution (1.6 g / l) was dropwise trains pass. The table below shows the composition of the liquid as the amount of CaSO ₄ added increases.

■
A. 6000
2.7016
11.2451
10.9048
0.3403 C. D. Added CaSO ₄ solution (cm ³ ). '. ■
CaO / 1 in the separated mother liquor ..
_{SO 4} equivalent belonging to CaO (g / l)
Actual SO ₄ value (g / l)
Difference (g / l) 3000
2.0306
8.4570
8.0732
0.3788 9000
2.8225
11.7482
8.8986
1.9496 12000
2.5654
10.6779
8.1899
2 ^ 4880

As a result, the specified SO ₄ difference remains dissolved in the form of calcium chloride CaCl ₂ in an amount equivalent to this difference. Since it gradually adds up, the following implementation takes place:

■ 2NaCl + CaSO ₄ = Na ₂ SO ₄ + CaCl ₂

Then the resulting Na ₂ SO _{4 combines with the CaSO 4} , which flows in later, and the double salt separates out. It turns out that CaO accumulates in the liquid, since CaCl ₂ remains in the liquid. In other words, this means that the _{amount of Na 2} SO ₄ equivalent to the remaining CaO has been converted into a double salt.

Furthermore, the composition of the salt solution for the production of said Poppel's salt must contain chlorine ions and a small amount of SQ ₄ ions. in admixture with sodium ions to work satisfactorily. The following are the results of experiments to carry out the method described:

'''■■' ■ ''''"'''■■' ■■■ '" \ Experiment A ^l ■' ■ ';-' ■ '. ■ ■ -. ■ ·

Salt solution (170 g / l) in the boiling state was _{subjected to constant volume evaporation with the addition of CaSO 4} solution (1.6 g / l). The separated material had the following composition:

, Na ₂ SO ₄ -CaSO ₄ ............... 33.192%

CaSO ₄ ...........- 66.808%

^ ⁰ attemptB;,

Saline solution (170 g / l) in the boiling state became a constant with the addition of sea water Subjected to volume evaporation. The separated material had the following composition:

Na ₂ SO ₄ - CaSO ₄ 49.875%

"■ CaSO ₄ - 5H ₂ O .................. 38.778%

Attempt C

. Sodium sulfate solution (200 g / l) in the boiling state was subjected to constant volume evaporation with the addition of seawater. The separated material consisted of 100% Na ₂ SO ₄ -CaSO ₄ .

Attempt D

Various concentrations of a salt solution in the boiling state were _{each subjected to constant volume evaporation with the addition of CaSO 4} solution (1.6 g / l). A comparison of the deposited substances is given in the following table:

5 Weight of the saline solution Na ₂ SO ₄ · 5CaSO ₄ · _{3H 2} O CaSO ₄ 4-H ₂ O 6th Na ₂ SO ₄ -CaSO ₄ ⁰ Be % % % '■ 14.1 25,571 74.429 0 ■; 21.4 35.999 64.001 0 22.9 . ■: '79.665 20,335 0 .: ■ V \ ;. 25.6 ^: '\ / 96.821 0 3.179 '. ·' ■ '■■' ': 26.1 ■■ ;.' ■ "■ 84,907 15.092 0 ^:; ·. '■' 27.5 · .. ■ .. ■: ■ '■ ■ 74.328 '■ ... 25.671

The following table shows a comparison of the composition of the deposited substances at constant Volume evaporation of boiling sea water. The extent of evaporation is indicated in each case.

concentration
■■ '... ° Be, "·' Na ₂ SO ₄ · 5CaSO ₄ · 3H ₂ O
% CaSO ₄ iH ₂ O
% '14.4
h- ^ ^: 'x: ■■ 19.4- ■■ ·. ■■ ■
_s: ,, <;;. ^v - ... ".23,1. ■;, '■■.
* '"■■■ _ ■■■' ■■ 28.7 .. - ■" ^: · 52.979
81.917
. ■ - 80.456
: 89.985 47.021
18.083
19,543 ', * !
10.015

Attempt F

Bitter liquor of the by-product of the salt production was diluted with water so that various concentrations of an aqueous bitter liquor solution were obtained. The samples were each heated to boiling and the boiling liquids were _{subjected to constant volume evaporation with the addition of CaSO 4} solution (1.6 g / l): The substances separated had the following composition:.

Weight of the heated CaSO ₄ - ^ H ₂ O " CaSO ₄ · 2H ₂ O Na ₂ SO ₄ · CaSO ₄ Na ₂ SO ₄ • 5CaSO ₄ ^; 3H ₂ O liquid 94.507 0 5,492 0 34.4 35,162 0 0 64.838 . 22.4 '50, 152 0 0 49.848 ..; .. ·; ■ 21.6 51.559 0 0 48.441 " 18.1 49.248 0 0 50.752 '13.6 Γ' "' 0 83.391 0 16.609 10.8

As can be seen from the above experiments, the best concentration for separating the double salt Na ₂ SO ₄ · CaSO ₄ is given when the boiling liquid contains a small amount of SO ₄ ions as well as sodium and chlorine ions from the outset and the sodium ion concentration is more than 3 η. According to experiment F, the degree of separation of the double salt is _{not high despite the higher content of chlorine and SO 4} ions in the bitter liquor in question. The appearance is due to the lack of sodium ion content in it.

If the concentration of the heated liquid in the evaporator is set to 3 to 4n with respect to Na ⁺ before the salt water to be enriched is fed in, the calcium ions in the incoming salt water are immediately _{blocked as double salt Na 2} SO _{4 · CaSO 4} inside the liquid in the evaporator will. That is, a crystal arises. seed of the double salt mentioned, and now newly deposited double salt attaches itself to the previously deposited double salt or crystal seed, so that a very large crystal is formed and the formation

is delayed by scale build-up on the heating surface of the evaporator. The invention can be used in a satisfactory manner in a device for obtaining fresh water from sea water or in a device for the extraction of salts from brine. >

The apparatus for carrying out the method may be of the conventional, convenient construction ' have, if it is also desired, special devices for carrying out the invention own. An expedient embodiment is shown in the drawing.

Fig. 1 shows schematically a water recovery system with the use of exhaust heat ;,

Fig. 2 shows schematically a multi-stage salt production plant.

In Fig. 1, 1 is a vent through which the exhaust gases of a diesel engine go off. 2 is a heating pipe, 3 is a Circulation pump, 4 an evaporator, in the interior of which the boiling vessel 5 is attached. 6 and 6 'are pipelines for the supply and return of the circulating liquid. 7 is an overflow pipe for the enriched liquid, 8 a pipe for the supply

7 8

guidance of the preheated sea water. 9 is one in the die in FIG. The device shown in FIG. 2 works like the upper part of the evaporator 4 attached impingement, as follows: Typical vacuum plates can be used for tanks I to IV. 10 is the preheater for the evaporator to be fed in for salt production. Sea water, 11 a reservoir for the sea water. Here, for example, tank I is used as an enrichment 12 is a cooler and 13 is a vessel for the receptacle 5 tank, while tanks II to IV are crystalline distilled water. 14 is a line for the sation tanks are. As indicated above, tank I is charged with vapor from evaporator 4. with the liquid which has three times the nor-die in FIG. 1 works like the malality of the concentration with regard to Na ⁺ and follows: 20% saline solution is prepared and as is produced by adding NaCl to the brine, circulating liquid through circulation pump 3 in the io boiler 21 generates superheated steam, which is in heating chamber 22 in Flue 1 attached heating pipe 2 passed. Which is directed from enrichment tank I. Heating chamber 22 liquid is heated by the hot exhaust gases (350 ° C) to the water content of the liquid from the diesel engine to 113 ° C. The heated tank I evaporate. Enriched liquid then flows through pipe 6 into evaporator 4. Liquid is passed through overflow pipe 29 into overflow vessel 30, where the liquid in vessel 5 boils. The generated 15 off. This evaporation is carried out carefully; Steam flows through a steam line 14 which, through the amount of brine fed in and the overflow, leads to the preheater 10 and cooler 12 to the vessel 13. the enriched liquid is used via steam line 14 to heat the circulating, so that the concentration of the liquid in the seawater in preheater 10. The overflow liquid tank I is kept at 3 to 4 η with respect to Na ⁺ , speed from boiling vessel 5 is through Pipe 6 'and pump 3 20 The monitoring is fed back to heating pipe 2 by measuring the density. The temperature of the overflow liquid in vessel 5 valve in the brine feed line can easily be carried through 100 ° C. The incoming seawater can lead.

be heated to 97 ° C by preheater 10. The so-called double salt Na ₂ SO ₄ -

distilled water then obtained contains less than 25 CaSO _{4 produced} in tank I. The enriched liquid

6 parts / million parts chlorine. speed in which the double salt is suspended goes through

The settled material and the circulating liquid overflow pipe 29 in overflow vessel 30, where two

show the following analysis: phases form that settling double salt Na _? SO ₄

CaSO ₄ and the supernatant clear liquid that free

Composition of the deposed Guts ^ _oF calcium ions, but rich in sodium chloride.

Mg (OH) ₂ 0.850% This enriched clear liquid flows into vessel 31

Na ₂ SO ₄ · 5CaSO ₄ · 3H ₂ O ''.'.'.'.'.'.'.93'δ33% over.

r en 1 u η η iaao / The exhaust steam from tank I is in heating chamber 23

Ca ^ u ₄ · ₂ W ₂ U u, i44 / 0 _{introduced inside the} second tank II, in order to

NaCl - 35 ren to heat; the exhaust steam goes out accordingly

Organic matter 5.172% is used in the second tank II in heating chamber 24

r,. j irjT-i-ii there as a heating source for the third tank III, and in

Composition of the circulating liquid of the same type, the fourth tank IV is heated

Weight 1.2067 (24.7 ° Be) heated chamber 25, into which the superheated steam from the

MgO 0.059 ₄ o previous tank III is passed. The steam from the

CaSO ₄ 1.710 last tank IV is passed into the condenser and

MgSO ₄ 15.270 condenses there to form water.

MgCl ₂ 26.713 Since all calcium ions in the brine are

KCl 4.178 stage of the first tank II are switched off, crystalline

NaCl 266,631 ₄₅ lises in the stages from the second to the fourth tank

only table salt out. The other ingredients go

In Fig. 2, I is an enrichment tank; II, III and IV all from the last tank as bitter liquor.

are crystallization tanks. 21 is a boiler for generating salt collector 37 at the bottom of bitter liquor vessel 36

supply of steam, which serves as a heat source; 22,23,24 regenerates the sodium chloride crystals that may

and 25 each in tanks I, II, III and IV are still suspended in the drained bitter liquor

brought heating chambers. 26 is a condenser, 27 is a stay, by settling.

Storage tank for brine, 28 a pump for feeding. If the salt water is a dilute solution, the brine in the first, the second tank provided with an overflow pipe 29 as an enrichment stage I 30 is an overflow. The enrichment stage can drain into the overflow pipe 29 opens. 31 is a 55 speaking of the concentration of the intermediate vessel to be treated for the enriched liquid, 32 salt water is increased or decreased,
A pump for feeding in the enriched The discharged liquids, which are pumped out of the heating liquid into the second, crystallization tank II, chambers 22, 23, 24 and 25, can of course be fed through a pipe 33.33 'which is a line for feeding in as pure water discharged or the liquid of the second tank II in the third 60 in boiler 21 are returned. With method and tank III, and 33 "is a corresponding conduit device according to the invention, in which between tanks III and IV. 34, 34 'and 34" are extraction of one ton of table salt compared to the bottom of tanks II and III and IV attached 16% coal one-salt collector to the conventional method. 35 is a pump for drawing off the saved.

Bitter liquor and conducts the liquid or bitter liquor 65.

from the last tank IV to the receptacle 36 for the characteristics described above and

the bitter liquor, which at its bottom with a wide range of

ren salt collector 37 is provided. changes to.

1 sheet of drawings 209508/343

Claims (1)

Claim: Process for the concentration of salt water containing calcium ions. in particular seawater, by heating the salty liquid in evaporator systems, extracting the vapor and discharging the highly concentrated solution, characterized in that the sodium ion concentration in the liquid to be treated is set to a normality of 3 to 4, expediently by adding NaCl solution , and maintains. · ^!