NL8320391A - PROCESS FOR PREVENTING MAGNESIUM AND CALCIUM (CONTAINING) DEPOSITS IN DESALIZATION DEVICES. - Google Patents

Description

8320391 Z v. Y't

Reg.Nr. 120520 / vdS / FM - 1 -

Method for the prevention of magnesium and calcium (containing) deposits in desalination equipment.

Field of the invention

The invention relates to a method for preventing the accumulation of magnesium and calcium (containing) deposits in evaporator systems used for desalinating water, for example sea water. In particular, the invention relates to a process in which silicon dioxide (SiO2) compounds are used as a means of preventing the accumulation of magnesium and calcium (containing) deposits in evaporation desalination systems. The desalination process primarily consists of the removal of water from salts by evaporation to make the water usable and is particularly necessary in dry, barren land regions where fresh water is limited and desalination of sea water is an important source of fresh water supply. In addition, desalination is used to remove salts from wastewater, so that the water can be recirculated and returned to natural watercourses. Background of the Invention Evaporation desalination plants are designed to operate at certain maximum temperatures ranging from (temperatures) near the normal boiling point of water at atmospheric pressure to about 130 ° C. Higher temperatures theoretically increase the efficiency of the installation, but this goes under pain of more severe deposit formation problems SX The deposit forming capacity increases for two reasons: 1. More carbon dioxide is removed from the water at higher temperatures causing the hydroxide ion content of the water increases and the formation of alkaline deposits (calcium carbonate and magnesium hydroxide) is promoted.

8320391 - 2 - 2. The solubility product of deposits-forming compounds (including calcium sulfate) decreases with increasing temperature.

Formation of deposits will therefore mainly occur on the hot metal surfaces where energy is supplied to the water, normally by heating with steam. The formation of deposits reduces the efficiency of heat transfer and eventually the installation has to be shut down for cleaning. The consequences of the formation of deposits 10 are: 1. Increased energy requirement (energy consumption).

2. Reduced fresh water production.

3. Increased operating costs for cleaning.

The salient chemical phenomena in the evaporator 15 that lead to the formation of deposits include the following reactions: 2- (1) 2hco3 = co3 + co2 + h2o

(2) HCO ~ = OH "+ CO

2+ 2- ^ (3) Ca + CO = CaCO (deposit formation)

2+ - - J

20 (4) Mg + 20H = Mg (OH) 2 (deposit formation).

It is often reported in the literature that at the higher desalination temperatures (110-130 ° C), magnesium hydroxide is the main or only solid that is formed. This situation can be represented by the following reaction that allows the complete removal of residual carbonate: 2+ 2+ (5) Mg + CaCO3 + H20 = Mg (OH) 2 + Ca + CO2>

The carbon dioxide is removed with the evaporated water.

Several methods have been proposed to reduce deposit formation and accumulation in desalination plants, including, for example, the use of chelating agents as disclosed in U.S. Pat. No. 2,782,162 and the use of other additives as taught in U.S. Pat. Nos. 3,981,779. 3,985,671, 35 3,042,606, 3,629,105, 3,630,930, and 3,363,975.

8320391 - 3 -

A commonly used method of controlling deposit formation at higher temperatures is to lower the brine pH by adding sulfuric acid. The carbonate equilibrium shifts such that much of the carbonate is in the form of carbon dioxide which is removed by aeration, usually above 50 ° C. There are two alternative methods of acid dosing: 1. Injecting enough acid to completely neutralize the bicarbonate alkalinity, aerating to remove carbon dioxide and raising the pH by adding alkali to 7.7-8.0 .

2. Inject acid so that approximately 15 mg / l of residual bicarbonate alkalinity remains.

Both of the above methods require precise control to avoid a low pH (less than 7.0) in the brine, which could cause corrosion. In practice it is not possible to remove all the bicarbonate at 50 ° C and the brine retains a certain ability to form 20 deposits via reactions 1 to 4.

Recently, low molecular weight polymaleic acids have been used to control (control) the formation of alkaline deposits. Assumed that the polymer functions as an inhibitor of the crystallization threshold and / or acts by deformation of the crystal lattice, thereby reducing adhesion to other crystals or metal surfaces. It may be that inhibitor control (control) alone cannot be used in evaporator designs with relatively low brine circulation rates or with insufficient carbon dioxide evacuation in the distillation steps. The highest temperature specifically mentioned for the use of inhibitors is 110 ° C. In other articles, it is reported that polymaleic acids at 25 ° C do not affect the crystallization of magnesium hydroxide.

8320391 - 4 -

In this regard, one of the most significant features of the present invention is that it is useful at higher temperatures than any known prior art systems. In practice, there is practically no upper temperature limit for the technique according to the invention.

Accordingly, the main object of the present invention is to provide means for overcoming the difficulties encountered in attempts to form magnesium and / or calcium (containing) deposits in desalination equipment by means of the prior art. technology.

Summary of the invention

According to the invention, the formation of deposits in desalination units is prevented by adding to the brine or salt water being treated, small but effective amounts of at least one silicon dioxide (SiCl 2) -containing compound selected from the group consisting of in water-soluble alkali metal silicates, in particular the sodium and potassium silicates, for example the meta and ortho sodium silicates and / or silicic acid. The alkali metal silicates can be characterized by the formula M ^ O X (SiC ^) where M represents the alkali metal, for example sodium or potassium, and X is a number ranging from about 3.9 to 1.7.

The preferred commercially available silicates are known as liquid alkali metal silicates or water glass.

The silicates are added to the seawater or brine in amounts ranging from about 5 to 500 parts by weight of the water-soluble silicate, calculated as SiCl 2, per million 30 parts by weight of the water, and preferably in amounts ranging from about 10 to 200 parts per million of the water-soluble silicate per million parts by weight of the water. It is necessary that the amount of alkali metal silicate or silicic acid added to the seawater be an effective amount, so that a magnesium silicate precipitate forms 8320391-5 - in which the molar ratio of the magnesium to silicon (Mg / Si) ranges from about 0, 75 to 2.0 moles Mg per mole Si and preferably the molar ratio Mg / Si is about 1.5.

In practice, a suspension or solution of the silicon dioxide-containing compound, for example, sodium silicate or colloidal silica, is added in an effective amount to the brine or brackish water, for example, sea water which is fed to the evaporator, where the water is converted into use water by removal of the 10 salts by distillation. It was found that the silica and / or the silicates react with the magnesium ions or magnesium hydroxide sufficiently quickly at evaporator temperatures, for example about 212 ° F, to form magnesium silicates that are non-depositing. The magnesium silicates, in finely divided form, pass through the evaporator and are discharged with the waste brine.

More specifically, at elevated temperatures, for example above 90 ° F, under vacuum, carbon dioxide is removed from the brine resulting in an increased concentration of hydroxide and carbonate ions. These ions react with the available calcium and magnesium ions to form the corresponding calcium carbonate and magnesium hydroxide which form a layer on or accumulate on heat transfer surfaces such as pipes, valves and pumps, thereby increasing the yield and efficiency of the evaporator Reduce.

By using the water-soluble silicon dioxide (SiCl2) -containing compounds or silica in the brine during evaporation, the silicon dioxide converts the magnesium into the silicate. In addition, during the formation of the magnesium 30 silicates, the amount of hydroxide ions is reduced, which in turn reduces the carbonate ion concentration and thus reduces the formation and precipitation of calcium carbonate.

The use of the water-soluble silicates and silicic acid according to the invention therefore significantly inhibits, if not completely prevents, the formation of deposits 8320391-6. Control of the amounts of silicon dioxide (sodium silicate) added to the water can be determined by periodic analyzes of the brine for (the content of 5) bicarbonate and carbonate ions and in case there is an excess of silicon dioxide in the system this is no problem, because silicon dioxide does not pose a health or environmental hazard.

In practice, it is preferred that the salt water to be treated is decarbonated, if necessary, by the addition of acid, aeration and the addition of alkali (hydroxide). With good control of the acid (addition) and efficient decarbonation, the addition of alkali (hydroxide) is not necessary. The water-soluble silicon dioxide-containing compound (water glass), in the desired concentration, is added to the salt water after the usual deaeration step. The amount needed is determined by the residual bicarbonate and carbonate concentrations that are periodically analyzed. The present invention has maximum utility in systems of the above type.

The general equations expressing the stoichiometry between the water glass, carbonate ions and a magnesium silicate of the serpentine type (Mg / Si = 3/2) are: (6) Na2 <0. X Si02 + (3X / 2) Mg2 + + (3X-2) HC02 “= (X / 2) Mg ^ iO ^ (OH) + (3X-2) CC> 2 + (X-2) / 2H20 + 2Na + 30 (7) Na20. X SiC> 2 + (3X / 2) Mg2 + + (3X-2) / 2 CO ^ ~ + XH 0 = (X / 2) Mg3Si205 (0H) 4 + (3X-2) / 2 CC> 2 + 2Na +

The silicate thus acts as a buffer for the removal of hydroxide ions. In addition to preventing the 832 0 3 9 1 i - 7 - accumulation of magnesium hydroxide deposits, the reaction is such that the pH of the system is kept at an optimal level, ie less than 9, meaning without the magnesium silicate reaction taking place , the pH would rise to higher values and more carbonate ions would be formed which promote the precipitation of calcium carbonate and of magnesium hydroxide. It has also been found that most of the bicarbonate decomposes to hydroxide ions and to CO 2 which escapes into the atmosphere (is vented) and thus the amount of calcium carbonate normally formed and deposited in the heat transfer unit. is reduced if not eliminated.

Specific examples

The seawater reactions described below were carried out in a 2L pressure vessel which had been modified and equipped with a 500 watt jacketed heating rod Incalloy, hereinafter referred to as HTS (heat transfer J * * <sensor, heat). transfer sensor). A type K thermocouple was placed in the center of the heating coil to monitor the internal temperature of the HTS. A jacketed thermocouple was placed in the water.

A 100-watt heater was also placed in the water to aid in temperature control.

The device was equipped with an external heater in the 1800 watt jacket, which heater was well insulated to improve temperature control. The temperature in the jacket was measured with a thermocouple. The heating device in the jacket and the water heating devices were operated by means of proportional temperature controllers.

In a typical run, 1.7 L of synthetic seawater (Table I) with or without added water glass (Table II) was deposited in the reactor and heated to 250 ° F (121 ° C). Steam was drained slowly to wash away carbon dioxide 35 and condensed to the concentration factor 8320391

- 8 - I

to measure that ranged from 1.1 to 1.8. After cooling, I

the suspended solids filtered out of the water I.

and analyzed for their base composition by I.

atomic absorption and colorimetry. If the HTS was used as I.

The heating device together with the jacket heating device was examined for deposit formation.

Table I

Synthetic sea water composition jq grams / liter

NaCl 23.50

Na 2 SO 4 4.00 KCl 0.68 H3B03 0.026 15 MgCl 2. 6H20 10.78

CaCl2. 2H2 <0 1.47

Na2SiC> 3. 9H20 0.030

NaHCO3 0.260 20 Table 11

ANALYSIS OF PHILADELPHIA

QUARTZ "N" liquid silicate weight percent

Q

Analysis Typical value 25 SiO 2 27.7a 28.7

Na20 8.6b 8.9

Molar composition Na ^ '3.32 SiC> 2 aAtomab s o rpt i bBase titration 30

Q

As stated in PQ literature.

8320391 - 9 -

Experiments A and B (Table III) in which no silicate was used show, as would be expected, that large amounts of magnesium hydroxide were formed. Calcium carbonate formed a component which was present in a lesser amount since about 88-90% of the alkalinity was distilled off as carbon dioxide.

From the general equation and the molar composition of the water glass (Table II), it can be seen that the stoichiometric ratio of 10 (8) (SiO 2) / (HCO 3 ~) = X / (3X-2) = 0.417 for the preparation of serpentine ( is).

The bicarbonate molarity in the synthetic seawater was 0.00328. The exact equivalent molarity of silicate as SiO2 is (0.00328) (0.412) or 0.00130. In the 15 runs C, D and E, a 65% excess of SiC> 2 was added above the amount required for serpentine formation. The result was the formation of a significant amount of talc (Mg / Si - 3/4 molar ratio). This response can be represented as:

3X 2+ 3X

20 (9) - Mg + Na 2 O. X SiC> 2 + (- - 2) HCC> 3 + ^ 0 = X 3x + v 4Mg3S14 ° 10 <OH, 2 + <- - 2) CO2 + 2Na + 2 V '

The solids in Run C were also analyzed by X-ray diffraction. Seven lines occurred which are characteristic of magnesium silicates.

Not present were Mg (OH) 2, SiC> 2 (all crystalline forms) and MgSO 2 (anhydrous, dihydrate, heptahydrate). The main crystalline component was NaCl (not listed in Table III).

30

Approximately 50% of the solid was amorphous material, probably amorphous silicon dioxide.

The stoichiometric ratio for talc formation is:

3X

(10) (SiO) / (HCO) = X - 2) = 1.11.

35 8320391 * -10-

In Run F, a 2% excess of SiO4 was used and only serpentine was generated.

1 Table III

CL · · '

Seawater solids formation j- added A B C D _E F

silicate (if SiC> 2) M 0 0f 0.00226d 0.00226 0.00226® 0.00140g time (min.) heating test 39 42 27 38 36 48g distillation (250 ° F) 32 39 8 35 18 50 10 cooling 90 90 90 90 90 67 final solution

Concentration 1.19 1.17 1.16 1.16 1.25 1.50 pH 8.7 8.65 8.2 7.9 7.1 7.2

Si (mg / l) 1.4 1.7 7.2 9.1 34 17.3

Alkalinity 18 15 17 16 9 5 ._ solids (mg)

Mg (OH '134 67 0 0 0 0

CaCO c 0.5 0.70 0 0 0

Serpentine 6 9 259 292 134 330

Talk 0 0 108 95 116 0

CaSO 4 C 0 0 7.5 16 3 4 ^ a Initial volume of sea water was 1.6 liters b

Calculated on initial volume c

The small amounts of calcium were randomly attributed to CaCO and CaSO, respectively. for pH above and below 8.5. ci “

Water glass was added after sea water was heated to about 200 ° F.

0

Water glass (PQ "N") was diluted 10 times and neutralized with H 2 SO 4 to pH 7.5 before being added to sea water.

^ The same amount of H2S ° 4 was added (^ as in tests D and E.

gThe total amount of silicate was added in three portions with three heating and cooling cycles before solids analysis. Times listed are average times.

83 2 0 3 9 1 4 - 11 -

Table IV gives figures for tests simulating full-scale evaporation desalination in which heat exchanger surfaces are in contact with sea water for a long time leading to build-up of deposition. The sea water 5 was partly evaporated, made up and again evaporated in three steps to obtain a high concentration factor, followed by isothermal heating for several days.

The heat was supplied by the HTS only, the internal temperature of which was kept at 250 ° F. The water temperature 10 was approximately 229 ° F.

At the end of the comparative test G (no silicate), the surface of the HTS was covered with a white layer. In the silicate test H, the surface of the HTS remained clean. It is seen in the table that the addition of silicate 15 prevents the formation of the deposit-causing substances, magnesium hydroxide and calcium carbonate. Little of the latter (compound) was present because most of the bicarbonate was removed by distillation.

83 2 0 3 9 1 - 12 -

Table IV

Formation of solids at higher concentration ratios_

G H

5 Millimoles SiC ^ 3 4.2 0 time (min.) Heating 57 57 57 40 113 41 42 60 distillation 105 80 65 - 62 69 50 isotherm 10 heating - - - 6240 - - - 4140 cooling 25 49 30 10 61 60 62 20

Concentration ratio 1.8 1.5 pH 7.3 8.3 Soluble Si (mg / l) 1.5 0.4 15 Alkalinity (ppm as CaCO3) 18 19 suspended solid (mg)

Mg (OH) 0 158

CaCC> 3 0 3

Serpentine 284 31 20 Talc 234 0

CaSO 4 9 0 at the beginning 8.1 millimoles of bicarbonate from seawater 2 ^ total volume / final volume.

In Run J, water glass was added in an amount equivalent to the alkalinity to form serpentine according to the equations reported in this application. However, 3q was formed as the main product of talc. This test differed from test F in that the water glass was not added in portions and the heating time was shorter. In any case, magnesium silicates were formed rather than magnesium hydroxide and calcium carbonate, and the heat transfer sensor 33 remained clean.

8320391 i - 13 -

Table V

SOLID FORMATION WITH (QUANTITY)

SILICATE EQUIVALENT WITH ALKALITY

J

Millimoles of SiC2.27

Millimoles HCO ^ 5.44 5 time (min.) Warming 78 distillation 103 isothermal heating (250 ° F) 18 cooling 78 10 concentration ratio 1.8 pH 7.00 soluble Si (mg / 1) 13 alkalinity 60 suspended solid (mg ) 15 Mg (OH) 2 0

CaCO3 0

Serpentine 122

Talk 75

CaSO. 4 4 20 For the production of serpentine (col. 6) ^ heat supplied by HTS.

2 ^ As indicated by the figures in the above tables, the use of silicates and / or silicic acid according to this invention in an evaporation desalination system makes it possible to keep the installations in operation for a longer period of time with a relatively high 3q efficiency .

In addition to the silicon dioxide-containing compounds for the prevention of magnesium and calcium-containing deposits, known additives may be used in combination with the silicates and include, for example, phosphonic acid and the 8320391 t-14 alkali metal or ammonium salts and polymers thereof. The latter compounds can be described, for example, as polymers in which at least 50% of the repeating units are derived from ethylenically unsaturated monobasic and / or dibasic acids and / or ethylenically unsaturated sulfonic acids. In particular, the polyanionic polymers having an average molecular weight ranging from about 1000 to 100,000 including polymers of monomers such as acrylic acid, maleic acid or its anhydride and methacrylic acid. In addition to the anionic polymers, various polycationic polymers can be used, including the compounds specifically mentioned in patents 3,738,945 and 3,288,770. Other known corrosion inhibitors can also be used as taught in U.S. Pat. Nos. 3,591,513, 3,518,204, and 3,644,205.

In the preferred practical embodiment of the invention, salt water being treated (being desalted) is subjected to the following procedure: 1. The salt water is acidified (preferably to a pH of about 4.7) by adding of sulfuric acid.

2. The salt water is then heated and vented and decarbonated (removal of CO 2).

3. If necessary, the salt water is treated to make it basic (preferably a pH of about 7.5).

25 4. The salt water is analyzed for its residual alkalinity (mainly bicarbonate).

5. An appropriate amount of the silicon dioxide-containing compound is then added to the salt water in an amount sufficient to produce non-depositing magnesium silicate in place of magnesium hydroxide.

While this invention has been described by a number of specific embodiments, it will be appreciated that other variations and modifications are possible without departing from the scope and scope of the invention as defined in the appended claims.

8320391

Claims (15)

1. Method for preventing the formation of magnesium and calcium (containing) deposits in evaporative desalination systems used for the treatment of water containing significant amounts of both magnesium and calcium, the method comprising adding to the salt water treated of an effective amount of at least one water-soluble silicon dioxide-containing compound selected from the group consisting of silicic acid and alkali metal silicates, which water-soluble silica-containing compound is added in amounts sufficient to ensure that non-scale magnesium silicate forming is preferentially formed instead of scale magnesium magnesium. Process according to claim 1, characterized in that the molar ratio of Mg / Si in the magnesium silicate ranges from about 0.75 to 2.0. 3. Process according to claim 1, characterized in that the salt water is treated at elevated temperatures and the alkali metal silicates and silicic acid are added to the water in amounts ranging from about 1.0 to 500 wt. parts per million parts by weight of water, depending on the residual bicarbonate concentration. Process according to claim 3, characterized in that the water-soluble silicate is an alkali metal silicate. A method according to claim 4, characterized in that θβη is that the alkali metal silicate / sodium silicate. 6. Process according to claim 1, characterized in that the water-soluble silicon dioxide-containing compound is silicic acid. 7. Process according to claim 2, characterized in that the water-soluble silicon dioxide-containing compound is added to the water in sufficient amounts to form magnesium silicate in which the ratio of Mg / Si 8 2 0 3 9 1 is about 1.5. * - 16 - A method according to claim 1, characterized in that the water-soluble silicon dioxide-containing compound is a mixture of alkali metal silicates. A method for inhibiting the formation of magnesium and calcium (containing) deposits in desalination systems by evaporation, characterized in that the salt water is treated at temperatures above about 200 ° F and at a pH below about 9, an effective amount of at least one water-soluble silicon dioxide-containing compound is added selected from the group consisting of silicic acid and alkali metal silicates which silicon dioxide-containing compound is added to the water in sufficient amount to form preferentially non-deposits magnesium silicate in place of magnesium hydroxide forming deposits. Method according to claim 9, characterized in that the pH of the water in the desalination system is kept between about 7 and about 9. 11. A method for preventing the formation of magnesium-containing deposits in evaporation-desalination systems in which the magnesium-containing salt water being treated is first subjected to an acidification treatment and then to a heating and aeration treatment, characterized in that to the salt water being treated an effective amount of at least one water-soluble silicon dioxide-containing compound is selected from the group consisting of silicic acid and alkali metal silicates, which water-soluble silicate-containing compounds are added in an amount sufficient to lower the residual alkalinity of the salt water to give preferential formation of non-deposits forming magnesium silicate. 12. Process according to claim 11, characterized in that the salt water is treated at elevated temperature 83 2 0 3 9 ΐ ΐ - 17 - and the alkali metal silicates and silicic acid are added to the water in amounts ranging from about 5.0 to 500 parts by weight per million parts by weight of water. 13. A method according to claim 12, characterized in that the water-soluble silicate is an alkali metal silicate. A method according to claim 13, characterized in that the alkali metal silicate is a sodium silicate. 15. A method according to claim 11, characterized in that the water-soluble silicate-containing compound is added in an amount sufficient to react with hydroxide ions present in the salt water to form a non-deposit-forming magnesium silicate material. 8 3 2 0 3 9 ï