WO1984002126A1

WO1984002126A1 - Process for the prevention of magnesium and calcium scaling in desalination apparatus

Info

Publication number: WO1984002126A1
Application number: PCT/US1983/001835
Authority: WO
Inventors: Lawrence B Magnusson; Robert F Motz; Richard G Tonkyn
Original assignee: Mogul Corp
Priority date: 1982-11-19
Filing date: 1983-11-18
Publication date: 1984-06-07
Also published as: DE3390252T1; GB2139995A; GB8416969D0; NL8320391A

Abstract

Scaling of heat transfer surfaces in evaporative desalination by the alkaline scales, calcium carbonate and magnesium hydroxide, is prevented by adding to the water being processed effective amounts of at least one water soluble silica-containing (SiO2) compound selected from the group consisting of the alkali metal silicates and silicic acid.

Description

PROCESS FOR THE PREVENTION OF MAGNESIUM AND CALCIUM SCALING IN DESALINATION APPARATUS

Field of Invention This invention is directed to a process for preventing the accumulation of magnesium and calcium scale in evaporator systems used for desalination of water, e.g., sea water. More specifically, the invention is directed to a process of utilizing silica- containing (SiO₂) compounds as a means of preventing the accumulation of magnesium and calcium scale in evaporation desalination systems. The process of desalination is primarily the removal of water from salts by evaporation to render the water useful and is particularly necessary in arid countries where fresh water is limited and desalination of sea water is an important source of supply of fresh water. In addition, desalination is utilized to remove salts from waste waters so that the water can be recycled and returned to the natural waterways.

Background of the Invention Evaporative desalination plants are designed to operate at certain maximum temperatures ranging from near the normal boiling point of the water at atmospheric pressure up to about 130°C. Increased temperature theoretically increases the efficiency of the plant but imposes the penalty of more severe scaling problems. The scaling potential increases for two reasons:

1. More carbon dioxide is removed from the water at higher temperatures, increasing the hydroxide ion content of the water and promoting the formation of alkaline scales (calcium carbonate and magnesium hydroxide). 2. The solubility product of scale-forming compounds (including calcium sulphate) decreases with increasing temperature.

Scaling thus tends to occur on the hot metal surfaces where energy is being supplied to the water, normally by steam heating. Scaling reduces the efficiency of heat transfer and ultimately the plant must be shut down for cleaning. The results of scaling are:

1. Increased energy demand.

2. Decreased production of fresh water.

3. Increased operating costs for cleaning.

The salient chemistry in the evaporator which leads to scaling involves the reactions:

(1) 2HCO₃- = CO₃ ^2- + CO₂ + H₂O

(2) HCO₃- = OH- + CO₂

(3) Ca²⁺ + CO₃ ^2- = CaCO₃ (scale-forming)

(4) Mg²⁺ + 2OH- = Mg(OH)₂ (scale-forming)

At the higher desalination temperatures ( 110- 130 °C) , it is frequently reported in the l iterature that magnes ium hydroxide is the main , or sole , solid formed . This situation can be expressed by the following reaction which can completely remove residual carbonate :

(5) Mg²⁺ + CaCO₃ + H₂O = Mg(OH)₂ + Ca²⁺ + CO₂ The carbon dioxide is removed in the evaporated water. Various methods have been proposed for decreasing the deposition and accumulation of scale in the desalination units and include, for example, the use of chelating agents as set forth in U.S. Patent 2,782,162 and other additives as taught in U.S. Patents 3,981,779; 3,985,671; 3,042,606; 3,629,105; 3,630,930 and 3,363,975.

A common process for controlling scale at the higher temperatures is to reduce the pH of the brine by adding sulfuric acid. The carbonate equilibria shift so 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 acid dosing methods:

1. Inject sufficient acid for full neutralization of the biocarbonate alkalinity, aerate to remove carbon dioxide and raise pH by adding alkali to 7.7 - 8.0.

2. Inject acid to leave about 15 mg/l residual bicarbonate alkalinity.

Both of the above methods require close control to avoid a low pH condition (less than 7.0) in the brine which could cause corrosion. In practice, it is not possible to removal all of the bicarbonate at 50°C and the brine retains a potential for scale formation via reactions 1 through 4.

Recently, low molecular weight polymaleic acids have been applied to the control of alkaline scaling. The polymer is believed to act as a crystallization threshold inhibitor and/or by distorting the crystal lattice to reduce adhesion to other crystals or metal surfaces. Control by inhibitor alone may not be applicable in evaporator designs which have relatively low brine circulation rates or inadequate venting of carbon dioxide in the distillation stages. The highest specifically cited temperature for inhibitor use is 110°C. In other works, at 25°C, it is reported that polymaleic acids have no effect on the crystallization of magnesium hydroxide.

In this regard, one of the most significant attributes of the present invention is that it is useable at higher temperatures than all known prior art systems. In fact, the technique of the invention essentially has no upper temperature limit.

Accordingly, it is the principal object of this invention to provide a means of overcoming the difficulties experienced when attempting to avoid magnesium and/or calcium scale formation in desalination apparatus by prior art means.

Summary of the Invention In accordance with this invention, the prevention of scale formation in desalination units is accomplished by adding to the brine or saline water being processed small but effective amounts of at least one silica (SiO₂) containing compound selected from the group consisting of water soluble alkali metal silicates particularly the sodium and potassium silicates, e.g., the meta- and orthosodium silicates and/or silicic acid. The alkali metal silicates may be characterized by the formula M₂O X (SiO₂) wherein M is the alkali metal, e.g., sodium or potassium and X is a number ranging from about 3.9 to 1.7. The preferred commercially available silicates are known as l iquid alkali metal silicates or water glass. The silicates are added to the sea water or brine in amounts ranging from about 5 to 500 parts by weight of the water soluble silicate, as SiO₂, per million parts by weight of the water and more 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 sea water be .an effective amount such that a magnesium silicate precipitate forms wherein the molar ratio of the magnesium to silicon (Mg/Si) ranges from about 0.75 to 2.0 moles of Mg per mole of Si and preferably the mole ratio of Mg/Si is about 1.5.

In practice, a suspension or solution of the silica- containing compound such as sodium silicate or colloidal silicic acid is added in effective amounts to the brine or brackish water such as sea water being carried to the evaporator where the water is converted to useable water by removal of the salts by distillation. It has been found that the silicic acid and/or the silicates react at evaporator temperatures, e.g., about 212°F sufficiently rapidly with the magnesium ions or magnesium hydroxide to form magnesium silicates which are non-sealant. The magnesium silicates, in finely-divided form, pass through the evaporator and are discharged with the waste brine.

More specifically, at elevated temperatures, e.g., above 90°F, under a vacuum, carbon dioxide is removed from the brine resulting in increased concentrations 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 build up or accumulate on heat transfer surfaces, such as pipes, valves, and pumps thereby reducing the output and efficiency of the evaporator. By utilizing the water soluble silica (SiO₂) containing compounds or silicic acid in the brine during evaporation, the silica converts the magnesium to the silicate. Moreover, during formation of the magnesium silicates, the amount of hydroxide ions is reduced which, in turn, reduces the carbonate ion concentration thereby decreasing the formation and precipitation of calcium carbonate.

Thus, by utilizing the water soluble silicates and silicic acid in accordance with this invention the formation of scale is substantially inhibited if not totally prevented. Control of the amounts of silica (sodium silicate) being added to the water can be determined by periodic analysis of the brine for bicarbonate and carbonate ions and in the event there is an excess of silica in the system there is no concern, since silica poses no health or environmental hazards.

In practice, it is preferable that the saline water being processed be decarbonated by the addition of acid, aeration and caustic addition, if necessary. With good acid control and efficient decarbonation, caustic addition is not necessary. The water soluble silica-containing compound (waterglass), in the desired concentration, is fed to the saline water after the usual deaeration stage. The amount required is determined by the residual bicarbonate and carbonate concentrations which are analyzed periodically. The instant invention finds maximum utility in systems of the foregoing 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 as follows: (6) Na₂O • X SiO₂ + (3X/2) Mg²⁺ + (3X-2)HCO₃- = (X/2)Mg₃Si₂O₅(OH)₄ + (3X-2)CO₂ + (X-2)/2H₂O + 2Na⁺

(7) Na₂O • X SiO₂ + (3X/2) Mg²⁺ + (3X-2)/2 CO₃ ^2- + XH₂O = (X/2) Mg₃Si₂O₅(OH)₄ + (3X-2)/2 CO₂ + 2Na⁺

The silicate thus acts as a buffer to remove hydroxide ions. In addition to preventing the accumulation of magnesium hydroxide scale, the reaction is such that the pH of the system is maintained at an optimal level, i.e., less than 9 which means that without the magnesium silicate reaction taking place, the pH would rise to higher levels and more carbonate ions would form promoting the precipitation of calcium carbonate as well as magnesium hydroxide. Also, it was found that most of the bicarbonate decomposes to hydroxide ions and CO₂ which is vented and thereby reduces if not eliminates the amount of calcium carbonate normally built up in the heat transfer unit.

Specific Examples The sea water reactions discussed hereinafter were conducted in a 2-liter pressure vessel modified to contain a sheathed 500 watt Incalloy heating rod designated herein as HTS (heat transfer sensor). A type K thermocouple was positioned at the center of the heating coil to monitor the internal temperature of the HTS. A sheathed thermocouple was immersed in the water. A 100 watt heater was also immersed in the water to assist in temperature control. The vessel has an external 1800 watt mantle heater which was heavily insulated to improve temperature control. Jacket temperature was sensed by a thermocouple. The mantle and water heaters were run by proportional temperature controllers.

In a typical experiment, 1.7 liter of synthetic sea water (TABLE I) with or without added water glass (TABLE II), is scaled in the reactor and heated to 250°F (121°C). Steam is vented slowly to purge carbon dioxide and condensed to measure the concentration factor which ranged from 1.1 to 1.8. On cooling, the suspended solids are filtered from the water and analyzed for elemental composition by atomic absorbance and colorimetry. When the HTS was used as a heater, in conjunction with the mantle heater, it was examined for scale.

In experiments A and B (TABLE III), in which no silicate was added, it may be seen, as expected, that large amounts of magnesium hydroxide were formed. Calcium carbonate is a minor component as about 88-90% of the alkalinity was distilled as carbon dioxide.

From the general equation and the molar composition of the water glass (TABLE II) it may be shown that the stoichiometric ratio

(8) (SiO₂)/(HCO₃-) = X/(3X-2) = 0.417

for the production of serpentine.

The molarity of bicarbonate in the synthetic sea water was 0.00328. The exact equivalent molarity of silicate as SiO₂ is (0.00328)(0.412) or 0.00130. In experiments C, D and E a 65% excess of SiO₂ was added, over that required to produce serpentine. The result was the production of considerable talc (Mg/Si mole ratio = 3/4). This reaction can be expressed as:

The solids of experiment C were also analyzed by x-ray diffraction. Seven lines appeared which are characteristic of magnesium silicates. Not present were Mg(OH)₂, SiO₂ (all crystalline forms) and MgSO₄ (anhydrous, dihydrate, heptahydrate). The major crystalline constituent was NaCl (not shown in Table III). About 50% of the solid was amorphous material, probably amorphous silica.

The stoichiometric ratio for the production of talc is:

In experiment F, a 2% excess of SiO₂ was used and only serpentine was formed.

Table IV gives data for runs simulating full scale evaporation desalination in which heat exchange surfaces are in contact with the sea water for long periods of time which results in scale buildup. The sea water was partially evaporated, replenished and evaporated again in three stages to give a high concentration factor, followed by isothermal heating for several days. The heat was supplied by the HTS alone with its internal temperature held at 250 °F. The water temperature was about 229°F.

At the conclusion of the control run G (no silicate), the HTS surface was covered with a white film. In the silicate run H the HTS surface remained clean. It may be seen in the table that the addition of silicate prevented the formation of the sealants, magnesium hydroxide and calcium carbonate. There was little of the latter because most of the bicarbonate is removed by the distillation.

In the experiment of J, water glass was added in an amount equivalent to the alkalinity for the formation of serpentine according to equations set forth herein. Talc appeared, however, as a prominent product. This ruα differed from that of experiment F in that the water glass was not added in increments and the heating time was less. In any event, magnesium silicates were formed rather than magnesium hydroxide and calcium carbonate and the heat transfer sensor remained clean.

As indicated by the data in the above Tables, the use of silicates and/or silicic acid in accordance with this invention in an evaporating desalination system enables the units to be operated over extended periods of time at comparatively high efficiency.

In addition to the silica-containing compounds for the prevention of magnesium and calcium scale, other known additives may be used in combination with the silicates and include, for example, phosphonic acid and their alkali metal or ammonia salts and polymers. These latter can be described, for example, as polymers where 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 average molecular weights ranging from about 1,000 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 may be used including those specifically set forth in Patents 3,738,945 and 3,288,770. Other known corrosion inhibitors may be used as taught in U.S. Patents 3,591,513; 3,518,204 and 3,644,205.

In the preferred practice of the invention, saline water being treated is subjected to the following procedure:

1. The saline water is acidified (preferably to a pH of about 4.7) by adding sulfuric acid thereto.

2. The saline water is then heated and deaerated and decarbonated (remove CO₂). 3. If necessary, the saline water is treated to render it basic (preferably a pH of about 7.5).

4. The saline water is analyzed for residual alkalinity (mostly bicarbonate).

5. An appropriate amount of silica-containing compound is then added to the saline water in an amount sufficient to cause non-scaling magnesium silicate to be formed instead of magnesium hydroxide.

While this invention has been described by a number of specific embodiments it is obvious that there are other variations and modifications which can be utilized without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims

The invention claimed is:

1. A process for preventing the formation of magnesium and calcium scale in evaporation desalination systems which are used to treat water containing significant amounts of both magnesium and calcium which process comprises adding to the saline water being processed an effective amount of at least one water soluble silica-containing compound selected from the group consisting of silicic acid and alkali metal silicates, said water soluble silica-containing compound being added in amounts sufficient to cause non-scaling magnesium silicate to be preferentially formed as opposed to scale forming magnesium hydroxide.

2. The process of claim 1 wherein the molar ratio of Mg/Si in said magnesium silicate ranges from about 0.75 to 2.0.

3. The process of claim 1 further characterized in that the saline water is processed 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 parts by weight per million parts by weight of water, depending on residual bicarbonate concentration.

4. The process of claim 3 further characterized in that the water soluble silicate is an alkali metal silicate.

5. The process of claim 4 further characterized in that the alkali metal silicate is a sodium silicate.

6. The process of claim 1 further characterized in that the water soluble silica-containing compound is silicic acid.

7. The process of claim 2 further characterized in that the water soluble silica-containing compound is added to the water in amounts sufficient to form magnesium silicate wherein the ratio of Mg/Si is about 1.5.

8. The process of claim 1 further characterized in that the water soluble silica-containing compound is a mixture of alkali metal silicates.

9. A process for inhibiting the formation of magnesium and calcium scale in evaporation desalination systems which comprises adding to the saline water being processed at temperatures above about 200 °F and at a pH below about 9, an effective amount of at least one water soluble silica-containing compound selected from the group consisting of silicic acid and alkali metal silicates; said silica-containing compound being added to the water in amounts sufficient to cause non-scaling magnesium silicate to be preferentially formed as opposed to scale forming magnesium hydroxide.

10. The process of claim 9 further characterized in that the pH of the water in the desalination system is maintained between about 7 to about 9.

11. In a process for preventing the formation of magnesium scale in evaporation desalination systems wherein the magnesium containing saline water being processed is first subjected to an acidifying treatment and then to a heating and deaerating treatment, the improvement which comprises adding to the saline water being processed an effective amount of at least one water soluble silica-containing compound selected from the group consisting of silicic acid and alkali metal silicates, said water soluble silicate-containing compounds being added in an amount sufficient to reduce the residual alkalinity of said saline water and thereby cause non-scaling magnesium silicate to be preferentially formed.

12. The process of claim 11 further characterized in that the saline water is processed at elevated temperatures 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. The process of claim 12 further characterized in that the water soluble silicate is an alkali metal silicate.

14. The process of claim 13 further characterized in that the alkali metal silicate is a sodium silicate.

15. The process of claim 11 wherein said water soluble silicate-containing compound is added in an amount sufficient to react with hydroxide ions present in the saline water to form a non-scaling magnesium silicate material.