Preliminary experiments have shown that magnesium hydrate obtained by precipitation had poor purity when desalination brine was directly used as the raw material, with a calcium content of about 3% - 7%. If a solution removed of calcium was operated previously, calcium content would be reduced to less than 1%. In an alkaline environment, carbon dioxide was allowed to preferentially react with calcium ion in the desalination brine, then the prepared decalcified mother liquor containing low calcium ion was reacted with alkali to produce the desired product magnesium hydrate. In this way, the purity of magnesium precipitate could be improved [3]-[5].

In the first set of experiments, 200.00 mL of brine was placed in a three-necked flat-bottom flask; speed of magnetic stirrer was controlled at 600 rpm; pH meter was inserted below the liquid level; and pH value was recorded after the reading stabilized. Next, carbon dioxide (flow rate of about 0.5 L∙min⁻¹) was passed through the solution; meanwhile, 2.5 mol∙L⁻¹ sodium hydroxide solution was added dropwise. Timing started when the pH of the system changed to 8 ± 0.2. After reacting for certain times (1 min, 2 min, 3 min, 4 min and 5 min), and at the same time, carbon dioxide and sodium hydroxide delivery were ceased. Afterwards, stirring was continued for 1 h, and then the mixture was allowed to stand for a certain time. After filtering through quantitative paper, concentrations of remaining calcium ion and magnesium ion in the clear liquor were measured and the amounts of substance were calculated, which were compared to the concentrations and molar amounts of calcium ion and magnesium ion in desalination brine before reaction to calculate the calcium removal rate and magnesium loss rate.

In the second set of experiments, conditions were set identically with the first set of experiments, except that stirring was terminated immediately after completion of reaction. After letting stand for a while, Ca²⁺ removal rate and Mg²⁺ loss rate were calculated.

Decalcified mother liquor was added with certain amounts of precipitant sodium hydroxide solution and coagulant aid PAM solution, then reacted, stood still and filtered to give magnesium hydroxide product. By controlling relevant parameters at different levels, the influences of sodium hydroxide addition amount, PAM addition amount, addition order of sodium hydroxide and polyacrylamide and standing time on precipitation effect were investigated.

100.00 mL of decalcified mother liquor was accurately pipetted and transferred to a 250 mL beaker used as the reaction vessel. pH and temperature of the reaction system were adjusted and measured. After certain amounts of sodium hydroxide and PAM solutions were added, pH and temperature, which stabilized after reaction, were recorded accurately. After letting stand for a while, the mixture was filtered, magnesium ion and calcium ion contents in the filtrate were determined. Optimal proportion was confirmed by continuously changing the molar amount of alkali solution added and the concentration of PAM.

(1) Effect of concentration of PAM solution on magnesium precipitation rate

PAM solutions with mass concentrations of 0.1%, 0.2% and 0.3% were prepared separately while other parameters remained unchanged. Through reaction, precipitation, standing, filtration, titration and calculation, the appropriate concentration of PAM solution was determined.

(2) Effect of addition amount of hydroxide ions on magnesium precipitation rate

Calcium ion and magnesium ion contents in decalcified mother liquor were accurately measured and theoretical molar amount of hydroxide ion required for magnesium ion in 100.00 mL of decalcified mother liquor was calculated, based on which the molar amount of actually added hydroxide ion was changed continuously. Through reaction, precipitation, standing, filtration, titration and calculation, the appropriate molar amount of hydroxide ion was determined.

(3) Effect of standing time on magnesium precipitation rate

After formation of magnesium hydroxide precipitate, contents of remaining calcium ion and magnesium ion in solution were determined at different standing times, in order to compare the magnesium precipitation rates at different standing times and analyze the effect of standing time on magnesium hydroxide yield.

Titration method was used to determine calcium ion and magnesium ion concentrations [6].

The amide groups on PAM were interacted by hydrogen bond and then adsorbed the sol particles. After adsorption, PAM chain segments rotated and moved, that is, bridging effect aggregated solid particles together to form precipitate. Therefore, the use of PAM would facilitate the production of magnesium hydrate precipitate.

As can be seen from Figure 1, when the same molar amount of hydroxyl ions was added, the magnesium precipitation rates for the reaction systems added with 0.1% and 0.2% PAM were close to that for the reaction system without PAM delivery. This indicated that PAM did not prompt more magnesium in the reaction system to react with hydroxyl anion. In the presence of PAM, the yield did not increase significantly.

In addition, it can also be found from Figure 1 that when the ratio of molar amount of actually added hydroxyl ions to the theoretical value was the same, the magnesium precipitation rate was between 70.48% - 75.66% in the presence of 0.3% PAM, which was less than that in the PAM-free reaction system (greater than 77%). When

the mass concentration of PAM solution reached 0.3%, the solution had high viscosity, which was mixed slowly and expanded moderately in the reaction system, thus affecting the results. Comprehensive analysis of Figure 1 and Figure 2 showed that the PAM solution well aided the coagulation when its concentration was controlled within 0.1% - 0.2%.

As can be seen from Table 2, when the ratio of n_OH-actual to n_{OH-theoretical} was 111.8%, magnesium hydrate yield was high. When the ratio of n_OH-actual to n_{OH-theoretical} increased to 130%, yield of Magnesium hydrate decreased instead; and at that time, calcium proportion in the precipitate rose.

Figure 3 shows changes in magnesium precipitation rate at different standing times. A. Magnesium hydrate yield was 50.25% - 59.15% at a standing time of 13 hours; and was 42.74% - 60.95% at a standing time of 18 hours. Moreover, the precipitation rate fluctuated widely, indicating insufficient standing and insufficient precipitate production. At a standing time of 60 h, yield was 70.48% - 77.22% when the addition amount of hydroxide ion was 101.5% - 130.2% of the theoretical value. At a standing time of 65 h, yield ranged from 71.48% to 74.40% when the addition amount of hydroxyl anion was 111.4% - 120.2% of the theoretical value, which was still slightly lower than that at a standing time of 65 h. At a standing time of 72 h, yield ranged 72.48% - 75.48%

when the addition amount of hydroxide ion was 111.4% - 120.2% of the theoretical value. Analysis of precipitation rates at standing times found that extension of standing time contributed little to the increase of precipitation yield at those times.

In addition, calcium ion did not show significant further reduction with increasing molar amount of hydroxide ion addition, indicating that the amount of calcium impurities in the product did not increase significantly. This was because after decalcification treatment, calcium ion concentration was low in the reaction material. Moreover, (5.6 × 10⁻¹², 25˚C) was far smaller than (5.5 × 10⁻⁶, 25˚C). Thus, magnesium hydrate precipitate was produced preferentially than calcium hydroxide.