Alkali feed

The pH is typically controlled by acid/alkali feeds. Dissolved oxygen and redox loops are controlled as a cascade loop utilising air flow, agitation, pressure, auxiliary feed or a combination of these controllers. 

Quantity and Concentration. Depending on the operating parameters, the degree of degradation, and the tightness of the liquor loop, the quantity of adipic acid required is quite small in relation to the alkali feed. At Shawnee, where a filter is normally used as the final sludge dewatering device, the adipic acid consumption rate is usually less than 10 lb/ton of limestone fed to the system, and sometimes as low as 2 lb/ton of limestone. These values correspond to only 0.6 to 3.0 tons of adipic acid per day for a 500 MW plant. 

The influence of alkali feed on the behavior of the crystallization of magnesium hydroxide was investigated. As alkali feeds, sodium hydroxide, potassium hydroxide, calcium hydroxide and barium hydroxide were used. Experimental conditions of series 1, that is, initial concentrations of magnesium chloride and alkali feeds, are listed in Table 1. ... 

From the experimental results in series 1, it was found that the particle properties of magnesium hydroxide depend on the type of alkali feed. To change the dominant particle sizes of primary and agglomerated particles, it may be effective to use the mixture of alkali feeds. In series 2, calcium hydroxide and sodium hydroxide were used as two kinds of alkali feed. Experimental conditions of series 2 are listed in Table n, where R means the mole fraction of sodium hydroxide in the mixture of calcium hydroxide and sodium hydroxide, that is, [OH... 

In general, particle size is reduced by additives. To investigate the effect of additives, sodium chloride or calcium chloride was added in the reaction system in which calcium hydroxide or sodium hydroxide was used as alkali feed. Experimental conditions of series 3 are listed in Tables HI —1 and HI —2. 

Figure 5 shows the relationship between dominant particle size Im and residence time 6. Dominant particle size decreases slightly with increasing residence time for the system of calcium hydroxide, whereas dominant particle size increases with increasing residence time in other alkali feed systems. This tendency corresponds to the difference in the inclination of G- 6 curve. 

The solubility and hydration number of alkali feed, diffusion coefficient of reacted ions and interaction of ion pair affect the reactive crystallization. As a result of investigation of a number of relationships between these properties and crystallization characteristics, the key property of alkali feeds may be considered as the following. 

Table IV shows the solubilities of various alkali feeds. In general, smaller solubility means that the movement of OH is controlled by ion pair of solute. So it may be difficult for Mg to collide with OH in calcium hydroxide which has the smallest solubility. From SEM photographs of magnesium hydroxide in Figure 3, primary particles are formed in the system of calcium hydroxide, whereas larger primary particles are formed and agglomerated easily in other alkali feed systems which have larger solubility. Then dominant particle size of agglomerated particle becomes larger. Therefore, the solubility may be the key property among the above-mentioned properties. This tendency was also observed in the reactive crystallizations of other carbonates, that is, calcium carbonate(9) and lithium carbonate(lO).

Figure 7 shows the relationship between 1 m and R. The dominant particle size increases with increasing R. By using two kinds of alkali feeds which have different solubilities, the size of primary particle and the frequency of agglomeration may be changed. Then the size of agglomerated particle may be controlled by R. 

Effect of additive. Figure 8 shows the relationship between dominant particle size 1 m and concentration of additive C a in each reaction system. The dominant particle size decreases in all cases of addition of chloride. But the concentration of additive does not affect the on dominant particle size to the same extent all systems. The difference of the dominant particle size is due to the alkali feed. Primary particles are agglomerated easily in the system of sodium hydroxide, whereas they are hardly agglomerated in the system of calcium hydroxide. 

In calcium hydroxide, Na and Ca hardly influence the dominant particle size. Then it is considered that the crystallization kinetics of magnesium hydroxide does not depend on the cation in the crystallizer, but on the properties of each alkali feed as shown in Figure 5. 

Figure 9 shows the relationship between the kinetic order i and the concentration of hydroxide ion in the crystallizer. The kinetic order i is less than 1 in the system of calcium hydroxide, whereas i is larger than 1 in other alkali feed systems. This phenomenon is explained as follows In the systems of alkali feeds except calcium hydroxide, birth rate is influenced more greatly than growth rate by residence time. Then the gradient of B ° curve is steep, hence i becomes large. 

Figure 10 shows the relationship between the kinetic order i and the concentration of hydroxide ion in the crystallizer in the systems of mixture of alkali feed and additive. The kinetic order i is less than 1 when calcium hydroxide exists in the crystallizer. 

Alkali feed influences the properties of magnesium hydroxide particles. In the system of calcium hydroxide primary particles are formed easily and the kinetic order i is less than 1. But, in other cases, that is, barium hydroxide, potassium hydroxide and sodium hydroide as alkali feeds, agglomerated particles are formed easily and kinetic order i is greater than 1. 

The dominant particle size may be controlled by the mole fraction of NaOH in the mixture of Ca(OH) 2 and NaOH, and additive. So the dominant particle size may be controlled by the mixture of alkali feeds, which are different in solubility. 

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