Calcium hydroxide, dissolution

Suitable catalysts include the hydroxides of sodium (119), potassium (76,120), calcium (121—125), and barium (126—130). Many of these catalysts are susceptible to alkali dissolution by both acetone and DAA and yield a cmde product that contains acetone, DAA, and traces of catalyst. To stabilize DAA the solution is first neutralized with phosphoric acid (131) or dibasic acid (132). Recycled acetone can then be stripped overhead under vacuum conditions, and DAA further purified by vacuum topping and tailing. Commercial catalysts generally have a life of about one year and can be reactivated by washing with hot water and acetone (133). It is reported (134) that the addition of 0.2—2 wt % methanol, ethanol, or 2-propanol to a calcium hydroxide catalyst helps prevent catalyst aging. Research has reported the use of more mechanically stable anion-exchange resins as catalysts (135—137). The addition of trace methanol to the acetone feed is beneficial for the reaction over anion-exchange resins (138). 

Sodium hydroxide (NaOH) (caustic soda) Potassium hydroxide (KOH) (caustic potash) Calcium hydroxide (Ca(OH)2) (slaked lime) Ammonium hydroxide (NH4OH) (aqueous ammonia solution) White deliquescent solid. Sticks, flakes, pellets. Dissolution in water is highly exothermic. Strongly basic. Severe hazard to skin tissue White deliquescent solid. Sticks, flakes, pellets. Dissolution In water is highly exothermic. Strongly basic. Severe hazard to skin tissue White powder soluble in water yielding lime water. Alkaline Weakly alkaline. Emits ammonia gas. Severe eye irritant... 

Since the surface silanol groups react weakly acidic, neutralization with strong bases can be used for their direct determination. However, care must be taken that no dissolution of silica takes place. Greenberg (1ST) found that the adsorption of calcium hydroxide was roughly... 

Von Getler et al. [23] were the first to consider these processes during spray absorption drying when they examined the material system of SO2 and Ca(OH)2. In the opinion of these authors, the absorption of sulfur dioxide is limited either by the dissolution of solid or by the gas-phase mass transfer in the first drying period of the drop. The product of this reaction causes substantial diffusion resistance for the absorbed sulfur dioxide and thus obstructs further reactions as the calcium hydroxide remains in the core. 

Experimental investigations and theoretical computations of SO2 absorption in a spray drier [47] showed that, with an excess of calcium hydroxide absorption is limited only by the gas-phase mass transfer. In addition, the flow in a spray drier could be described by the model of an ideal stirred vessel. Newton et al. [70] considered the subprocesses of mass transfer of SO2 from the gaseous phase to the drop surface, the absorption, the dissociation of SO2, the diffusion of the produced species and the dissolution of calcium hydroxide particles in the drop. 

Chemical leach tests on the bulk settled dust samples showed that the dusts are quite chemically reactive. Leach solutions have high alkali-nities, due to the rapid partial dissolution of calcium hydroxide from concrete particles. Indoor dust samples produced higher pH levels (11.8-12.4) and alkalinities (—600 mg CaCOa) than outdoor dusts (pH 8.2-10.4 alkalinity —30mgL CaCOa), indicating that outdoor dust samples had reacted with rainfall or other water prior to collection. Thurston et al (2002) found that the leachate pH of the dusts decreased with decreasing particle size. Some metals or metalloids in the dusts (aluminum, chromium, antimony, molybdenum, barium, copper, zinc, cobalt, nickel) are readily leached by deionized water many of these form oxyanion species or carbonate complexes that are most mobile at the alkaline pH s generated by the leachates. 

During electrolysis, per mole of potassium permanganate one mole of potassium hydroxide is produced, which has to be recovered. This can be achieved, for example, by evaporating the mother liquor to 750 g KOH per L, whereupon the dissolved potassium manganate(Vl) and calcium hydroxide crystallize out and are removed. The potassium hydroxide can be returned to the dissolution step. Other dissolved impurities from the ores, such as silicates or aluminates, have to be removed from the alkali cycle. 

As dissolution is usually an endothermic process, an increase in the solubility of solids with a rise in temperature is the general rule. Therefore, most graphs of solubility plotted against temperature show a continuous rise, but there are exceptions, for example, the solubility of sodium chloride is almost invariant, while that for calcium hydroxide falls slightly from a solubility of 0.185 g/mL at 0°C to 0.077 g/mL at 100°C. 

In addition to ion exchange with rock surfaces, alkali can react directly with specific rock minerals. When divalents, Ca and Mg ", exist, alkali will react with them and precipitation can occur. One example is the incongruent dissolution of anhydrite or gypsum in the rock to produce the less soluble calcium hydroxide (CaS04(s) -F NaOH Ca(OH)2(s) + Na2S04). Another simple example is Ca -F COs " CaC03(s). Alkali can also dissolve other minerals from a rock, for example, silica. These reactions could cause plugging. 

The process is complex and involves simultaneous dissolution of calcium hydroxide and carbon dioxide, and crystallisation of calcium carbonate. Carbonation is generally carried out in a series of reactors under closely controlled pH, temperature and degree of supersaturation, to produce the required PCC morphology and particle size distribution (see section 31.2.3). Crystallisation can occur on the surface of the calcium hydroxide particles (producing scalenohedral crystals), in the aqueous phase (producing rhombohedral crystals) and at the gas-liquid interface. 

Others have confirmed that light-curable calcium hydroxide materials are resistant to dissolution in water [42]. However, as we have seen, this is not the only crileiion to be considered. Given that these materials are known to promote the formation of reparative dentine as a consequence of their high pH, a property which requires at least partial dissolution in order to be manifested, it is likely that the kind of solubility properties shown by the material Dycal makes them more suitable for this particular application. Certainly, a resin-based material that showed absolutely no solubility or degradation in dentinal fluid would not be acceptable clinically, because it could not... 

Finally, there are the experimental data supporting the hypothesis corKeming the supersaturation of hquid phase with calcium hydroxide as a cause of induction period. Barret et al. [7, 12] have formd that the rate of C3S, dissolution decreases with growing concentration of calcirrm iorrs in the solverrt filtrated through the C3S layer. The release of calcium ions into the solution with the formation of calcirrm vacancies occrrrs according to the scheme ... 

The soft water attack occurs in the case of mountain torrents, and for example in the Scandinavian countries the water dams were destroyed by this process. The calcium hydroxide and other hydrates dissolution are involved in this corrosion mechanism. Ca(OH)2 is dissolved before the other phases sodium and potassium hydroxides are quickly leached out, because of their high solubility. However, according to Taylor... 

The stoichiometry of calcium zincate has also been reported in the literature as Ca(OH)2 2 Zn(OH)2 2 H20. This reference also contains a detailed study of the kinetics of calcium zincate formation in potassium hydroxide solutions. This study concluded that the rate of formation of calcium zincate was not a function of the dissolution and transport of zinc oxide or of the transport of water, but is a function of the calcium hydroxide concentration. 

Cells containing calcium zincate electrodes can be manufactured in at least two different ways. Calcium hydroxide can be added to the zinc oxide electrode mixture. In this case, the calcium zincate is formed in situ as the electrode is cycled in the ceU during the electrochemical formation process. Another method is to form calcium zincate in a separate step and then use this material in the electrode fabrication process. Calcium zincate can be prepared, purified and identified by its X-ray diffraction pattern, shown in Fig. 31.6. This process produces electrodes with a more uniform distribution of calcium zincate and overcomes the problem of zinc dissolution that occurs during the first few cycles before the calcium-zincate has fully formed in the in situ method. This serves to increase the overall cycle Ufe and performance of the battery. In either case, an important advantage of the plastic-bonded calcium zincate stmcture is the reduction of the tendency to form zinc dendrites. The zinc active materials are embedded in the three-dimensional stmcture of the PTFE nanofibers. This increases the stability of the electrode. Although it is possible for some zinc dissolution and migration to occur in the cell, dendritic deposits which penetrate the separator are rarely seen. 

CaCOs in the form of PCC, as true for GCC, is naturally soluble under add conditions and therefore requires a near neutral to slightly alkaline pH environment Residual calcium hydroxide Ca(OH)2 in the PCC can require extra measures for pH control of the final product and the paper mill wet end system. Despite reported developments in providing so-called acid tolerant PCC, intended to remain stable against dissolution under acid (< pH 7) conditions, papermakers, it seems, continue to make the conversion to slightly alkaline or alkaline papermaking when considering the use of CaCOs. 

An extensive work was carried out by Uchikawa l on the conduction calorimetry of superplasticizers, such as SNF (NS in the figure), lignosulfonate (LS), a co-polymer of acrylic acid with acrylic ester (PC), and a three dimensional polycondensate product of aromatic aminosulfonic acid with trimethyl phenol (AS) (Fig. 11). The first peak in the calorimetry corresponds to the heat of dissolution of alite, the heat of formation of the AFt phase, and the calcium hydroxide formation from free lime. The second peak corresponds to the heat of hydration of alite. The admixtures were found to accelerate the formation ofthe ettringite phase. At w/cratios ofO.3 and 0.5 and a later addition of the admixture, the appearance of the second peak was significantly delayed and the peaks were of lower intensity. Most retardation occurred with polycarboxylic acid and amninosulfonic acid-based admixtures (Fig. 11). DSC was used to determine the amount of lime formed at different times. The DSC results show that the addition of admixtures at different w/c ratios generally decreases the amounts of lime in the presence of superplasticizers (Fig. 12). 

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