Calcium carbonate temperature effect

First Carbonation. The process stream OH is raised to 3.0 with carbon dioxide. Juice is recycled either internally or in a separate vessel to provide seed for calcium carbonate growth. Retention time is 15—20 min at 80—85°C. OH of the juice purification process streams is more descriptive than pH for two reasons first, all of the important solution chemistry depends on reactions of the hydroxyl ion rather than of the hydrogen ion and second, the nature of the C0 2 U20-Ca " equiUbria results in a OH which is independent of the temperature of the solution. AH of the temperature effects on the dissociation constant of water are reflected by the pH. 

The solubility of calcite and aragonite increases with increasing pressure and decreasing temperature in such a way that deep waters are undersaturated with respect to calcium carbonate, while surface waters are supersaturated. The level at which the effects of dissolution are first seen on carbonate shells in the sediments is termed the lysocline and coincides fairly well with the depth of the carbonate saturation horizon. The lysocline commonly lies between 3 and 4 km depth in today s oceans. Below the lysocline is the level where no carbonate remains in the sediment this level is termed the carbonate compensation depth. 

In a review of the course and mechanism of the catalytic decomposition of ammonium perchlorate, the considerable effects of metal oxides in reducing the explosion temperature of the salt are described [1], Solymosi s previous work had shown reductions from 440° to about 270° by dichromium trioxide, to 260° by 10 mol% of cadmium oxide and to 200°C by 0.2% of zinc oxide. The effect of various concentrations of copper chromite , copper oxide, iron oxide and potassium permanganate on the catalysed combustion of the propellant salt was studied [2], Similar studies on the effects of compounds of 11 metals and potassium dichromate in particular, have been reported [3], Presence of calcium carbonate or calcium oxide has a stabilising effect on the salt, either alone or in admixture with polystyrene [4],... 

Calcium carbonate solubility is also temperature and pressure dependent. Pressure is a 6r more important fector than temperature in influencing solubility. As illustrated in Table 15.1, a 20°C drop in temperature boosts solubility 4%, whereas the pressure increase associated with a 4-km increase in water depth increases solubility 200-fold. The large pressure effect arises from the susceptibility of the fully hydrated divalent Ca and CO ions to electrostriction. Calcite and aragonite are examples of minerals whose solubility increases with decreasing temperature. This unusual behavior is referred to as retrograde solubility. Because of the pressure and temperature effects, calcium carbonate is fer more soluble in the deep sea than in the surfece waters (See the online appendix on the companion website). 

Low-pH cleaners are typically used to address calcium carbonate scale and iron oxide deposition. These cleaners are usually formulated using only acid, such as acetic, hydrochloric, or sulfamic. Figure 13.5 shows the effects of temperature and pH on the removal of calcium carbonate from a membrane.7 As the figure shows, lower pH, and higher temperatures are more effective at restoring permeate flow than higher pH and lower temperatures. 

Figure 13.5 Effects of temperature and pH on removal of calcium carbonate from an RO membrane. Courtesy of Dow Water and Process Solutions.

Distinguishing between calcium carbonate precipitates adjacent to actively metabolizing cyanobacteria versus cell directed mineralization, i.e., via bioorganic molecular templates, is often moot. It is probable that both processes take place intermittently if not sequentially with shifts in temperature and composition, alkalinity or salinity, of the surrounding fluid media, especially when the waters are effectively saturated with respect to carbonate and calcium ions. Where the waters are in rapid motion, as in springs and waterfalls, CaCOs deposition may be aided by evaporation while bioprecipitation of carbonates is also taking place (Golubic, 1973, 1983). 

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