Calcium carbonate from water softening

The calcium carbonate recovered can be calcined to give lime for recycle to the treatment process as well as by-product lime for marketing. In addition, removal of the calcium carbonate, the major ingredient in sludge from water softening treatment plants, reduces the disposal problem to a fraction of that previously required. 

Calcium carbonate sindge is a precipitated by-product (e.g. from water softening, the sulfite paper-pulp process and from the purifieation of sugar). 

RE-CARBONATION - A process using carbon dioxide gas to stabilize and thereby prevent precipitation of calcium carbonate from cold process lime or lime-soda softened water. 

Makeup. Makeup treatment depends extensively on the source water. Some steam systems use municipal water as a source. These systems may require dechlorination followed by reverse osmosis (qv) and ion exchange. Other systems use weUwater. In hard water areas, these systems include softening before further purification. Surface waters may require removal of suspended soHds by sedimentation (qv), coagulation, flocculation, and filtration. Calcium may be reduced by precipitation softening or lime softening. Organic contaminants can be removed by absorption on activated carbon. Details of makeup water treatment may be found in many handbooks (22—24) as well as in technical Hterature from water treatment chemical suppHers. 

Sodium carbonate is used in detergent formulas as a water softener, to lock up calcium and magnesium from the water. Calcium and magnesium would otherwise combine with the soap or detergents and form an insoluble scum that would stick to the clothes and the washing machine. 

Hoi-Lime Zeolite Softening. In this process hydrated lime is employed to react with the bicarbonate alkalinity of the raw water. The precipitate is calcium carbonate and is filtered from the solution. To reduce silica, the natural magnesium of the raw supply can be precipitated as magnesium hydroxide, which acts as a natural absorbent fur silica. These reactions are carried out in a vat or tank that is located just head of the zeolite softener tank. The effluent from this tank is filtered and then introduced into the zeolite softener. There is always some residual hardness leakage from the hot-process softener to be removed in the final zeolite process. The hot lime process operates at about 220T (I04°C). At this temperature the potential for the exchange of sodium for hardness ions is greater than at ambient temperature, and the result is a lower hardness effluent than is achieved at ambient temperatures. This system is shown schematically in Fig. 2. 

The investigations of materials obtained from nano-sized particles showed that they have unknown properties or enhanced characteristics compared to common materials. Many alkaline carbonates have take in especially application in practice. Calcium carbonate is widely used for production of toothpaste, binding agents, etc. [12,13]. Barium carbonate is used mainly in glass production industries, as well as water softening agent, etc. Both substances are crystalline, with rhombic lattice and practically insoluble in water [12]. 

Scale is a rock-hard crust that can form in pipes and pots that are used with hard water. Before the general availability of household water softeners, scale was a much more common experience. Insoluble scale forms from calcium ions when carbonate ion is present. This fact highlights, once more, the versatility of carbonates. We have seen carbon dioxide form carbonates and hence carbonic acid in water we have used sodium bicarbonate (baking soda) as a base and finally we have pointed out that carbonates can make fairly insoluble solids. These many talents of the carbonate ion make baking soda good for more than cooking. Baking soda makes an excellent deodorizer because it can react with both acidic and basic smelly compounds and can form nonvolatile, and hence non-smelly, compounds with many more. A lot of chemistry in a little box ... 

The direction of the flow of the water through the softener is shown by the arrows. It emerges through a wood-wool filter, intended to catch any remaining undissolved calcium carbonate, into the soft water section from which it is withdrawn as required. 

Temporary hard water may be made soft by removing the excess of carbonic acid, which preserves the acid carbonate from decomposition, by boiling the water, as stated above, or by neutralizing the acid with a base, such as ammonium hydroxide or calcium hydroxide. Both kinds of hard water may be softened by the addition of a reagent which will form an insoluble compound by reaction with the salt which is causing the hardness. Borax is often used for this purpose. 

Removal of the ions that cause hard water is called water softening. In the lime-soda process used for large-scale municipal water-softening operations, the water is treated with calcium hydroxide (prepared from lime) and, sodium carbonate (sometimes called... 

In the lime soda process at one time used in large scale municipal water softening, calcium hydroxide prepared from lime and sodium carbonate are added to precipitate Ca - as CaC03(s) and Mg + as Mg(OH)2 (s) ... 

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