Calcium oxide

(endorsed 2005)

Calcium oxide is used (after hydrating to produce ‘slaked lime’) to correct pH and adjust alkalinity, for coagulation optimisation, corrosion control and water softening. It can also be used to assist in the dewatering of sludge.

General description

Calcium oxide, $\text{CaO}$, is also known as calx, quicklime, unslaked lime and burnt lime. It is a grey-white, hard, odourless solid, which sometimes has a yellowish or brownish tint due to the presence of iron. It crumbles on exposure to moist air and is soluble in acid. Calcium oxide reacts with water to form calcium hydroxide (slaked lime), releasing heat as it does so.

Calcium oxide is available in several grades, and is the least expensive way of obtaining calcium hydroxide. Quicklime has a specific gravity of 3.2–3.4. Its bulk density is 1030 kg/$\text{m}^{3}$ for pebble quicklime or 1050 kg/$\text{m}^{3}$ for powder quicklime; it usually contains approximately 94% calcium oxide.

Appropriate handling materials for calcium oxide include fibreglass-reinforced plastic, polyethylene, polyvinyl chloride, cast iron and steel. Rubber-lined containers can also be used.

Chemistry

Calcium oxide is formed by calcination of limestone, and it can also contain magnesium oxide, MgO. Before being used in drinking water treatment, calcium oxide must be hydrated or ‘slaked’ to calcium hydroxide or slaked lime:

$$\text{CaO}+\text{H}_{2}\text{O}\rightarrow\text{Ca(OH)}_{2}$$

Slaked lime is added to water to provide hydroxide ions to raise pH and alkalinity, and to neutralise free carbon dioxide or carbonic acid. It reacts with carbon dioxide to form calcium bicarbonate:

$$\text{Ca(OH)}_{2}+\text{2CO}_{2}\rightarrow\text{Ca(HCO}_{3}\text{)}_{2}$$

To remove carbonate hardness, hydroxide ions are used to raise the pH of water. This causes precipitation, as bicarbonate ions are converted to the carbonate (at pH > 10), precipitating calcium carbonate.

$$\text{H}_{2}\text{CO}_{3}+\text{Ca(OH)}_{2}\rightarrow\text{CaCO}_{3}\text{(s)}+\text{2H}_{2}\text{O}$$

Typical use in Australian drinking water treatment

In production of drinking water, slaked lime is used:

• at the start of the water treatment process, to adjust pH and boost alkalinity in order to assist coagulation

• at the end of the treatment process, to adjust final pH and alkalinity, and to minimise corrosion

• to soften hard waters by raising the pH, thus precipitating calcium carbonate;

• with carbon dioxide, to increase soft water’s resistance to pH changes during distribution and to decrease its corrosivity

• to reduce the moisture content of sludge — if the concentration of calcium hydroxide is sufficiently high it will collapse the sludge structure, helping to reduce the water content of the sludge.

Slaked lime is usually made up as a solution or a slurry of up to 10% concentration; a slurry with a concentration of 1–5% is most commonly employed.

Typical slaked lime concentrations used in drinking water treatment depend on the quality of the water to be treated and the purpose of the treatment (e.g. water softening, pH adjustment or alkalinity increase). Slaked lime concentrations can vary from 5 to 500 mg/L, and the appropriate concentration should be determined by laboratory trials.

Contaminants

The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the source of the raw materials and on the manufacturing process. The following chemical contaminants may be present in calcium oxide (JECFA, KIWA 1994, NRC 1982):

• aluminium

• arsenic

• barium

• cadmium

• chromium

• fluoride

• iron

• lead

• magnesium

• manganese

• mercury

• nickel

• selenium

• silica

• silver

Residual and by-product formation in drinking water

When employed in drinking water treatment, calcium oxide should be used in such a way that any contaminant or by-product formed by the use of the chemical does not exceed guideline values in the Australian Drinking Water Guidelines.

Adding slaked lime to water can significantly raise the turbidity and the concentrations of iron, aluminium and manganese. Thus, it is often best to add slaked lime at the start of the water treatment process, if possible, so that any impurities added with it can be removed during the treatment process.

Status

Calcium oxide was endorsed by the NHMRC for use as a drinking water treatment chemical in 1983. The revision undertaken in 2003 did not change the status of this chemical for the treatment of drinking water.

References

ANSI (American National Standards Institute)/AWWA (American Water and Wastewater Association) Standard no B202-02. AWWA CD-ROM (April 2003). Available at <www.awwa.org>

Clesceri LS, Greenberg AE and Eaton AD (eds) (1998). Standard Methods for the Examination of Water and Wastewater, 20th edition. American Public Health Association, Washington, DC.

JECFA (Food and Agriculture Organization of the United Nations (FAO)/World Health Organization (WHO) Joint Expert Committee on Food Additives). Compendium of Food Additive Specifications. FAO Food and Nutrition Papers 52 (two volumes). Available at https://www.who.int/foodsafety/publications/jecfa/en/

KIWA (1994) Guideline quality of materials and chemicals for drinking water supplies. Inspectorate of Public Health and Environmental Planning, Publication 94-01. Rijswijk, The Netherlands.

National Lime Association (1992). Chemical Lime Facts, 6th edition. National Lime Association, Washington, DC.

National Lime Association (1995). Lime: Handling, Application and Storage, 7th edition. National Lime Association, Arlington, Virginia.

NRC (National Research Council) (1982). Water Chemicals Codex. Committee on Water Treatment Chemicals, Food and Nutrition Board, Assembly of Life Sciences, NRC.