Dolomite: What It Is and Where It Is Used

Limestone is calcium carbonate. Dolomite is calcium magnesium carbonate — the same rock family with magnesium built into the crystal, in roughly equal measure with the calcium. That one difference gives dolomite a different set of uses from limestone and lime, and a different set of things to specify. This article is the short introduction a buyer needs before ordering it.

What dolomite is

Dolomite the mineral has the formula CaMg(CO₃)₂; dolomite the rock is made mostly of that mineral. Compared with a high-calcium limestone, a dolomite carries much more magnesium and correspondingly less calcium. Burnt in a kiln it loses its carbon dioxide like limestone does, giving calcined dolomite or dolomitic lime — a mixture of calcium oxide and magnesium oxide. Slaked, that becomes a dolomitic hydrated lime. So there are three materials to keep apart:

• raw dolomite — the carbonate rock, crushed and graded;
• calcined (burnt) dolomite — the oxides, reactive like quick lime;
• dolomitic hydrated lime — the hydroxides.

A buyer asking for "dolomite" usually means the raw rock. Say which you mean on the enquiry.

Where it is used

Steelmaking and refractories

Basic-oxygen and electric furnaces lined with magnesia refractory add a magnesium source to the slag so the slag is saturated with MgO and attacks the lining less. Raw dolomite or calcined dolomite charged with the high-calcium lime flux does this job. Sintered and fused dolomite are also refractory materials in their own right. The lime in steelmaking article covers the flux side.

Glass

Dolomite supplies both calcium and magnesium oxide to a glass batch, where the magnesium improves the glass's resistance to chemical attack and adjusts how it works at temperature. Glass makers specify iron content closely, because iron colours the glass.

Construction and road aggregate

Crushed dolomite is a hard, durable aggregate for concrete, road base and railway ballast, and a filler in some products. For this duty the physical properties — strength, abrasion, grading — matter more than the chemistry.

Agriculture

Ground dolomite is spread to correct soil acidity where the soil is also short of magnesium; it supplies both calcium and magnesium carbonate, and it acts more slowly and more safely than lime.

Chemical and other industries

Dolomite and calcined dolomite are sources of magnesium for magnesium compounds, and are used in some water-treatment, filtration and flue-gas duties where a mixed calcium–magnesium alkali is wanted.

What to specify

• MgO and CaO content — the defining numbers, reported on the carbonate rock or on the calcined material as the order requires. For steel and glass, minimum MgO and the balance of CaO.
• Silica, iron and alumina — the impurities; a maximum on each, with iron specified tightly for glass.
• Loss on ignition — on the raw rock, a check that it is a carbonate; on the calcined material, a check on how well it was burnt.
• Size fraction or mesh — a lump band for furnace charging, a ground product for glass and agriculture, a graded aggregate for construction. Deshwali Minerals supplies dolomite graded to the size or mesh the buyer's process needs, with the fraction agreed at order.
• Certificate of analysis per lot, as for lime, against named methods.

The product page for dolomite states what the company confirms against the test certificate for a consignment; figures the buyer plans on are those agreed at order.

Dolomite or high-calcium lime?

If the process wants calcium alkalinity or calcium flux and nothing else, high-calcium quick lime is the material, and magnesium is an impurity to limit. If the process wants magnesium — refractory protection, a glass batch, a magnesium-deficient soil — dolomite or dolomitic lime is the material, and the MgO content is the specification. The quick lime vs hydrated lime article covers the calcium side of the family.

Sources

• Bureau of Indian Standards — Indian Standards for limestone, dolomite and lime are listed on the BIS standards portal: services.bis.gov.in
• Deshwali Minerals — dolomite product page