Introduction

Just as lime is produced by calcining limestone:

CaCO₃                           CaO   + CO₂
Limestone                      lime        carbon dioxide

so dolomitic lime is manufactured from dolomitic limestone (dolomite). 

CaCO₃.MgCO₃            CaO +MgO  +2CO₂

Thus dolomitic lime is a mixture of lime and magnesia. 

Dolomitic limestone (dolomite) has normally been formed by the replacement of some of the calcium in limestone with magnesia, a process known as dolomitisation.

Theoretically dolomitic lime should be a stoichiometric mixture of CaO and MgO, i.e. 58% CaO and 42% MgO. In practice, because of impurities and differing levels of  dolomitisation,  commercial products normally contain 35 - 40 % MgO .

Dolomitic lime is also known as:                        

calcined dolomite
dolime
lightly calcined dolomite
burnt dolomite

Production

Dolomitic lime may be produced in much the same way as lime, i.e in either shaft kilns or rotary kilns.

However much of the dolomite in the world is unsuitable for burning (calcining) at all, because it is highly crystalline.  This causes it to decrepitate (break up explosively) when calcined, as the CO₂ has difficulty escaping. Most dolomites are not physically as strong as many limestones, making shaft kiln firing impractical.

Lastly for many of the applications for dolomitic lime, a product low in residual CO₂, or a densified product is required.

For these reasons, most dolomitic lime is produced using rotary kilns.

Dolomitic lime, also known as soft burnt dolomite, or calcined dolomite is produced in rotary kilns fired with natural gas, coal, petroleum coke, and a range of recycled material (wood, tyres, solvents etc.) at temperatures up to about 1400 deg. C.  This gives a reactive product.

However by firing dolomite at high temperature (up to 2000 deg C) with a longer residence time in the kiln, the material begins to densify. The small crystals of MgO grow larger and the pores in the structure disappear. The bulk density may increase from around 1600 to over 3100 kg./m3. The resulting material is known as dead burnt or sintered dolomite and is used as the basis for a range of  refractory products.

Uses

Plants producing dolomitic lime are much less common that those producing quicklime .  This is partially because dolomite is less widely distributed than limestone, but also because of the more restricted uses.

The principal use of dolomitic lime is in the steel industry as a flux together with conventional lime. (see Iron and Steel industry)  In the UK around 250,000 tonnes per year are used for this purpose.

Dolomitic lime is also used in other applications as a source of lime and magnesia, or as an alkali to raise pH.

• as a slag component in the production of alloys such as ferro-chrome, ferro-manganese etc.
• as a binder for  pellets/sinter in iron making.
• as a source of lime/magnesia in the glass fibre industry
• as a source of MgO and a  pH modifier in agriculture and horticulture
• as a neutralising agent in water treatment
• as a reagent for producing magnesia from seawater  and brines
• as a pH modifier in drilling muds
• as a pH modifier in sewage sludge treatment.
• as a component in traditional mortars.

Dead burnt dolomite is the base material for the production of dolomite refractory bricks, and a wide range of monolithic and gunning materials. These are used principally in the steel industry, but also in kilns used in the manufacture of lime, dolomitic lime and cement.