Dolomite bricks

Includes magnesite, magnesite—chrome, chrome, chrome—magnesite brick, and dolomitic brick. 

Dolomite bricks are a cheap refractory construction material with limited application possibilities... 

Other industrially important basic products are forsterite and dolomite bricks. Forsterite bricks are manufactured by firing naturally occurring low iron olivine, a mixed crystal of 2Mg0 Si02 (forsterite) and 2FeO Si02 (fayalite). 

The starting material for refractory dolomite bricks is calcined dolomite, which is produced from dolomite by deacidification at temperatures of ca. 1000°C and then firing at 1500 to 1800°C. They are very much cheaper than magnesite products, but are also less slag-resistant. To prevent hydration, dolomite bricks have to be impregnated with tar. 

Dolomite bricks are used in the linings of furnaces in the cement and steel industries. Pure dolomite contains 45.7% MgCOg and 54.3% CaCOg by mass. Determine the empirical formula of dolomite. 

The cost of the lining amounts to between 1 and 2% of the cost of construction if the rate of consumption of refractory material is about 0.5-1.5kg per tonne of Portland cement clinker produced. About 35% of the lining in a modern kiln typically consists of dolomite brick, 35% of magnesite-chrome brick, and the remainder (30%) of fireclay brick, lightweight refractory brick, special brick and monolithic refractories. 

Munchberg, W.,/Opitz, D./Stradtmann, J.. The wearing of burnt dolomite bricks in cement kilns. - In World Cement Technology 8/1977/39 —46. 

According to Konopicky (1957), the expansion joints should, as a rule of thumb, be designed to absorb half the thermal expansion that develops up to the service temperature of the refractory lining. Approximate values to be adopted are 0.5% for fireclay and silicon carbide brick, and 1.2% for magnesite-chrome and dolomite brick. The expansion joints are filled with cardboard, which burns away when the kiln is heated, leaving the joints clear. In general, it is more favourable to provide a larger number of narrower expansion joints than a smaller number of wider ones. 

A cheaper variety of dolomite brick is known as semi-stabilized dolomite. This is prepared from the calcined dolomite without the separate addition of silica. Fe203 and AI2O3 are added to the calcined dolomite for bonding purposes. Heavy oils are also added that uniformly spread the Fe203 and AI2O3 particles in the mixture. These oils also keep out moisture. Thus, a reaction between moisture and calcia is prevented until the bricks are fired. The firing is done is a kiln at a temperature of 1400°C. If the fired bricks are not put into service immediately, they are coated with a tarry material. 

Dolomite refractories were used in AOD service in the early stages of development of the AOD process. Dolomite brick contain islands of MgO crystals in a continuum of CaO. The hot face region of a used dolomite brick after AOD service is shown in Figure 29. In this photomicrograph, the immediate hot face is to the right of the photomicrograph. The central area of the photomicrograph is the affected zone where corrosion has resulted in reaction of... 

Figure 29 Dolomite brick after AOD service (130x).

The principal use of forsterite refractories is in Europe, but apphcations throughout the world have been reported. Its main uses are as steel-making refractories for tundish boards, sprays, and back fiUings ladle safety linings for dolomite brick and in electric arc furnace safety linings and roof brick. In industrial furnace applications, forsterite is employed in alloy foundries and glass tank regenerators. 

In the early 1960s, fired dolomite bricks had been developed for use in the burning zone of rotary cement kilns. Dolomite is chemically compatible with the cement-making process and readily acquires a protective coating with the clinker in the burning zone. The need for chrome-free linings in the cement-making process added impetus to penetration of dolomite refractories in this application. 

As indicated previously, fired dolomite brick has been used in cement kiln burning zones since the early 1960s, and in order to improve product performance, formulations have been developed incorporating Zr02 additions to improve the resistance to thermal shock. The use of zirconia for this purpose was first developed for cement kiln brick. 

---