Fire Clay

Though no standard definition exists, the term fire clay refers to secondary clays that are not ball clays or china clays, but can be used to produce refractory bodies [3]. Fire clays are often found in proximity to coal deposits, but this is not true for all fire clays or for all coal deposits [6]. The main sub-types of fire clays, in the order of increasing alumina content, are plastic fire clays, flint fire clays, and high-alumina fire clays. The compositions of typical fire clays are summarized in Table 6 [22]. Among the attributes common to the different varieties of fire clays are their relatively low concentration of fluxing impurities (alkalis, alkaline earths) and their non-white color after firing. Through the 1970s, refractories made from fire clays set the standard for performance in metal processing applications due to their low cost, high corrosion

Table 5 Typical compositions (weight percent) of some ball clays [3,24]

Location

SiO2

AlA

Fe2O3

TiO2

CaO

MgO

K2O

Na2O

H2O

Tennessee

57.6

28.1

1.1

1.4

Trace

Trace

0.9

0.1

10.6

Tennessee

51.7

31.2

1.2

1.7

0.2

0.5

0.4

0.6

12.1

Kentucky

57.7

28.5

1.2

1.5

0.2

0.2

0.1

1.2

9.5

Type

SiO2

Al2O3

Fe2O3

TiO2

CaO

MgO

K2O

Na2O

H2O

Plastic fire clay

58.1

23.1

2.4

1.4

0.8

1.0

1.9

0.3

8.0

Flint fire clay

33.8

49.4

1.9

2.6

12.0

Diasporitic fire clay

29.2

53.3

1.9

2.7

12.0

resistance, and excellent thermal stability. However, higher processing temperatures and increasingly stringent batch chemistry requirements have driven most industries to alternative refractory linings such as high-alumina castables, basic brick, or carbon containing materials. In spite of the shift in industry needs, fire clay refractories are still used extensively. Current uses for fire clay refractories include insulation behind hot-face materials, low heat duty furnace linings, and specialty applications such as laboratory crucibles and setters.

Plastic fire clays have a composition similar to china and ball clays, except for the elevated Fe2O3 and TiO2 contents. Because of their composition, plastic fire clays have similar plasticity, dried strength, and fired strength when compared with china clays. Plastic fire clays range in color from gray to red or even black in the raw state. Like other fire clays, plastic fire clays produce buff-colored bodies when fired.

Flint fire clays have a higher alumina content than plastic fire clays, ball clays, and china clays, in addition to having slightly elevated levels of Fe2O3 and TiO2 (Table 6) [22]. Flint fire clays have lower plasticity (compared with china clays) when mixed with water and, consequently, develop lower dried and fired strengths. Because of the lower plasticity, the drying and firing shrinkages tend to be very low [25]. Processing of flint fire clays can require plastic additives such as ball clays or bentonites. In the raw state, flint fire clays range in color from gray to red and flint fire clay deposits tend to be harder than other clays [3].

High-alumina fire clays found in the U. S. contain substantial amounts of alumina minerals such as diaspore, in addition to the aluminosilicate clay minerals present. High-alumina fire clays can have much higher alumina content than other common clays (Table 6). These clays produce refractory bodies when fired, but have compara­tively low plasticity when mixed with water. Like flint fire clays, high-alumina fire clays undergo little shrinkage when dried or fired. In addition, the dried strength of bodies produced from high-alumina fire clays is poor. High-alumina fire clays tend to be gray to reddish-brown or brown in the raw state and produce buff-colored objects when fired.