The raw materials used to make bricks are the most important part of the process. The quality of the brick and the success or failure of the brick plant can often be traced to the raw materials. The planning and implementation of a raw materials program is the backbone of a well-run brick plant. Brick plants are often built in close proximity to the primary raw materials to be used for decades. The quality and consistency of these raw materials normally vary throughout the mine property. When the characteristics of some mined materials are too far out of specifications it may be necessary to avoid these raw materials thus reducing the raw material reserves. Some of the characteristics and some of the ways to optimize the usage of raw materials are described below.
The characteristics of raw materials that can vary include but are not limited to the following:
- Fired Color – The color of the fired clay will determine the body color of the fired finished products. This is especially critical where the bricks will be used on commercial buildings where the color has been carefully selected. Colors can be managed through the careful mixing of different raw materials or through the use of natural or engineered pigments.
- Shrinkage – In order to meet the size specifications for fired brick, raw material shrinkage during the drying and firing processes have to be understood. In some brick plants the dry brick size is important as the setting equipment is designed to grip and maneuver within specific size ranges. Dry bricks that are too big or too small will not be able to be handled. The fired shrinkage and resulting total shrinkage must be understood so that the resulting brick meet the size specifications required by industry standards. /shrinkage can be partially controlled through the use of non-plastic materials such as ground fired brick, calcined clay, or sandy materials.
- Contaminants – Raw materials may or may not contain contaminants such as carbon, sulfur, soluble sulfates, lime, or iron pyrite. These contaminants can cause common brick making issues such as black core, bloating, scumming, efflorescence, face pops, or bleed-outs. These problems can be intermittent as the contaminants may or may not be present throughout a mining season and may vary in severity depending on the level of contamination. Black core and bloating can be managed through opening up the brick body with non-plastics or by adjusting the firing curve to allow for oxidation. Scumming and efflorescence can be eliminated through the use of additives such as barium or by firing to a low cold water absorption. Lime pops and bleed-outs can be avoided by grinding to a fine enough mesh where the lime or pyrite are broken up to a point where they do not pop or bleed out.
- Sand Content – The sand content of a raw material will vary throughout a geological formation. Sand is classified as any particle that is larger than 75 microns. The sand sized particles are non-colloidal and can help reduce shrinkage. The desired range of sand sized particles for a brick body mix is 30% - 35%. Planned mining using the weighted average values for particle size can help keep the raw material mix within specifications.
- Silt Content – Silt is classified as having a particle size of greater than 2 microns and less than 75 microns. These particles may or may not be colloidal and may or may not contribute to shrinkage. Raw materials or raw material mixes with high silt content sized particles can be difficult to use as drying issues and unpredictable characteristics are common. The desired range of silt sized particles for a brick body mix is 20% to 25%. Planned mining using the weighted average values for particle size can help keep the raw material mix within specifications.
- Clay Content – Clay is classified as having a particle size of less than 2 microns. These particles account for the shrinkage in a clay body and the strength of the dry and fired brick. Clay sized materials are normally colloidal and have additional properties such as plasticity that contribute to how brick raw material mixes behave. The desired range of clay sized particles for a brick mix is 40% to 45%. Planned mining using the weighted average values for particle size can help keep the raw material mix within specifications.
OPTIMIZATION AND VARIATION
The full usage of raw materials from a mine site is the goal when a raw material usage plan is made. Through exploration efforts and the characterization of the properties’ raw material reserves a written plan can be made that will utilize the range of raw materials to make a quality brick. Mine planning can be done that will result in the desired color, avoid contaminants, and contain the right ratios of sand, silt, and clay sized particles.
The plans for raw material utilization and optimization can still result in enough variation that the quality of the brick will be affected adversely. The sand, silt, and clay content can vary which can result in a less than optimized raw material mix. Particle size ratios that are not within specifications can lead to poor extrusion characteristics, low plasticity, and low green and dry strength. The natural variation of raw materials in a geological formation can make it difficult to use all of the materials in a formation.
IMPROVING RAW MATERIALS
In order to utilize the entirety of a mining area it may be necessary to enhance the raw materials with a binder and/or plasticizer. This can make marginal raw material mixes perform much better than they would without the use of an additive. The use of a clay conditioner at the correct dosage can help a brick plant extend the reserves of clay by making the marginal materials perform well on a consistent basis.