In the past, potters used to “age” their moist clay in damp cellars or storage containers for years to achieve the effects that are now possible after a few minutes of clay mixing.
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.
In today’s heavy clay market there is becoming a push for more product in faster lead times. This need is putting extra pressure on manufacturers especially to increase throughput times. Although kilns and sometimes driers can be the bottleneck, there is added pressure on the moulding equipment to produce clay products quicker.
Why does an efflorescence appear?
In chemistry, efflorescence is defined as the migration of a salt to the surface of a porous material, where it forms a coating. The essential process usually involves the dissolving of an internally held salt in water. The water, with the salt now held in solution, migrates to the surface, then evaporates, leaving a coating of the salt. These efflorescent salt deposits tend to appear at the worst times, usually about a month after the building is constructed, and sometimes as long as a year after completion.
Before bricks and tiles can be fired the majority of water used in its moulding needs to be removed from the clay product. Any excess residual water or moisture left in the article as it enters the kiln to be fired will be removed very quickly leading to major problems such as blow out, warpage or in the extreme – as has been seen on numerous occasions – to explode causing serious damage to the kiln and production.
Generally, refractories are classified as basic, high alumina/silica, fireclay and insulating. There are also classes of “special refractories” which include Silicon Carbide, Silicon Nitride, Graphite, Zircon, Zirconia, fused cast and several others. Most refractory materials are supplied as preformed shapes. However, they also are manufactured in the form of special purpose clays, bonding mortars and monolithic such as castable, plastic refractories, ramming mixes and gunning mixes. In fact, many refractories materials have been developed specifically to meet the service conditions of a particular process. The characteristic properties of each refractory class are a function of both their raw material base and the methods used to manufacture the refractory products.
Plasticity is the characteristic behaviour of a ceramic material to become permanently deformed after the application of an external force. This property is the most characteristic one in clays, and there are some factors that influence the plasticity and they should be considered in plasticity measurements:
- Water physical characteristics like, viscosity, surface tension…
- Particle size distribution of the solid sample and its specific surface
- Chemical and mineralogical composition of samples
- Effect of the additives added to the clay/water system
- Sample temperature
- The way to prepare the sample, particularly the energy used to mix and to process clay, water & additives
Plasticity is defined as the capacity to be deformed without being broken. Many methods available to measure it, and they are classified into two groups: direct and indirect methods.
Table1. Direct and indirect plasticity methods.
The first process required to begin the production of clay bricks or roof tiles is to make available the raw materials required for the process. The major material used in manufacture are clays and as such brickworks will have been built as close to a clay quarry as possible although some clays are still trucked in to aid in varying colours and technical characteristics.
Breakage and rejection are two of the most common problems in ceramic manufacturing processes and It can come from a wide range of issues. Most ceramic manufacturers try to reduce their breakage and rejection as low as possible because small defect in a ceramic product can devalue the product by more than 50% or even mean a loss for a premium grade producer.
Drying is one of the most critical parts of making quality brick. There are a number of things that can be done to improve drying and reduce drying defects. In order to reduce cracking there needs to be an understanding of the drying process and understanding of the brick body that is being dried.