Sintered brick production technology

1. Aging

To ensure the stability and physical properties of raw materials, long-time weathering and homogenization is necessary for the raw materials, for better physical properties and well-distributed chemical composition. In the treatment stage, crushing and homogenization are particularly important, which will directly determine the final product quality. By comparing internal tests among some factories, the particle size of raw materials after crushing and out of cage screen, directly affects the shrinkage between brick shaping and firing. During the production according to the new national standard, we found that the most prone to deviation is the shrinkage, variation of cavity size, rib deformation and small cracks. When other processes and raw materials unchanged, smaller is the particle size after cage screen, the less happened this phenomenon is. In addition, by comparative tests, we found the raw materials newly mined and used directly after fineness treatment are prone to the above phenomena. Therefore, we suggest that factories implementing the new national standard must stack raw materials for long time, weathering and homogenizing the raw materials, and control the fineness of the crushed particle sizes. Though reducing the particle size will inevitably increase power consumption and equipment wear, this raised production cost will be rendered by the higher qualification rate after this adjustment.

Aging is a very important process, whose purpose is to allow the water to penetrate into the particles inside, realizing maximum uniform and consistent of water distribution, improving the surface brightness, strength, qualification rate of green bricks and reducing the product shrinkage at the same time. Four elements of aging are particle size, moisture, time and temperature. Particle size determine aging time and effect: the finer the particle size, the easier the moisture will penetrate, the better the aging effect, the time will be shortened accordingly. Via tests, the residue after 0.2mm screen will decrease by 3.51% after 48 hours aging. Relatively speaking, the higher the aging moisture content is, the better the aging effect will be. However, generally the aging moisture control is determined by the mold water rate, and it cannot be higher than the molding water rate, normally less than 16%. Time is also important to the aging effect: the longer the aging time, the more powder there will be.

The aging temperature is relatively easy to be solved in southern China, for instance, the general temperature in Guangxi is 10-25 ℃, which has little impact on the aging. However, as to the cold and frozen northern China, the impact of temperature on aging effect will be reflected quickly and heat sources should be used for aging if necessary. Factories should have an aging warehouse with more than 4 days storage. In case the aging warehouse is small, it has to adopt stricter crushing process, increasing the processes of crushing, rolling and multistage mixing and etc. The purpose is to reduce the particle size, increase specific surface area, make the mud materials fully contact with water, shorten the water soaking path and wet the mud materials evenly and fully.

2. Crushing

In the sintering brick production processes, the raw materials crushing equipment widely adopts two types: counter rolling crusher and hammer crusher. Normally, counter rolling crusher equipment need to configure with multi-stages counter roll crushers, realizing coarse, medium and fine three stages crushing, to meet the requirement of high output and fineness. In the production process, the counter roller crusher has good adaptability to the types of raw materials and high crushing capacity, and even if the moisture content reaches about 20%, it has little impact on the capacity and fineness. However, the application of three stages of coarse, medium and fine crushing increases the crushing processes and improves the difficulty of production management. Besides, rollers gap will become bigger after the surfaces is worn, thus the output size becomes bigger and coarser. So the roller surfaces shall be repaired in time, which is the shortage of counter roller crushers applied in practice. When adopt hammer crusher, the normal raw material treatment process is single stage hammer crusher plus rotary screen. The allowed moisture content of raw materials into the hammer crusher is less than 8%, otherwise, it is easy to block materials.

The parameters of hammer crusher like main shaft rotation speed, rotation diameter, number of hammer heads, hardness of hammer head, clearance between hammer heads and lining plates, number of grate plates, and etc, have great impacts on crushing capacity, and powder ration after the screen. In modification of sintered brick factory technologies, for the treatment of the same materials, the selection of crushing equipment is determined by the moisture content of the raw materials. For raw materials with high natural moisture content, it is recommended to use counter rolling crushing equipment; for raw materials whose moisture content can be controlled within 10% and with small fluctuation of moisture content, it is recommended to use single-stage hammer crusher and rotary screen.

3. Shaping

After the selected brick making raw materials treated and prepared, they enter the shaping workshop. China sintering brick molding method is mostly plastic extrusion. Plastic extrusion could be classified into three types: plastic extrusion molding, semi hard plastic extrusion and hard plastic extrusion, which are distinguished according to the different molding moisture content. When the moisture content of wet billet is over 16 %( dry basis, same as below), it is plastic extrusion. When the moisture water content is 14-16%，it is semi hard plastic extrusion. When the moisture water content is 12-14%, it is hard plastic extrusion.

Green brick shaping includes: feeding, mixing, adding water and wheel-rolling equipment after entering shaping workshop and before entering extruder; after the extruder, forming the qualified mud bar and green bricks. Shaping is to achieve the shape and structure of product. Therefore, shaping is the basis. That is to say, the product external shape and structure are formed by shaping process, which is the definition of ‘molding is the basic of brick making processes. Because the quality of the green bricks after shaping is directly related to the final product appearance quality, these wet green bricks need to be dehydrated and dried out from the shaping workshop.

In the sintering brick making processes, there are two ways of drying: natural drying and manual drying. Natural drying is to stack the wet green bricks on ridges of natural drying place, adopt manual inverted flower rack, and dry the wet green bricks into dried bricks with natural air. Manual drying means adopt manual dry chamber to realize the dry process，which could be divided into three types: large section tunnel drying chamber, small section tunnel drying chamber and room drying chamber. Whichever of these three drying types is adopted, the green bricks are stack on kiln cars by manual or by machine, and then the kiln cars with stacked wet bricks are moved into drying chamber for drying. The heat medium of the drying chamber generally comes from the waste heat of the sintering kiln or the hot blast stove. No matter which drying type or which manual drying type is adopted, it must be followed that the green bricks do not deform nor crack during the drying process. In case any problem in the drying, the appearance quality of the product would not be guaranteed, the scrap rate would be higher, the output capacity would decrease, the cost of finished product would increase, and the economic benefit of the factory would naturally be bad. Therefore, the green brick drying is often called ‘guarantee’, which emphasizes the importance of wet green bricks drying in brick making process.

After drying, when the residual moisture content is less than 6%, the green bricks will enter the sintering kiln. Rotary kiln and tunnel kiln are widely adopted for bricks sintering. In Rotary kiln, the green bricks are stacked in the kiln path by manual and the fire runs in the kiln for the baking. In tunnel kiln, bricks are stacked onto the kiln cars by manual or by machinery. The kiln cars with stacked bricks enter from the kiln head and exit from the kiln tail, and during the process, the green bricks on the kiln cars are roasted into sintered bricks. The fire in the tunnel kiln doesn’t move, while the kiln cars with bricks move in the kiln. No matter adopting rotary kiln or tunnel kiln, two points must be followed: 1. mustn’t burn the bricks not long enough, occur raw fire bricks   2. Mustn’t burn the bricks too much, occur over-burnt bricks. Therefore, it is often said that firing is the key of brick making processes, illustrating the importance of firing.

The new standards have higher technical index requirements to the hole rate, and the hole size requirement of rectangular hole improves the difficulty of brick shaping, which is relatively high requirement for vacuum extruder. The mud is evenly stirred by the spiral reamer of mixing machine, and fully homogenized in the conveying process, which avoids the uneven shrinkage of the adobe caused by uneven mixing and prevents the adobe cracks during drying and firing process. The mixed mud is vacuumed to remove the air in the holes, which further improves the mud’s plasticity. When producing rectangular perforated bricks, the extrusion pressure and vacuum degree of the extruder shall be appropriately increased. The shaping moisture shall be generally controlled to be 1-3% lower, normally no more than 18%. Since the moisture content is lower, to achieve the extruding strength, it is necessary to improve the extruding pressure and vacuum degree. The selection of extruder should be above model 50, with extrusion pressure no lower than 3.0MPa, and extrusion pressure more than -0.08Mpa. Only in this way, the green bricks strength and quality could be ensured, which is the basis of stacking bricks.  Otherwise, the increase of holes rate will cause the bottom green bricks strength decrease, thus causing the bottom bricks deform and collapse, and further affecting the stack effect and firing effect.

In the production of rectangular perforated bricks, reasonably core tooling is also very vital. The core tooling plays the role of perforation and adjusting the mud move speed at each part, which has a direct impact on whether qualified bricks could be extruded. By adjusting resistance of each related parts like the tool holder, core rod, core head, and etc. balance the mud’s extrusion speed as much as possible.  According to the shrinkage of raw materials, select the appropriate mouth size and brick thickness. In addition, the appearance quality of green bricks could be improved by reducing the size of cutting steel wire. One thing to note is the core tooling’s head and rod is rectangular, with different shape and different stress direction. So in case not fixed firmly, the core head could easily twist and have displacement caused by the deviation of stress, which will affect the shape of the extruded blank , failed meet standard and causing unqualified products. So when installing the core head, measures have to be found to fix it firmly.

3. Drying

3.1 One time stack drying

In sintering brick production process, one time stack is widely used due to its advantages of reduced operation steps, lower labor cost, improved product appearance quality and etc. In the technologies upgrades of sintering brick factories, who will adopt one time stack drying and sintering, the importance of drying tunnel kiln should be fully understood. The economic benefit of sintering brick factory mainly depends on high output: if the output is low, the benefit is poor. While the output basically determined by the drying process. In the sintered brick production processes, drying is between shaping and sintering and adopting one time stack drying kiln will enhance the continuity of whole production processes. In drying process, the wet green brick’s drying process is affected by raw materials properties, moisture content, brick structure, drying medium, drying equipment and etc. The drying cycle is quite different with the production rhythms of shaping and firing, which will have a great impact on the running of shaping equipment, firing cycle, production cost and etc.

One time stack drying kiln has certain requirements for the raw materials treatment: the fineness of raw materials must be controlled, the mixing proportion of waste slag must be stable, and the mixed materials must distribute evenly. Otherwise, reject rate will increase during drying and firing processes. One time stack drying tunnel kiln’s performance should meet the requirements of drying quality that the wet bricks should not deform, not crack, dry evenly, have short drying cycle, and consume less heat. In view of the rhythm difference between drying cycle of wet brick and the shaping process and firing process, many present one time stack drying kiln bring in static stop dehydration of natural drying, stacking the wet bricks on kiln cars and park them on the static stop line for some time.  At this time, the moisture content of the wet brick decreases, the moisture uniformity inside the wet brick increases, and the strength of the wet brick increases. The decrease of moisture content of wet bricks could appropriately increase the drying speed, thus improving the drying kiln output. Static stop dehydration, as an auxiliary method of one time stack drying kiln, is feasible.  It should be noted that when the production scale is large, the time for static stop dehydration after shaping is longer, therefore need more kiln cars and parking spaces. With more parking spaces of static stop dehydration, the number of finished product kiln car parking space and empty car space should increase simultaneously. Otherwise, the extruder production system will be will be restrained and the advantage of high output will have no place to play.

One time stack drying tunnel kiln is determined by the kiln section, kiln length, the distance from kiln car platform surface to the kiln roof. The wet green bricks are normally stacked no more than 14 layers. For the production line with annual output of 60 million pcs (converted into standard bricks), two lines of drying kiln could be adopt, with a cross section over 3.6m, length about 80m. Moreover, attention should be paid to the selection of fans for the drying kiln. The air pressure and air volume of heat fan and moisture exhaust fan have a big impact on the drying effect.

3.2 Notes for stacking green bricks

In production lines of all kinds of building bricks, kiln car’s stacking is a key link of brick factory production process. It is to stack green bricks by a certain stacking method and density on the kiln cars. Whether the stacking method and density are reasonable will not only directly affect the quality of drying and firing, but also affect the kiln’s production efficiency and firing quality. In the tunnel kiln production process, there’s a saying of ‘seven points for stacking and three points for firing’. In actual production practice, it is necessary to explore for long period to determine the suitable stacking scheme. The stacking scheme shall be determined according to the specific conditions of raw materials, guided by the principle of facilitating the firing, and shall ensure the smooth ventilation of brick stacks. A reasonable stacking scheme can not only make the air in the kiln distribute uniformly, but also can increase the heating area of the bricks in drying and firing, improving the drying and firing efficiency and product quality. There are two ways of stacking: stack with fire path and stack without fire path. Fire path means the gaps between stack and stack, and, bricks and bricks, reserved as the channel of fire and air flow. Whether to preserve fire path is decided by specific requirement of product type and sintering process. Stacking density is determined by the calorific value of the raw materials, and the detail number of one stack could be adjusted (slightly and cannot often) to the caloric value. For internal fired bricks, the consistency of calorific value of bricks on each kiln car in certain period of time shall be ensured. This requires that the calorific value of raw materials entering the production line be measured in time, and adjust the bricks number stacked on kiln car according to the change of calorific value, reducing the temperature fluctuation in the kiln caused by the fluctuation of calorific value and effect on quality of finished product. No matter adopting which kinds of stacking scheme, below points must be complied: the side shall be dense while the middle shall be thin, the stack shall be neat and stable, the air path shall be consistent as far as possible, and the brick stacks shall not be tilted in case any accident happened in the kiln. The two sides of the brick stack on the kiln car are important. It must be well controlled, cannot tilt, nor exceeding the kiln car structure, resulting in unstable stack or brick stack rubbing the kiln wall.

3.3 Moisture removal and drying of green bricks

The process of moisture removal and drying is to discharge moisture from the green bricks, which includes moisture removal stage and drying stage. Drying stage is the main stage, which could de divided into heating, constant speed drying, slower drying, and balance stages. Moisture removal mainly refers the air flow at room temperature take away the extra moisture on the surface of green bricks greater than air humidity, which is general short process. Tunnel kiln normally adopts drying chamber for moisture removal and drying, and the waste hot air from preheating zone is used for moisture removal and drying. The air flow direction is opposite to the direction of brick entering the kiln. The hot air firstly enters the bricks in balance stages, which has strict requirements for the air ‘s temperature, flow, speed. First of all, it requires appropriate coal amount. Normally, it is better to control the heat of green brick at 260-360kcal / kg, and the best is 290-330kcal / kg. In this way, under normal conditions, the firing temperature is generally controlled at about 950-1050 degrees, so the waste heat temperature will not be too high. The rectangular perforated brick produced according to the new national standard has big hole rate and relatively small rib. The dehydration process of raw materials is also called drying shrinkage. Drying shrinkage rate and drying sensitivity are indexes that shall be pay attention to. Generally, the shrinkage rate greater than 6% will bring a lot of cracks and affect the finished product rate. Figure 3 and Figure 4 show the relationship between drying shrinkage curve and brick temperature rise curve. In operation, attention shall be paid to ensure or coordinate the normal operation of the drying system, including checking whether each fan operates normally and the condition of the kiln car in and out. Generally, the drying chamber could only allow one cart of wet green bricks. Not entering wet bricks cart for long time or continuously entering several carts in short time should be forbidden. It is best to pushing out one cart of dried bricks and pushing in one cart of wet bricks in a few minutes. Secondly, When pushing the wet bricks carts into the drying chamber or pushing the dried or fired bricks from the drying chamber or firing kiln, the shorter the opening time of drying chamber door and the firing kiln door, the better, that is quick open and quick close. Thirdly, when loading the wet bricks, pull gently and push slowly to protect the stability of brick stacks from damaged. Before entering the drying chamber of firing kiln, The condition of the brick stacks shall be checked and whether comply with standards. In case the bricks stacks are inclined and easy to fail, protruding on two sides and top, and too high, measures shall be taken to repair one by one. No matter the bricks are stacked by manual or by machine, the key point is to facilitate drying and firing, firm and stable.

(1) Operate according to the fire path scale to ensure the smooth flow of vertical and horizontal fire paths. It is forbidden to throw blank blocks and other sundries in the fire path.