Manufacturing process
Step 1 – Raw Material Preparation
Very first step of AAC blocks manufacturing process is raw material preparation. List of raw materials and relevant details are mentioned below.
Fly ash or sand
Key ingredient for manufacturing Autoclaved Aerated Concrete (AAC) blocks is silica rich material like fly ash or sand. Most of the AAC companies in India use fly ash to manufacture AAC blocks. Fly ash is mixed with water to form fly ash slurry. Slurry thus formed is mixed with other ingredients like lime powder, cement, gypsum and aluminum powder in quantities consistent with the recipe. Alternately sand can also be used to manufacture AAC blocks. A ‘wet’ ball mill finely grinds sand with water converting it into sand slurry. Sand slurry is mixed with other ingredients just like fly ash slurry.
Limestone powder
Lime powder required for AAC production is obtained either by crushing limestone to fine powder at AAC factory or by directly purchasing it in powder form. Although purchasing lime powder might be little costly, many manufacturers opt for it rather than investing in lime crushing equipment like ball mill, jaw crusher, bucket elevators, etc. Lime powder is stored in silos fabricated from mild steel (MS) or built using brick and mortar depending of individual preferences.
Cement
53grade Ordinary Portland Cement (OPC) from reputed manufacturer is required for manufacturing AAC blocks. Cement supplied by ‘mini plants’ is not recommended due to drastic variations in quality over different batches. Some AAC factories might plan their captive cement processing units as such an unit can produce cement as well as process lime. Such factories can opt for ‘major plant’ clinker and manufacture their own cement for AAC production. Cement is usually stored in silos.
Gypsum
Gypsum is easily available in the market and is used in powder form. It is stored in silos.
Aluminum powder/paste
Aluminum powder/paste is easily available from various manufacturers. As very small quantity of Aluminum powder/paste is required to be added to the mixture, it is usually weighed manually and added to the mixing unit.
Step 2 – Dosing and Mixing
AAC blocks manufacturing process of dosing and mixing defines the quality of final products. Maintaining ratio of all ingredients as per the selected recipe is critical to ensure consistent quality of production. This is accomplished by using various control systems to measure and release the required quantity of various raw materials.
A dosing and mixing unit is used to form the correct mix to produce Autoclaved Aerated Concrete (AAC) blocks. Fly ash/sand slurry is pumped into a separate container. Once the desired weight is poured in, pumping is stopped. Similarly lime powder, cement and gypsum are poured into individual containers using screw conveyors. Once required amount of each ingredient is filled into their individual containers control system releases all ingredients into mixing drum. Mixing drum is like a giant bowl with a stirrer rotating inside to ensure proper mixing of ingredients. Steam might also be fed to the unit to maintain temperature in range of 40-42℃. A smaller bowl type structure used for feeding Aluminum powder is also attached as a part of mixing unit. Once the mixture has been churned for set time, it is ready to be poured into molds using dosing unit. Dosing unit releases this mixture as per set quantities into molds for foaming. Dosing and mixing process is carried out continuously because if there is a long gap between charging and discharging of ingredients, residual mixture might start hardening and choke up the entire unit.
In modern AAC manufacturing plants, entire dosing and mixing operation is completely automated and requires minimum human intervention. This entire operation is monitored using control systems integrated with computer and CCTV cameras. As with any industrial operation, there is provision for human intervention and emergency actions integrated inside the control system.
Lime and Cement
Lime and cement are both individually dosed by precision dosing screws into the batcher(s).
Water Content
The total weight of water is important in the mix formula. Based on the required density the computer calculates the water already contained in the slurries and adjusts the water batcher to the required residual water.
Slurries
The production slurry is pumped to the slurry batcher and dosed as first component. Returns slurry is pumped into the batcher and dosed as second component. The slurries are dosed in quantities which contain exactly the weight of dry material required by the mix formula. That means the actual density of the slurries is calculated as the base. The slurry quantity is quickly new designed for each mix-batch by Ruiguang and automatically adjusted in case of differences in the slurry density. This guarantees a consistent quantity of dry materials at all times.
Temperature
The mix temperature is important to get consistent uniform cakes.
The temperatures of all materials are measured to receive the actual heat content. This will be compared with the necessary heat content for the cake to rise to the required height. The preselected slurry temperature is achieved by adding hot and/or cold water which is automatically adjusted by the PLC control system.
Aluminum
Fine aluminum particles are used to create the hydrogene bubbles forming the porous structure of AAC. Aluminum is mixed with water in one of the aluminum slurry mixers. This slurry is pumped to the aluminum slurry.batcher above the main mixer. (In some factories aluminum is dosed batchwise.)
Mixing Sequence
• starts by dropping the slurries into the main mixer
• cement or lime or both together and the residual water are added (details depend on the mix formula and the raw material quality)
• preselected main mixing until a good blend is achieved. At the end of the main mixing cycle, aluminum slurry is added to the mixer
• followed by cleaning water from the aluminum dosing equipment
Once the aluminum is well distributed in the slurry, the mixer discharges the mixed slurry into the mould
Step 3 – Casting, Rising and Pre-curing
AAC blocks manufacturing involves casting, rising and pre-curing. Once
desired mix of raw materials is ready, it is into moulds. This process has different names. It is called casting, pouring or moulding. For current discussion, we’ll call it casting.
After thorough mixing, slurry containing fly ash (or sand), lime powder, cement, gypsum and Aluminum is poured into moulds. Moulds can be of various sizes depending upon installed capacity. Standard moulds for 160-500 m3/day are 4.2m x 1.2m x 0.65m in size. Some
Autoclaved Aerated Concrete (AAC) blocks plant might have moulds of either 4.8m or 6m.
Before casting, moulds are coated with a thin layer of used oil. This is done in order to ensure that green-cake does not stick to moulds. Once slurry is mixed and poured into greased moulds, Aluminum reacts with Calcium Hydroxide and water to form Hydrogen. Millions of tiny Hydrogen bubbles are released due to this reaction. This leads to formation of tiny unconnected cells causing slurry mix to expand. Such expansion may be twice its original volume. It must be noted that bubbles generated during AAC manufacturing process are unconnected and closed bubbles unlike open bubbles of CLC blocks. Bubble size is usually 2-5mm. These cells are the reason behind light weight and insulating properties of AAC blocks. Once rising process is over, green-cake is allowed to settle and cure for some time. This ensures cutting strength required for wire cutting.
Usually rising and pre-curing process takes around 60-240 minutes. Rising is dependent on raw material mix and weather conditions. It should be noted that weather is a major factor affecting rising process. As reaction is affected by weather conditions, it is recommended to maintain constant temperature in pre-curing area. Due to this, pre-curing is also referred as ‘heating room pre-curing’. This can be achieved by deploying radiating pipes for indirect heating. It should be ensured that green-cake is not subjected to vibrations during pre-curing or else it might develop cracks.
At end of pre-curing process, green-cake is hard enough to be wire cut as per requirements. Pre-curing is not a complicated process, but it should be monitored constantly. Operators must monitor the slurry change during rising. Constant feedback must be provided to dosing, mixing and casting operators. Pre-form defects (cracking, sinking, etc) mainly occur during the process.
Step 4 – Cutting
The trolley of the cutting line transport system picks the cake for cutting.
The horizontal (cross)cutter cuts the vertical sides of the cake to achieve the block length (or panel width) and profiles tongue and groove, if required.
The vertical cutter slices the cake pre_precisely to the required block or panel thick_ness.The cutting wires of the thickness cutter are stepwise off-set to minimize cutting stress in the green cake.
Handholds facilitate lifting of blocks by the mason, a tremendous benefit for heavier blocks
Extremely short and thin cutting wires improve thickness accuracy.
The cross cutter cuts vertically the block height (or panel length). Precise block height is the most important feature for thin bed laying.
The cut-offs are dropped into the returns mixing system underneath.
Conveyors bring the returns into the mixing tank. Once the returns mixing tank is full and the slurry has the required density it will be pumped into one of the storage tanks for returns slurry.
The cut cake remains on the autoclave pallet or autoclave grid and is ready to be placed onto the autoclave trolley by the loading machine.
Step 5– Autoclaving and transport
The loading machine loads a number of cut cakes on autoclave pallets or grids, with the fresh cakes onto autoclave trolleys. These go on storage tracks into a covered and heated room in front of the autoclaves.
A full set of autoclave trolleys is wheeled into the autoclave. After closing the door the autoclave is set under vacuum.
The steam hardening process starts by feeding steam and gradually increasing the pressure up to approx. 12 - 13 bar overpressure (188 psi) equivalent to 194℃ (381℉). This pressure / temperature is kept for several hours for hardening and forming calcium-silicate hydrates.
At the end of the autoclave cycle the pressure/ temperature is gradually reduced to ambient pressure. The temperature difference ，especially at the beginning of the autoclave cycle causes condensate. The condensate is automatically drained by the autoclave drainage system.
With continuously rising energy cost, the condensate water should be collected, the heat used and then the condensate water can be cooled for reuse.
The whole autoclave cycle is controlled by PLC system, computerized autoclave control operating with special motorised valves.
After autoclaving the door opens. The autoclave trolleys are one by one pulled out by the transfer car and pushed to an extra track ready for unloading.
Usually there are approx. 2 autoclave cycles in 24 hours. The autoclaves should always run 24 h/day, even if the production of the factory runs on less hours.
Step 6– Steam boiler plant
Energy Efficiency
With rising energy cost and to conserve environment all heat must be used to opti_mize energy efficiency.
The steam boiler plant for approx. 16 bar (230 PSI) with burner, chimney, feed water preparation, heat exchange, piping system, electric control and more accessories supplies steam for:
• autoclaving
• hot water (Dosing and Mixing)
• heating of the precuring chamber
• heating of the waiting area in front of the autoclaves
• the heating of the factory buildings
Part of the heat can be taken from the heat contained in the condensate.
Step 7– Handling ,sorting and palletizing
The autoclave trolleys with the autoclaved cakes on autoclave pallets are moved to the loading/ unloading machine which takes each autoclave pallet with the hardened cake from the autoclave trolley and places it onto the autoclave pallet transport.
When all cakes are unloaded the empty autoclave trolley is grabbed by the loading machine and placed onto a storage track in front of the autoclave to be re-loaded with fresh cakes on autoclave pallets.
The hardened cake on the autoclave pellet moves into the separating machine where all layers are separated one by one.
The sorting and palletizing machine removes the complete cake from the autoclave pallet and places it onto wooden pallets.
The sorting and palletizing machine can also sort two or more different block thick_nesses, a tremendous benefit of the whole system.
The empty autoclave pallet is cleaned and returns to the autoclave pallet feeder at the tilting machine.
The blocks on the wooden pallets move forward by the pack transport to the film wrapping machine and further out. forklift brings the packs to the storage yard.
Brick block machine
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