1. Ball mill for milling sand
The sand is usually brought to site and stored in a sand bunker. A front-loader dumps the sand into a feeding hopper, from where a conveyor belt with weighing system will feed the sand to the ball mill.

The sand will be milled with water to a fresh sand slurry; a slurry pump pumps the slurry from the sand mill outlet into the slurry tank(s).
Note: In some cases it is required to dose the gypsum (anhydrite) into the ball mill as well and this will be considered in the relevant design.
2. Slurry tanks
The fresh sand slurry from the ball mill as well as the return slurry (green waste material dissolved in water) from the cutting system are separately stored in slurry tanks. Standard design agitators are single shaft agitators, but on request planetary type agitators are also available.
3. Silos for lime, cement and gypsum
The binder materials such as cement, lime and gypsum (anhydrite) are usually brought to site as dry powder in silo trucks with a compressor.

The truck blows the materials up into one of the storage silos on top of the mixer tower.

The silo size is approx. 120m³ and each silo is equipped with filters, overpressure protection, level and temperature indicators and fluidising elements to ensure a controlled discharge of the binder material from the silo.
4. Dosing and mixing
All the components are batch wise weighed and dosed according to the required recipe. The recipes are stored in the control system of the mixing tower and will vary depending on the required product quality, binder material properties as well as production circumstances. The control system automatically adapts to variations in temperatures of the raw materials, densities of the fresh sand slurry and the return slurry and other influences in order to ensure that high quality AAC can be produced.

Depending on the type of factory and the required production capacity the slurries can be dosed in separate weighing hoppers in parallel (KBT plant version) or directly into the mixer (KBT-S plant version).

A special patented fluidising dosing system doses the binder powders to ensure high dosing speed at very high accuracy. This system requires less maintenance than conventional screw conveyors. The components are weighed in either a combined weighing hopper (KBT-S plant version) or dedicated weighing hoppers (KBT plant version).

Hot and cold water are weighed in a dedicated water scale; a part of the water is pressurized and released into the mixer through spray nozzles in order to clean the mixer internals every cycle to prevent any material build-up.

The aluminium is usually delivered as dry powder in barrels or as paste. In case powder is used, the aluminium powder will be batch wise mixed with water to a dispersion for every single casting cycle. Optionally it is possible to use two types of aluminium (coarse and fine) simultaneously. The dispersion is pumped into a holding hopper over the mixer, to permit the release of the aluminium in the mixer at just the right moment.

All components are released into the mixer in a predetermined sequence, taking the local circumstances and raw materials into consideration and are mixed by the high-speed mixer to a homogenous mix.
5. Casting
A conveyor moves the empty and oiled mould with inside dimensions LxWxH of 6,16 x 1,58 x 0,69 m to the casting position under the mixer.

The casting outlet lowers into the mould and the mixer flap opens to pour the mix into the mould, filling up the mould only partially.
The design of the pour chute will reduce the entrainment of air in the poured mix and will prevent dripping of the mix after the mould is conveyed from the casting position.
6. Reinforcement preparation and inserting
HESS AAC plants offer the possibility to produce large size (up to 6 m long) reinforced elements. The welded steel reinforcement cages are coated with an anti-corrosion agent using the special HESS coating system. Depending on the required capacity it is possible to use either
a basic small capacity system or a fully automated advanced system
to produce up to 100% of the
production capacity as reinforced material. The reinforcement is assembled and positioned with the aid of holding frames. The reinforcement crane lowers the holding frame with the coated reinforcement into the casting mould immediately after the mix
is poured.
7. Rising/pre-curing
A traverser moves the mould with the fresh mix onto a rising position in the rising area. In approx. 2-3 hours time the fresh mix rises and stiffens to a firm green cake with the volume of the mould.
8. Position for the extraction of reinforcement holding pins
In case reinforced products are made, the holding frame for positioning the reinforcement shall be removed before cutting of the cake. The operator unlocks the pins attached to the holding frame to release the reinforcement cages from the holding frame and the reinforcement crane pulls the holding frame from the mould. The holding frame with pins returns to the reinforcement preparation system where a new set of reinforcement will be attached to the holding frame.
9. Tilting the cake with tilting crane
Once the green cake is firm enough to be handled and cut, a traverser moves the mould out of the rising area and a conveyor moves the mould to the tilting crane. The tilting crane tilts the mould with cake by 90 degrees, positions the mould side (cutting platform) onto the cutting wagon in the cutting line, unlocks and detaches the mould body from the cutting platform.
The cake is standing vertically on the cutting platform. The tilting crane moves the empty detached mould body to an empty and cleaned cutting platform that is available on the cutting platform return line. The tilting crane locks the platform to the mould body and tilts the assembly flat while positioning the mould on the conveyor to the mould-oiling unit.
10. Mould oiling
The inside of the mould requires to be oiled to prevent sticking of the green cake to the mould during the rising and de-moulding process. A fully automated oiling unit will spray a thin film of oil onto the inner surface of the mould. As an alternative the moulds can be oiled manually.
11. Vertical cutting and profiling
The cutting wagon carries the cake on the cutting platform through the various cutting and profiling stages. In the first stage a set of wires trims one side of the cake (the top side during rising) to obtain roughly the right cake width; a set of knives on both side of the cake will cut the final dimension. The (tongue and groove) profile in the blocks can be cut by using a knife blade with the corresponding shape; the groove is cut at the side that was the bottom side during rising.
12. Horizontal cutting
After vertical cutting and profiling is completed, the cake passes through the horizontal cutter to cut the block thickness. Stationary wires that are pneumatically tensioned cut the cake horizontally in the required dimensions.

The wires are at an angle to the direction of movement of the cutting wagon to avoid the pull out of material at the trailing end of the cake.
13. Cross cutting and handgrip milling
After horizontal cutting is completed, the cake is transported to the cross cut position. A lifting station will lift the platform from the cutting wagon. The cutting wagon moves back to take the next cake down the cutting line, while the transport wagon moves under the cake lift to receive the platform with cake after completion of the cross cut. The lifting frame of the cross cutting machine will move the cutting frame down and up through the cake. The cutting wires are oscillating via two reciprocating moving cutting shafts on both sides of the cake. Pneumatic spanners tension the wires and 200 mm wide segments with guiding grooves at a 5 mm pitch guide the wires. This allows blocks with a height of 200 mm or more to be cut with individually tensioned wire to prevent any form of double cutting and to achieve narrow tolerances on product dimension.

A vacuum hood lifts the top layer from the cake and dumps it into the channel under the cross cutter after the cutting wagon has moved from the cross cutting position. The crusts at the front end and at the trailing end are blown into the slurry channel under the cutting machine; a scraper will clear any remaining waste from the cutting platform.

A milling unit attached to the cutting frame of the cross cutting machine will cut the handgrips corresponding with the block thickness in the green stage. On each side of the cake two milling units with a number of cutting blades pitched at block height (cross cut dimension) on a rotating shaft move in and out of the cake. At the position of the block thickness the cutting machine holds the cutting frame at the required position for milling the handgrips.
14. Back-tilting onto a cooking frame
After the cutting is completed, the back tilting table tilts the cake back by 90° onto a cooking frame. The cooking frame with the bed waste layer (the layer between the platform and the first layer of product) tilts back to its original position for transport to the bed waste removal unit. Autoclaving the cake on the cooking frames allows efficient autoclave loading and most importantly, will prevent most of the sticking of the layers, which is a typical disadvantage of the traditional tilt-cake systems in which the cake is autoclaved in the vertical position.

Both the HESS KBT-S and KBT systems have combined the advantages of the tilt-cake and flat-cake system. In the HESS system no part of the mould or platform used for cutting go into the autoclaves.

HESS SYSTEMS PRODUCE NO PROCESS RELATED WASTE!
15. Bed waste removal
A combined scraper and brush remove the green bed waste from the cutting platform. The waste is collected and a belt conveyor will move the waste to the cutting pit for further dissolving into return slurry.
A conveyor system transports the cutting platform from the platform crane via the bed waste remover to the position in which the tilt cake will lock a mould body to the platform.
16. Stacking and buffering of green cake
Roller conveyors move the cooking frame with green product to the multi functional crane.
br> The multifunctional crane stacks three cooking frames with green product on top of one another. Stacking pins are manually inserted as spacers between the cooking frames. A bogey carries the stack of green product down the buffer track until loading into the autoclaves can take place.
17. Autoclaving
The traverser in front of the autoclaves moves the bogeys into the autoclave. This traverser is capable of pushing and pulling a complete train of bogeys with stacked cakes into or out of the autoclaves.

In the autoclaves the cakes are cured with steam at a pressure of approx.
12 bars.
The pressure and cycle time depend on the raw materials and the product quality to be made. On average the production capacity of the plant can be calculated based on two autoclave travels per day
(24 hours). A steam boiler supplies fresh steam to the autoclaves.
18. Buffering and destacking of hardened cakes
After the curing process in the autoclaves is completed, the traverser in front of the autoclaves moves the bogeys with cured product one by one to the buffer tracks in order to allow for a short unloading time of the autoclave.

The multifunctional crane takes the
cooking frame with cured product one by one from the bogey to the roller track leading to the unloading crane. The operator will remove the stacking pins from the cooking frame and store these into the brackets in the cooking frame.
19. Unloading from cooking frames
The unloading crane takes the white product from the cooking frame and positions the product on the white separator (if this option is included) or positions the product directly onto the packaging line. The empty cooking frame is moved into a buffer or
returned to the back tilting table.
20. Separating
Although the sticking between the horizontal cut layers is minimal due to the orientation of autoclaving in the HESS KBT-S and KBT system, it is recommended to install a separation machine. Especially thinner blocks (100 mm and less) can be sensitive to sticking.
A green separator may serve as an alternative to a white separator; the green separator separates the layers carefully after the back-tilting system has put the green material on the cooking frame. The separator repositions the product layers with small gaps on the cooking frame. This will eliminate sticking completely and will enhance the curing process in the autoclave.
21. Packaging lines
The unloading crane moves the cured and separated product to the packaging line. Marker requirements will determine pack size, product orientation in the pack, requirements for strapping as well as the requirement to sell product with or without wooden pallets, and with or without foil.
Packaging lines are for this reason always customized to fit the needs of the individual customer.