GIPS AD has been active in the gypsum and plaster market in Bulgaria for many years. It sells not only calcined plaster directly to the market but also raw gypsum from its own underground mine to building material companies. The old calcining plant from the early 1970s was based on Russian kettle calcining technology with a heavy fuel oil hot gas generator and electrostatic filter.
To fulfil European emission legislations, GIPS AD decided to build a new green-field facility next to the old one. The plant lies in Koshava near Vidin on the bank of the Danube River in the northwest corner of Bulgaria. Here semi-turnkey provider Claudius Peters Projects GmbH gives a detailed run-down of the new plant from the gypsum feed to the packed plaster product.
The Koshava plant produces plaster of different qualities that is dispatched in paper bags, big bags and silo trucks. Claudius Peters was selected for the plaster production as well as for the rock crushing and packing installation. The gypsum calcining systems are designed to process natural gypsum from the local underground gypsum quarries.
A green-field project
To take over production from the old three calcining kettles, the new calcining line was designed for a plaster capacity of 14t/hr, utilising a Claudius Peters EM 47-568 Gypsum Calcining Mill. The complete calcining package was awarded to Claudius Peters as a semi-turnkey project for supply and installation from the gypsum feed to the packed plaster product.
The scope of supply included the complete engineering as well as delivery of all of the equipment for raw material handling, calcining system, packing system, low voltage switch gear and automation system. Supervision of the erection and commissioning was also part of the contract.
Raw material handling
The raw material handling system includes all equipment necessary to handle the rock gypsum coming from the mine and supply it at the required quality and quantity to the plaster calcining system.
The rock gypsum is dumped by a wheeled loader into the hopper of a vibro feeder. It is then dosed into an impact crusher, where it is reduced to the right size for the grinding process. The crushed material is discharged via a belt conveyor, while metal impurities are removed using an electromagnetic belt separator. In addition, a metal detector is installed as a back-up to protect the grinding process from malfunction.
A double belt conveyor feeds the gypsum into the raw material silo. This technique was chosen because it allows for the inclusion of flue-gas desulphurisation (FGD) gypsum at a later date. For the same reason a mechanical extraction unit at the silo bottom was installed instead of a standard single-sided cone silo.
This unit feeds the Claudius Peters Troughed Chain Feeder (TCF), which supplies a constant gypsum rock feed into the Claudius Peters mill. The advantages of the TCF are its wear resistance and air-tight design.
Plaster calcining system
To enable plaster production in Koshava, the plant is equipped with an innovative Claudius Peters Gypsum Calcining System using a ball ring type mill and a new homogeniser. The EM mill has been installed in more than 100 gypsum applications and is a well-known machine for plaster production. This experience is the basis of the state of the art grinding and calcining technology.
The processes of grinding, drying, calcining and classifying are all executed in one single machine. Auxiliary equipment is needed for hot gas generation, product dust collection from the system air stream and for air flow supply. The mill design is based on hot gas inlet temperatures of up to 650°C, which allow low airflow volumes. This results in excellent energy performance.
Exhaust gas recirculation to the hot gas generator supports the thermal energy performance and provides the optimum atmospheric conditions for consistent product quality. Based on the direct calcining principle, the plaster quality produced provides the desired characteristics for making finished products.
The rock enters through a chute into the mill, dropping into the centre of the grinding element set. The grinding elements, consisting of five grinding balls arranged between the upper and lower grinding rings, crush the gypsum while the material migrates towards the outside of the elements, finally dropping into the hot gas air stream at the nozzle ring.
The hot gas picks up the ground particles, dries and calcines the gypsum and transports it to the internal classifier. The static classifier is a proven technology for the grinding requirements of the gypsum wallboard industry. The fine particles pass out of the classifier in the hot gas stream, while the denser, coarse particles fall back down into the grinding circuit.
The mill does not have any internal bearings or lubrication points and requires little maintenance and few operating personnel. By combining grinding, drying, calcining and classifying into one (high capacity) unit, floor space requirements are also reduced. This keeps land and building costs to a minimum. The main features of the grinding mechanism are as follows:
- No bearings and lubricating points in the grinding area,
- No sealing air required for mills working at negative pressure,
- Low specific energy consumption for grinding,
- Air inlet temperatures of up to 620°C are possible,
- Grinding element contour is self-optimising due to its spherical rolling motion,
- No adjustment of the contours by welding is required,
- Constant throughput capacities throughout the complete lifetime,
- Longest grinding element lifetimes,
- Optimum and constant material calcining for reliable highest product quality,
- Even material distribution in the grinding area for optimum quality,
- The highest resistance against foreign particles,
- Automatic foreign particle rejection without halting the mill.
Post calcining treatment
Downstream from the calcining system the plaster passes to a Claudius Peters Homogenizer. With the innovative CP Homogenizer Claudius Peters has introduced a method for post-treatment of β-hemihydrate allowing cost efficient plaster quality improvement.
A property of freshly-calcined β-hemihydrate (HH) is that it changes its characteristics during extended storage or handling. This effect is known as ‘natural aging’ (or ‘aging’ for short), and is based on the reaction of the gypsum with moisture.
Calcined gypsum from a technical process is not a pure gypsum hemihydrate, but also consists of water-free gypsum, mostly in the form of AIII (soluble anhydrite), as well as un-calcined remains in the form of gypsum dihydrate (DH). The presence of moisture allows the reconversion of AIII into HH and, given sufficient moisture level and contact time, even back into DH. This reconversion may occur preferably at cracks and pores of the fractured HH, resulting in covering and filling of these voids. A measureable result of the specific surface according to BET is reduced and in consequence the characteristics of the final product change.
In natural aging this process may take hours or even days, making the quality parameters of the product fairly hard to predict. For this reason many gypsum processors prefer to operate directly with the freshly-calcined gypsum in the downstream process.
A reduced specific surface area of the plaster indicates a compact crystal structure, such as expected in α-hemihydrate, which directly reduces the water demand of the plaster. At the same time it has been found that this water demand is also related to product strength, where a reduction in water demand generates products with improved, higher compressive and flexural strength.
Following the ideas of artificial aging, Claudius
Peters has developed processes that allow improvement of the following parameters in a gypsum production facility:
- Homogenising product quality,
- Stabilising the product,
- Reducing water demand,
- Reducing production cost,
- Improving product strength.
The key component in this process is the Claudius Peters Homogenizer. Freshly-calcined gypsum is taken directly and continuously from the system filter at full calcining temperature and introduced into the homogeniser. At the same rate the treated gypsum is discharged from the homogeniser, in this case to a downstream cooling process. The level in the homogeniser allows the treatment of the gypsum at the required temperature for a constant defined retention time. Moisture is supplied depending on the modification of the homogeniser, as humidity carried in the process gases from the calcining unit.
Gases introduced into the homogeniser are vented into the process dust collector. The homogeniser consists of a vertical reactor, where the reactor floor is covered by a fabric allowing even fluidisation gas distribution (see Figure 12). In this way the gypsum bed can float freely in the reactor and is transferred into intense contact with the carried moisture. In addition a central riser pipe receives additional amounts of fluidisation air, allowing gypsum transport from the floor to the upper section of the homogeniser, intensifying the product mix and homogenisation.1-2
|Property (example)||Before homogeniser||After homogeniser||Improvement|
|Specific surface according to BET (m2/g)||09/12/2013||~7||22 – 42%|
|Water/plaster ratio according to strewing method (kg/kg)||0.65 – 0.75||~0.6||8 – 20%|
|Compressive strength (N/mm2)||approx. 11||~16||~45%|
|Combined water content (%)||5.5 – 6.2||~6.2||0 – 13%|
|Soluble anhydrate content (%)||05/10/2013||~1||80 – 90%|
Table 1: Plaster properties
From the homogeniser the plaster is dosed by Claudius Peters Flow Control Gate into a pneumatic suction cooler. The cooling system not only reduces the plaster temperature from the calcining temperature of approximately 155°C down to 80°C by direct cooling with ambient air, but conveys it to the cooling filter.
From there it is discharged through a rotary airlock directly into the plaster storage silo. Further conveying equipment such as screws or bucket elevators are not required.
The packing plant consists of three independent packing and loading options: A Big Bag packing installation, a packing system for Valve Bags with palletising and a silo truck bulk loading system. They are all fed by the same plaster storage silo and can work simultaneously if required. The storage capacity is 500t of plaster.
Packing system for big bags: The bag packing system is designed for four sling big bags with a spectrum of 500kg to 2000kg. The filling is accomplished in coarse and fine flow by a screw conveyor. The rotation speed of the screw conveyor is adjustable using a frequency converter. The system is controlled by the electronic weighing system, Claudius Peters Pactron.
Packing system for valve bags: Bag filling is performed by a Claudius Peters in-line packing machine. The packing machine has two modular filling units.
It features modular design, horizontal filling impeller, upstream dedusting, a coarse / fine flow filling process, automatic tare setting before filling starts, check-weighing of the bags on the spouts and self-optimisation of the filling parameters by the Pactron weighing system.
Design capacity is 500bags/hr with 40kg bags but the machine is currently producing 25kg bags at a capacity of 600bags/hr. Other filling weights are freely selectable.3 From the inline bag packing system the bags are conveyed to the palletising installation. A bag roller conveyor is designed to transport and clean the bags pneumatically. They are flattened on a following belt conveyor. The bags coming from the bag flattener enter the palletiser via a bag infeed. A bag turner arranges the bags in rows according to the control programme. The row pusher shifts the individual rows to form complete layers. The stripping plate deposits these layers on the pallet hoist to form a stack.
The palletiser is fitted with the most sophisticated control facilities and offers a variety of possibilities for the operator to adjust the machine to changing operational requirements.
Bulk loading system for silo trucks: For larger quantities a bulk loading system for silo trucks has been installed. A flow control gate doses the plaster coming from the storage silo into an aeroslide. The plaster is conveyed with the aeroslide to a Claudius Peters Stationary Loading Device. There the plaster is loaded into the truck, while dedusting is accomplished by a central dedusting filter unit (red pipe in Figure 17).
Claudius Peters engineered and delivered the complete automation system as well as the low voltage switch gears. The automation system consists of IT-infrastructure, programmable logic controller system (PLC-system), computers and human-machine-interface (HMI). The visualisation was programmed with WinCC.
The project began in summer 2008 when GIPS AD was surveying the gypsum calcining market for a reliable partner. Due to a financing programme of the European Union an official tender procedure was carried out, in which Claudius Peters was awarded the project in December 2009.
After some clearing and engineering time, the project contract was signed in July 2010. For the first ground breaking in August 2010 a priest sanctified the location. In February 2011 the erection of Claudius Peters equipment started. Erection took six months. The first calcined material was produced in September 2011, followed by the plant performance acceptance in October 2011. The official plant opening ceremony was in October 2012.
|Plant parameter||Performance data|
|Product fineness||Residue on 200µm sieve, ≤1%|
|Product fineness (9t/hr)||Residue on 100µm sieve, ≤3%|
|Total fuel requirement||≤940kJ/kg|
|Total electrical power||≤25kWh/t (inc. cooling)|
|Initial setting time||6-9min|
Table 2: Calcining plant performance data
From project development through engineering, equipment dispatch, erection and commissioning, a close relationship has been kept between GIPS AD and Claudius Peters. This allowed the input of best practice in all stages to provide an innovative calcining facility to the Koshava facility. In addition, the supply of the complete system by Claudius Peters, from raw material handling to packing installation, allowed GIPS AD to minimise its own human resource commitment to the project.
The fully-automated system requires only a few operators and provides consistent product quality for the downstream processes. This results in efficient production of high-quality plaster with excellent characteristics. This is the basis of GIPS AD’s successful and environmentally-friendly operation that can flexibly supply the preferred product at low prices in a competitive market.
This project is financed by European Union and therefore any publication shall contain the following statement: “This document was created with the financial support of the Operational Programme for Development of the Competitiveness of Bulgarian Economy, co-financed by the European Union through the European Regional Development Fund. GIPS AD is solely responsible for the contents of the document and it can in no case be considered to be the opinion of the European Union and the Contracting Authority.”
1. Wetegrove, H. ‘The Claudius Peters Homogenizer: Cost cutting plaster production technology,’ within ‘Global Gypsum Magazine – November/December 2009,’ December 2009, 24-26.
2. Hilgraf, P. ‘Quality improvement of β-plasters,’ within ‘Zement Kalk Gips,’ 65. Volume (2011) No 6, 38-50.
3. Lübbert, B. ‘Packing Machines for Gypsum,’ presentation at ’11th Global Gypsum Conference, & Exhibition,’ 17-18 October 2011, Las Vegas, USA.