Produces a High Strength Beta Plaster
Processes a wide size range of materials
Processes wet feed materials
Fine Temperature Control
Controlled Material Residence Time
Direct or Indirect Firing
It is generally accepted that beta plasters produced by conventional means (kettles, rotary kilns etc) provide only relatively low strength plasters with high water take-up during setting.
Alpha plasters, typically calcined under steam pressure in autoclaves, provide a high strength material with low water take-up during setting. These plasters, however, are not generally economical to use for such end products as Rapidwall.
The early development work on the Rapidflow Calciner indicates that it is capable of producing a significantly higher strength beta plaster while still operating under atmospheric pressure. The reason for this is that, while the design of the Rapidflow Calciner is based on the fluidised bed concept, it differs significantly from other fluidised bed calciner designs in that the introduction of hot calcining gases into the fluidised bed of material is better controlled.
Gypsum (calcium sulphate dihydrate) is a material that calcines at a relatively low temperature (150°C - 180°C) to produce the required hemihydrate product. The proportions and forms of dihydrate, hemihydrate and anhydrite in the plaster product are very sensitive to calcining conditions, particularly temperature.
The Rapidflow Calciner design provides for very limited temperature gradients across the bed of material being processed with the result that the calcining conditions can be more readily controlled to provide the required plaster product specification. This is achieved primarily by the calciner distributor design that provides for rapid heat transfer from the hot calcining gases to the calcining gypsum and prevents any material being subjected to temperatures in excess of that required for optimum calcination. The essence of the distributor design is that it provides a pronounced velocity gradient across the lower region of the fluidised bed with the effect of keeping the whole bed of material in suspension. Under these circumstances there is no opportunity for material to settle out or partially defluidise and thus be subjected to over-temperature conditions.
As indicated above the Rapidflow Calciner incorporates a novel distributor plate design. This also allows it to fluidise, and therefore calcine, a wide size range of materials. For natural gypsum, a top size of 10 - 12 mm is applicable while synthetic gypsum can be handled down to less than 100 micron sizing.
The advantage of the design is that, within wide limits, it is not sensitive to feed sizing. This in turn provides for more economical feed preparation systems and increased plant operation versatility.
An added advantage is to have the ability to accept feed materials with significant quantities of free moisture. In the case of natural gypsum this would typically be 5 - 10% while synthetic gypsum may typically contain 15 - 20% free moisture. The advantage is that a separate drying stage is not required.
The design of the Rapidflow Calciner provides for a self-cleaning feature by way of an air sweeping effect at the distributor plate level. Conventional fluid bed distributor plate designs have no specific fluidising air directionality related to material flow through the Calciner. In this case the fluidising air introduced to the fluidised bed of material via the distributor plate is directed toward the calciner product discharge. This has the effect, upon emptying the calciner bed, of sweeping any remaining material from the unit thereby allowing feed material of a different type to be introduced quickly without cross-contamination or the need for operator intervention.
In kettle type gypsum calciners, for example, the process of calcining gypsum with the consequent evolution of water vapour brings about a state of natural fluidisation that assists with heat transfer through the bed of material. This natural state of fluidisation, however, can only be poorly controlled and this can compromise the degree of process control required to meet particular product specifications.
In the case of the Rapidflow Calciner the fluidisation of the material is established without relying on the evolution of water vapour or the natural fluidisation characteristics of the gypsum. It can be readily controlled to suit the physical characteristics of the material and thereby ensure controlled and repeatable process conditions.
The gypsum calcination process is very sensitive to temperature in terms of product specification achieved. To ensure that a product of a particular specification is consistently produced the calcination process has to provide minimal temperature gradients through the bed of calcining material. This is readily achieved in the Rapidflow Calciner due to the controlled fluidisation providing excellent mixing and heat transfer from the calcining gases to the material.
Also, the distributor plate design ensures that all of the material in the bed remains suspended in the fluidising gas stream thus ensuring that no material is exposed to over-temperature conditions. The Calciner allows temperature control to ±1°C - this provides for specific process operating parameters to be established to achieve required product specifications from various feed materials.
The Rapidflow Calciner is designed to operate continuously under closely controlled conditions. Its design allows a wide variety of feed materials to be processed and it is important in the case of the finer synthetic type gypsums that sufficient residence time is provided to allow the completion of the calcination process.
In other designs the finer materials may be entrained in the exhaust gas system where the exposure to the required process conditions is limited. The Calciner design in this case provides for a fines recycling system that can be controlled to ensure that the minimum necessary residence time at required process conditions is established.
The Calciner design allows for either direct or indirect firing. Where clean fuels such as natural gas are available a direct firing system can be used thus simplifying the calciner design. Where other fuels or waste heat streams are to be used the calciner design readily adapts to indirect firing. The design is also readily adaptable to heat recovery from the calciner exhaust stream. This feature has important implications for plant operating costs and end product embodied energy.
As evidenced by market surveys of alternative gypsum calcination systems, the design of the Rapidflow Calciner will allow it to be built economically from readily available materials local to most processing plant sites and by using standard ancillary equipment thus minimising investment costs.
Its operating costs are potentially very attractive particularly where drying and calcining can be taken advantage of in a single-step process. Being based on a fluidised bed design the inherent maintenance costs can be expected to be minimal with most maintenance requirements centred on ancillary equipment such as dust collection and material handling equipment.