SOLID-LIQUID CALCINATION (SLC) TECHNOLOGY: COMMISSIONING AND DEMONSTRATION OF A FULL-SCALE UNIT
Alumina Espanola, S.A.
PO Box 71, 27890 San Ciprian,
Martin A. Gayol
Alumina Espanola S.A., Spain
Alcan International Limited, Canada
FFE Minerals Denmark A/S
Aughinish Alumina Limited, Ireland
The Solid-Liquid Calcination Technology (SLC), forms the basis for a new, user friendly process jointly developed by Alcan , FFE Minerals (sister company to F.L.Smidth), Alumina Espanola and Aughinish Alumina with the specific aims of:
This paper reports on the commissioning and start-up experiences of the Full-Scale, 120 MTD, SLC Demonstration Unit built at the Alumina Espanola facilities at San Ciprian (SPAIN).
drying; calcination; salt-cake recovery; bayer liquor purity.
SOLID-LIQUID CALCINATION (SLC) TECHNOLOGY: COMMISSIONING AND DEMONSTRATION OF A FULL SCALE UNIT
Jose Pulpeiro, Martin A. Gayol, Andrew Carruthers, Jens Fenger, Liam Fleming
Both the Aughinish and San Ciprian alumina refineries were designed in the mid 70s with side stream salting out evaporation units to maintain liquor purity. At that time, there was no appropriate technology for salt cake recovery, and as a result, the material was sent to the mud stack for disposal. This was environmentally undesirable, and in the mid 80s both plants evaluated and tested various methods of recovering the value of the saltcake.
The calcination of sodium carbonate with aluminous materials to form sodium aluminate is a technology which was in use before the development of the Bayer process. However, as generally practised in rotary kilns it is an unpleasant unfriendly process, unsuitable for application in the 21st century. Never the less, it was selected as the most positive approach to the salt cake disposal problem, and in 1992 a partnership was set up to develop a "user friendly" version of the process. The partners were Alumina Espanola, Aughinish Alumina, Alcan International and FFE Minerals (Billiton contributed to the early stages of the process, but withdrew before the pilot plant stage).
After extensive laboratory and equipment testwork a 24 MTD Pilot Plant was built in Dania, Denmark, and successfully operated on a variety of feedstocks. This work has been previously reported (Fenger 1996, Alvarado 1996). During the Dania trials it became obvious that the equipment has applications beyond simple salt cake recovery, into liquor burning and even digestion of diasporic bauxites, in addition to slurry drying applications outside the alumina industry.
The partners decided to build a 120 MTD Prototype for further development at San Ciprian plant site (now Alcoa-Inespal). This paper describes the Prototype and provides initial results. The Prototype first went on feed on 7 September 1998. Publication deadlines for the papers limit the amount of operating data included, but the presentation in March 1999 will cover the results of the first six months operation.
2.0 PROCESS DESCRIPTION
The intent of the design was to produce an operationally robust system, highly automated and with low maintenance costs. It must meet all present and anticipated environmental standards, and its capital and operating costs should be lower than any competitive system. We believe these objectives have been achieved.
The prototype unit is sized to treat all San Ciprians current and medium term projected salt cake production, with a margin to allow for some liquor calcination to improve liquor purity. This equates to a nominal sodium aluminate production rate of 120 MTD.
For descriptive purposes the process (SLC area) can be broken into two sections:
2.1 Wet Section
The Wet Section comprises the collection and blending of salt cake, bauxite slurry and concentrated spent liquor to yield a pumpable slurry of molar ratio (Al + Fe)/Na of 1.0. The importance of accuracy in this blending operation has been noted elsewhere (Boily, 1999).
All these materials present handling problems. The principle adopted is to maintain a flow of each material past a batching station, and to extract the required quantities of each material from the streams to make up a batch. This principle enables us to eliminate "dead legs".
The salt cake filter discharge is conveyed to a transfer tank where it is re-pulped by strong agitation. It is then pumped to a storage tank in the Dry section. The salt cake is continously re-circulated around the batch mixing station, together with bauxite slurry and concentrated spent liquor. When a new batch of slurry is required a "batch blending programme" automatically meters preset quantities of each component into an agitated blending tank. On completion of a preset mixing time the batch is dropped into a large slurry holding tank. This tank re-circulates around a feed header tank which provides constant suction to a bank of four slurry feed pumps, which feed the atomising nozzles in the dryer. Off-line, a second computer program calculates the target quantities of each material for each batch, using the most recent laboratory information.
The Wet Section also features at the end of the process, where cooled sodium aluminate granules are metered into a dissolution tank and absorbed in a controlled flow of spent liquor. The cycle is completed by pumping this material back into the main Bayer circuit.
2.2 Dry Section
Feed slurry is injected into the throat of a Gas Suspension Dryer (GSD) where it contacts hot gases and dries to form granules. The hot gas at a set temperature and velocity is produced from a Fluid Bed Calciner (FBC) and a Product Cooler, supplemented by a low pressure heat (hot air) generator.
Product size granules automatically discharge from the bottom of the GSD and enter the FBC. Intermediate size material re-circulates internally within the GSD, whilst fine material is elutriated out of the GSD and captured in a Bag Filter. This fine material is recycled through the Recycle Bin into the lower section of the GSD where it meets the slurry spray and is agglomerated. The use of internal recycle to protect the walls from scaling combined with a powerful agglomeration mechanism to convert fines into strong product size material are key features which allow robust operation.
Dried product size granules discharge automatically from the GSD and enter the FBC, where they are heated to around 1000° C to yield sodium aluminate. The calcined product exits the FBC and enters a grate cooler where the particle temperature is reduced below 100° C. Cooled granules are metered into a Dissolution Tank, where the sodium aluminate is dissolved in spent liquor, leaving an undissolved mud residue from the bauxite. This slurry is pumped back to the main plant.
Cooled gas exits the GSD and enters a Bag Filter. Specially designed bags, assisted by a filter aid, capture the fine dust. The cleaned exhaust gases pass an online monitoring station for Total Organic Carbon (TOC), particulate concentration and temperature before discharging to atmosphere.
Solid-Liquid Calcination Unit Legend
3.0 EQUIPMENT DESCRIPTION
(We cover only items which are not obvious from the Process Description).
The prototype has been built into a 70 m tall vertical arrangement to provide maximum simplicity for gravity transfer of solids. Space has been allowed vertically and horizontally for possible equipment modifications. Future units may be folded in two or three sections to reduce the height, at the expense of more complicated solids transfer systems.
3.1 Wet Section
3.1.1 Batch mixing
The salt cake is fed from pre-existing filters to a specially designed re-pulping tank from which it is pumped at intervals to the salt cake storage tank. The contents of the latter circulates over the batch mixing tank, and the salt cake slurry is mass-flow metered into the mixing tank as required. Concentrated spent liquor is mass-flow metered in the same way. The bauxite slurry is weighed into a metering tank placed on load cells,from where it is discharged into the mixing tank.
3.1.2 Feed slurry holding tank
This tank is sized for a nominal 24 hour run. The feed slurry is pump circulated, and topped-up with fresh batches as required.
3.1.3 Dryer feed pumps
Four variable speed Mono pumps are used.
3.2 Dry Section
3.2.1 Gas Suspension Dryer (GSD)
The GSD consists of seven main parts as follows:
Air heater The low pressure air heater is a standard FLS unit of 4MW thermal output, fueled by low sulphur heavy oil. It operates slightly below ambient pressure. Hot gas from the heater is mixed with cold ambient air and the off-gases from the FBC and cooler to provide gas at target temperature for the throat of the GSD. This heater delivers 50-75% of the total heat input to the plant.
Throat This section acts as a classifier and shapes the spray inside the dryer. Its dimensions and geometry are critical for successful operation. Construction is of heat resistant steel with external insulation for reasons of flexibility.
Slurry nozzles Four high performance slurry nozzles are available, though not all are needed for reduced throughput. These nozzles atomise the slurry, and guide the droplets into the bottom of the dryer vessel with a narrow angle of dispersion which helps to keep the droplets off the walls of the dryer.
Dryer vessel The dryer vessel is a cylindrical mild steel vessel with a defined L:D ratio, externally insulated. The transition from the throat is a conic section, angled to encourage appropriate flow patterns. At the upper part of the vessel there is a transition piece leading to the baghouse.
Baghouse The Filtax Jet Pulse Filter Baghouse operates above the dew point of the gas and above the hygroscopicity point of the solids. The properties of the dust make it necessary to operate at loadings below 1 m3/m2/minute. All the baghouse catch is returned to the recycle bin.
Recycle bin and feeder The mass flow recycle bin provides a 30 minute surge, and enables the recycle rate to be controlled. The bin is mounted on load cells, and is rated to operate at 200° C.
I.D. Fan The ID Fan is a standard centrifugal unit with varispeed drive.
The function of the calcination unit is to rapidly heat the dried pellets to a temperature of 950-1200° C, and to hold them at that temperature until the following reactions have taken place:
- Decomposition of sodium organates with liberation of carbon dioxide
- Decomposition of sodium carbonate with liberation of carbon dioxide
- Combination of sodium compounds and aluminium compounds to form sodium aluminate (Boily, 1999)
The calciner operates continuously and consists of the following sections:
Fluid bed - The fluid bed is cylindrical with a refractory lining and a perforated cast refractory distribution plate. Residence time is 30-60 minutes, and energy for the reactions is provided by four heavy oil burners firing directly into the bed.
Free board - Above the fluid bed is an expanded, refractory lined section designed to minimise entrainment of bed material in the off gases, and to provide time at temperature for organics destruction.
Air Heater - The fluid bed requires preheated air at a moderate pressure to fluidize its contents. This is provided by an FLS unit modified for moderate pressure duty. Heavy fuel oil is used.
Roots Blowers - Compressed air to the air heater and fluid bed is provided by variable speed Roots blowers.
The sodium aluminate clinker is cooled to 80° C by a standard FLS Coolax grate cooler. The clinker is dispersed on grate plates through which cold air is blown. The cooling air is applied by centrifugal fans with louvre control.
A key feature of the plant design is the degree of automation. In principle, the plant can be started and stopped from the Control Room. Groups of associated equipment can be started with one command and the full plant brought into operation in this manner. Appropriate interlocks, motion detectors etc., are incorporated to ensure proper operation of key equipment. When shutting down, injection systems are incorporated to ensure proper cleaning of equipment. With this design philosophy in mind, the plant is very much amenable to future improvements in ADVANCED CONTROL techniques. The inherent robustness of the process design underpins this high degree of automation.
5.0 PRELIMINARY RESULTS
As was mentioned earlier, feed was put to the unit on 7 Sept, and at "Press-time" (30 Sept) the unit has not been stabilised enough to quote consumption and efficiency figures. However, there has been sufficient operation to reach some major conclusions and to reveal some minor difficulties.
- The entire wet side is fully satisfactory: operation is automatic and trouble-free, and the accuracy and repeatability of the batch composition is excellent.
- We have successfully demonstrated the fundamental principle of the unit the transition from pumpable slurry to dried agglomerated pellets at a scale-up factor of 5 from the original pilot unit in Dania.
- We need to modify the recycle bin outlet control system to prevent surges in the discharge.
- We need to modify the emergency water spray nozzles to prevent wetting of the dryer walls. (The system is necessary for emergency cooling in the event of slurry feed failure).
- We appear to have a mis-match between the primary elements and the transmitters in the air-flow measurement systems, which will have to be corrected.
We expect to complete this work in the next few weeks, and will present a full report of the first six months operation at the Workshop in Bunbury.
Fenger, J. et al, "Operation of a 24 MTD Solid-Liquid Calciner for Liquor Purification in a Bayer Plant", Light Metals, (1996), 161-168.
Alvarado, J-M et al, "Solid-Liquid Calcination (SLC) Process for Liquor Burning", Fourth International Alumina Quality Workshop, Darwin, 2-7 June (1996), 383-391.
Boily, H. et al, "The Chemistry of Solid-Liquid Calcination (SLC)", Light Metals, (1999).