Fertilizer International 502 May-June 2021
31 May 2021
The latest filtration technology
FILTRATION & THICKENING
The latest filtration technology
Vacuum filtration is a key process step in phosphoric acid production. We highlight the main equipment options and recent project installations.
Thickening, clarification and filtration are integral to the phosphoric acid production process. In phosphoric acid plants, thickeners are used to concentrate phosphate rock slurry prior to the reactor stage, while vacuum filtration, in combination with clarifiers, removes gypsum from the phosphoric acid product (Figure 1).
Thickeners and filtration equipment are also used to dewater phosphate tailings (see separate article on page 55).
For new plants, selection of the phosphoric acid process is the primary design consideration, as is production output – with an industry trend towards constructing larger capacity phosphoric acid plants (1,0001,500 t/d P2 O5 ) since the early 2000s. Once the production process has been selected, and the scale of operation decided upon, the selection of suitable vacuum filtration equipment then becomes a vital technical and economic matter for any phosphoric acid plant1 .
Filtration options
Essentially, there are three options when it comes to selecting vacuum filtration equipment for phosphoric acid plants:
- Table filters
- Tilting pan filters
- Horizontal belt filters.
Although they vary in terms of capital outlay, operating/maintenance costs and performance characteristics, all three types of filter can produce phosphoric acid efficiently and economically (Fertilizer International 487, p43).
It can be tempting to select the lowest capital cost equipment. However, a range of other factors – such as filtration performance, equipment quality, installation cost, and ongoing operational and maintenance costs – also need to be considered when selecting filtration equipment1 .
Horizontal belt filters
Horizontal belt filters have become increasingly commonplace due to their operational flexibility, large throughput and ability to handle corrosive slurries. They have become a popular alternative to table filters and tilting pan filters in many industrial applications.
The horizontal belt filter combines a perforated rubber drainage belt, travelling continuously in a loop around two rollers, with an underlying longitudinal vacuum box (Figure 2). The top of this box forms a moving seal with the bottom of the belt. Slurry is fed onto a filter cloth overlying the drainage belt.
Most manufacturers offer a choice of belt materials including natural rubber, SBR (HT or LT), EPDM, Chlor-Butyl and Neoprene. Pneumatic systems for lowering and raising the vacuum box are also commonplace, enabling fast and reliable maintenance and cleaning.
Filtrate is drawn down through the filter cloth under vacuum, entering channels holes in the drainage belt, and then flows into the vacuum box beneath. Spray nozzles or a flood weir box are used for filter cake washing. The filter cake eventually separates from the cloth surface at the far end of the filter as the belt moves downward over the discharge roller.
Horizontal belt filters are used to filter calcium sulphate from phosphoric acid (counter-current washing) and to dewater phosphate rock (co-current washing).
Notable manufacturers (Fertilizer International 487, p43) include:
- ANDRITZ
- TAKRAF/DELKOR
- FLSmidth/EIMCO
- Hasler/Filtres Philippe
- Gaudfrin
- Outotec/Larox
- WesTech.
DELKOR, part of TAKRAF Group, has been manufacturing horizontal belt filters (see photo) for more than 40 years, supplying more than 950 units globally over this period with a combined filtration area of 37,500 m². The solid/liquid separation company is also the supplier of the world’s largest individual horizontal belt filter unit. DELKOR has often led the way in horizontal belt filter design and was the first manufacturer to introduce:
- Ripple curbing with superior vacuum sealing and longer life
- Wear strips and wear belts for vacuum sealing.
DELKOR recently installed a large horizontal belt filter (110 m² area ) at a phosphoric acid plant on behalf of Coromandel International Limited, India’s second largest phosphate producer. The filter was successfully commissioned at the company’s Visakhapatnam plant in Andhra Pradesh in February 2020, ahead of schedule.
The belt filter was supplied to Coromandel as part of an engineering, procurement and construction (EPC) package. This covered the design, engineering, supply, construction and commissioning of the horizontal belt filter and ancillary equipment – including filtrate pumps, wash pumps, air blowers, valves and piping. This ‘Industry 4.0’ filtration system comes ready equipped for advanced control technology.
The highly corrosive nature of phosphoric acid means DELKOR needs to design and manufacture belt filters using corrosion resistant materials, such as stainless steel for the frames, vacuum boxes, feeders and air boxes, an EPDM rubber transporter belt and GRP (glassreinforced plastic) for the fume hood.
Detailed planning and scheduling substantially reduced the on-site equipment erection time. This included a mock assembly of the major components of the belt filter and plant piping at DELKOR’s India fabrication plant in Bangalore.
At the inauguration of 450 t/d expansion to the Visakhapatnam plant, Vinay Kumar Sandur, DELKOR’s vice president, projects, said: “The safe and successful implementation of such a complex project clearly demonstrates our horizontal belt filter capabilities. With this achievement, we continue to entrench ourselves as a leading global provider of dewatering equipment.”
Tilting pan filters
Tilting pan filters consist of series of independent trapezoidal pans mounted on a rotating circular support structure (Figure 3). Pans are kept horizontal and under vacuum as they rotate during the filtration cycle. The cake is washed countercurrently during filtration, generally in three stages, with filtrates collected by separate vacuum receivers. The cake is eventually discharged dry at the end of the cycle. Following cake discharge, pans are tilted through 180 degrees and pass over radial manifolds fitted with high pressure jets which clean the cloth and dislodge any remaining cake.
Prayon, through its PROFILE equipment division, is a leading supplier of tilting pan filters to the phosphates industry. Around 250 of the 300 tilting pan filters sold by Prayon since the 1950s have been installed in phosphoric acid production plants worldwide.
According to Prayon the main benefits of tilting pan filter include:
- High level of reliability due to their robust design, with some 1960s filters still in operation
- On-stream factor of over 95 percent
- Excellent capacity/extraction yield ratio, due to batch filtration and a high maximum vacuum level
- Energy efficiency
- Cake discharged contains very little free water
- Prayon’s proven track record with very large filters, e.g. the PROFILE 30-240 filter with a surface area of 275 m².
Prayon has improved its tilting pan filter design over the years. The main developments are:
- A special compact vacuum box for separating gases and filtrates
- A fast-drain filtration cell (pan) to increase filter capacity and filtration yield
- An automatic system to keep the pans horizontal
- A new tilting-track design for higher rotation speed and filtration capacity
- Support rollers designed for heavy loads
- A robust rotating frame with a replaceable wear plate
- A new waterproof cloth-fixing system for quick cloth replacement
- A high filter surface-to-base-area ratio.
- Stainless steel rotating frame with replaceable wear plates
- Fibre-reinforced plastic (FRP) hood and side protective barriers
Paul-Henri Legros, the general manager of Prayon’s PROFILE division, explains the company’s approach and ambitions:
“PROFILE wants to remain the showroom for excellency in performance and engineering in P2 O5 filtration, so we decided to refocus our resources on the Prayon tilting pan filter. The actual production trend in the market is for large capacity (surface area) filters – and we clearly keep the edge with our tilting pan filter, in comparison with the belt filter where the available surface is limited by mechanical contingencies.”
Demand for large capacity filtration equipment in the phosphates industry, says Paul-Henri, is being driven by two main factors:
- The ability to increase production throughput provides a quicker return on capex
- As the quality of available phosphate rock decreases, larger volumes need to be handled and treated to achieve comfortable investment returns.
“No single type of vacuum filter can outperform all others in every application. Table, tilting pan and horizontal belt filters all have strengths and weaknesses.”
“We continue improving every aspect of our equipment, and are also developing and investing for the future,” comments Legros.
“We are currently designing and engineering a new filter model with 36 filtration cells, for example, and are also optimising the design of tilting pan filters by carrying out studies on the flow of liquids within filtration cells in collaboration with the University of Liege.”
He continues: “One of Prayon’s main objective is sustainability and modifying our impacts on nature – a subject which will guide my team and myself in upcoming years. To deliver on this, the questions are: how can we spare expensive raw materials, how can we save on spare parts, how can we decrease the movement of goods and people worldwide?”
PROFILE has secured a number of major phosphate industry equipment contracts in recent years. These include a large-scale project with OCP Group for its Jorf Lasfar production complex in Morocco. This involved the supply of:
- Eight 30-220 model tilting pan filters (four lines at 1,500 t/d P2 O5 ) for the ODI project
- Eight 30-220 model tilting pan filters (four lines at 1,500 t/d P2 O5 ) replacing the eight existing table filters at MP3 and MP4
- Four 30-220 model tilting pan filters (two lines at 1,500 t/d P2 O5 ) for Line E and Line F at MP3 and MP4.
As part of this project, PROFILE manufactured 18 filters between mid-2011 and end-2014, then shipped four filters every six months from mid-2012 to end-2014. These were then erected ready for start-up of the different production lines between 2013 and 2018.
The last two filters were manufactured in 2018 and shipped in 2019. Their erection has been finalised although start-up is currently on hold.
PROFILE successfully overcame multiple challenges to deliver this large-scale filtration project for OCP, as Paul-Henri Legros makes clear:
“The replacement, in the same footprint, of table filters by tilting pan filters without major modification of the civil work of the building is an engineering challenge. There’s also the procurement and organisational challenge of following up manufacture with on-site erection in parallel.
“Finally, the logistical challenge was the supply and storage of more than 800 boxes of equipment with a combined volume of 12,500 metres cubed and weighing 3,200 tonnes. To repeat a famous slogan: yes we can and yes we did!”
Table filters
Table filters, also known as horizontal pan filters, are designed for continuous vacuum filtration of relatively coarse, granular, fast-settling solids. Their main applications include the filtration washing of slurries in bauxite and phosphate processing.
Table filters consist of a circular filtration surface rotating in a horizontal plane. The filtration surface is excellent for both cake washing and drying. After moisture has been reduced by an applied vacuum, the cake is removed by a wear-resistant cake discharge scroll.
Originally developed in the early 1960’s, the UCEGO® filter marketed by Technip is specifically designed to filter phosphogypsum slurry generated by the dihydrate (DH) and hemihydrate (HH) production processes. The filter has been adopted throughout the global phosphates industry with worldwide sales to some 28 countries. More than 90 UCEGO® filter units have been installed in plants responsible for over 35 percent of the world’s phosphoric acid production.
The UCEGO® filter is also suitable for retrofits at existing plants. Its compactness and high per-unit-area filtration capacity helps to optimise throughput for a given space. According to Technip, replacing certain types of tilting pan filter with an UCEGO® filter can increase filtration area by up to 100 percent for the same construction footprint. Such retrofits only require a short shutdown and the installations costs are minimal, suggests Technip.
Summary
No single type of vacuum filter can outperform other types in every application. All three kinds – table filters, tilting pan filters and horizontal belt filters – have their own strengths and weaknesses (Fertilizer International 487, p47). Instead, vacuum filtration equipment needs to be selected according to individual plant needs using the best engineering solution available, while aiming for the lowest lifecycle cost1 . The main considerations when choosing equipment are:
- Equipment quality
- Service quality
- Spare parts availability
- Expected equipment life
- Capital, installation, operating and maintenance costs.
References
