Under pressure:
Carly Lovejoy looks at filter presses – their main design features and
applications. There are a number of stages in the mining process that require
the separation of liquids and solids. As particles become progressively finer,
the removal of liquid becomes more difficult. When gravity dewatering is no
longer viable, mechanical methods that use over pressure or under pressure to
force water from interstatial pores within the mass of solids are often used as
an efficient and economical technique.
Filter presses separate liquids and solids by forcing the liquid fraction of a
slurry through a permeable filter cloth. The separation takes place in chambers
between the recessed faces of plates, which are clamped together in a steel
frame. The plates are usually designed with corrugated-drainage surfaces in the
chamber recesses and ports for slurry feed and liquid drainage. Each face is
covered with a filter cloth and the plates are clamped together using a
hydraulic ram.
Slurry is pumped in under high pressure, filling the chambers with solids and
pushing liquid out through the filter cloth. When no more solids can be forced
into chambers, the feed pumps are turned off and compressed air is used to
create a differential pressure across the filter cake and force out the liquid.
When the desired residual moisture content has been achieved, the filter is
opened, cake is removed and the procedure is repeated.
In addition to this basic cycle, residual slurry can often be removed from the
feed channel using compressed air and/or water, air purging of filtrate drain
channels can be undertaken and cakes can be washed to remove soluble impurities.
Applications:
Filters presses are applicable at a variety of steps in the mining flow sheet.
While some dry processes utilise size, magnetic or gravity separation, they are
in the minority. Often, wet processing, such as flotation or hydrometallurgy, is
used to recover valuable minerals. Water is added at the milling stage, and
later recovered using filtration to dewater products for storage or transport.
Filter presses are primarily used in the dewatering of tailings and to obtain
the desired moisture content or transportable moisture level (TML) for mineral
concentrate in base metal and iron ore processing – this could be for transport
purposes or to meet the next step in a processing circuit.
They are often used to filter base metal concentrates from flotation circuits,
such as copper, zinc, lead, tin, molybdenum, iron, gold and silver; to dewater
clean fine coal and phosphate slurries; and in hydrometallurgical streams, such
as the Merrill Crowe process for gold extraction, cementation solutions and
polishing.
Other applications include:
• Solvent extraction (SX) crud and regeneration streams;
• Dewatering slimes and crud in electrowinning and electrorefining;
• Smelting dusts and acid recovery;
• Treating acid mine drainage;
• Furnace blow-down solids filtration;
• Dry stacking of tailings including red mud, coal refuse and other general
tailings applicable to above streams; and,
• Cyanide recovery and destruction in precious metals tailings.
Reuben Neumann, global product group manager for filter press technologies at
FLSmidth explains to MM: “There are other benefits as well in certain tailings
materials. For instance, in gold counter current decantation (CCD) processes,
additional cyanide can be recovered from the tailings with the additional step
of destroying residual cyanide prior to deposition of the tailings.
“Another example would be to remove impurities that may be harmful to the
environment in operations where sea water is used as process water.”
Robert Bosgraaf, product manager – dewatering and mining at Siemens Water
Technologies, says: “The primary purpose [of filter presses] in mineral
concentrate and tailings applications is to dewater the slurry to a defined
moisture goal, at a throughput rate and level of equipment uptime that is
consistent with the mine’s output.
“With both mineral concentrates and tailings, the moisture goal is the more
critical parameter than it may be with other processes. If it is too wet, the
material may cost too much to transport, or specifically for mineral
concentrates, the energy costs to operate the smelter will increase.
“For tailings, the requirement is to have a filter cake product that is dry
enough to transport, spread properly and to be structurally stable when dry
stacking. If it is too dry, the product could dust and create a material
handling issue.”
In recent years, filters have also become more widely used for the dewatering of
tailings in paste backfill applications and dry stacking.
“In hydrometallurgical operations, filtration can be used to dewater products
and residues, but the focus is often on the recovery of soluble metals and
minimisation of water consumption,” explains Jason Palmer, global business
manager for filtration at Outotec.
However, the recovery of water from processing and waste water filtration is
often just as important, be it for additional value, water shortages, or
environmental discharge.
Design choices:
There is a wide and varied range of features available for filter presses, such
as: horizontal or tower arrangements; different materials of construction; plate
configurations (recessed, membrane, plate and frame); filter media; media
washing; discharge assist systems; plate opening systems; drip containment;
filtrate sampling; control systems; and, cake conditioning options, such as
blowing, washing and squeezing.
“The selection of features can be complex, and is often driven by testing,
experience and/or customer preference,” explains Mr Neumann. “A simple example
might be whether to use membrane compression or not. For materials that are
non-compressible, such as many minerals concentrates, expensive and
maintenance-intensive membrane systems may not be required to achieve the
desired results.
“While for materials that exhibit compressible-cake characteristics or that
contain soluble salts that can cause cake shrinkage during the washing or even
blowing step, membranes are important to prevent cake cracking and increase the
efficiency of the air blowing or cake washing steps.”
The design chosen will also depend on the mining process used at each site and
the number of desired filtration cycles per day.
Bill Kearney, vice president of sales at ErtelAlsop, says: “Filter units are
available from totally manual operations to fully automated PLC controlled units
that are monitored on-site or from remote locations. Semi-automated and fully
automated plate shifters are available to eliminate the need to manually shift
the plates, usually on the larger size presses. Bombay drip doors can be used to
keep any cloth/plate wash water from entering the dry cake areas, fully
automated frontal valves eliminate the need for the operator to manually open
and close valves during the filtration cycle, and automatic cloth washers
eliminate manual cloth washing. High speed closures are also available when
shorter cycle times are needed.”
Andrea Pezzi, North America and Canadian sales manager at Diemme Filtration,
adds that factors such as throughput capacity, duty and operational conditions
must also be taken into account when making a filter selection.
Evolution:
Like most mineral processing equipment, filtration technology has developed to
suit the demands of the mining industry. Along with advances in techniques and
mechanical designs for traditional applications, new and emerging markets have
driven significant advances in the size and application of filter presses.
Mr Palmer says: “The most obvious change is the scale of equipment which has
quadrupled in the past 20 years. There have also been significant improvements
in energy efficiency and safety from suppliers. Outotec, for example, has
focused on automation and equipment design to reduce the labour required to
operate and maintain filters.
“The increasing difficulty and complexity of orebodies and higher throughputs
required has partially disguised such improvements in filtration technology.
Additionally, increased demands on mineral processing efficiency require finer
grinding, producing slurries that are more difficult to filter.” He adds that
future developments should include better integration with upstream thickening.
Anders Söderman, vice president for process equipment at Metso, agrees that the
main change in filter presses has been the development of larger sizes to meet
higher throughput capacities. This is a direct result of declining orebody
grades globally, and the finer grind sizes needed to increase or maintain yields
from many processing facilities.
Mr Bosgraaf says that to meet this challenge, filter presses have become more
automated and have higher uptime demands. To achieve higher throughput, cycle
times have been reduced, and larger filter plates have been introduced to
increase the capacity of material produced per press cycle. Reliability and
serviceability are also key considerations.
Environmental regulations and scarcity of water are also driving the industry to
recycle as much water as possible and reduce the use of tailings ponds. Dry
stacking is therefore becoming more prevalent, which requires additional
dewatering capacity at mines.
Mr Neumann says: “Government regulations are rapidly driving dry stacking of
tailings so as to mitigate the hazards associated with ground water
contamination, water usage and recovery and dam failure. In the US and many
other countries, obtaining permits to construct a traditional tailings-pond
system is almost impossible. High tonnage rates demand very large heavy-duty
filter presses that can be operated with minimal operator and maintenance
attention.”
Mr Söderman adds that the development of high-performance filter fabrics and
control instrumentation has also improved the results of filter pressing.
Automation:
Fully automated filter presses are now becoming a necessity for new mining
installations to assist in plant and operational management. The main saving is
labour, however, technologies are becoming so sophisticated that the information
and feedback generated allows for continuous equipment and process optimisation.
Mr Bosgraaf says: “Full automation of dewatering equipment has historically been
the requirement. This is due to the large volume of material to be dewatered per
day and the rapid cycle times. A typical filter press dewatering mineral
concentrates cycles 150-175 times per day.
“It would take as many as six operators to run one filter press 24h/d. With
automation, half the number of operators can run four to five presses. With
limited manpower, especially at remote mine sites, the labour-cost savings can
be dramatic.”
Filter presses used on smaller volume applications do not cycle as frequently,
and the degree of automation required will depend upon the application, total
throughput per day required, capital cost (CAPEX) and operating costs (OPEX).
Many manufacturers offer fully automatic, semi-automatic and manual options to
suit different applications.
Mr Neumann adds: “The increasing use of certain hydrometallurgical extraction
processes, which use chemicals that are often hazardous to personnel, is another
driver for automation.”
While operators are often the best option to manage process disruptions, the
majority of filters are required to run with minimal operator intervention. Mr
Palmer explains: “Outotec has invested heavily in R&D targeted towards improving
availability and assisting troubleshooting and maintenance. While the general
trend is more isolated operations, there is a developing requirement for remote
support with data collection, diagnostics and monitoring assistance for site
personnel. Beyond equipment control, automation is being utilised for site and
virtual training to improve operator skills.”
Alternatives:
There are a number of other technologies that can be used for mineral
concentrate and tailings dewatering. These include: vacuum disc, vacuum drum,
belt, capillary, ceramic disc, tower press and hyperbaric filters.
The use of filter presses to dewater mineral concentrates, in particular, has
increased over the past 20 years. The main change has been the shift from the
use of vacuum filters in these applications to pressure filters. Pressure
filters can achieve the required moisture content in one step and eliminate the
thermal drying typically required with vacuum filters. Vacuum filters and
thermal heating are also more energy intensive than mechanical pressure
filtration, as are centrifuges.
The selection of a technology is based on a variety of factors. These include:
the quantity of material to be filtered per day; type of material to be
dewatered; specification requirements for final moisture content; dewaterability
of the slurry; initial capital cost; operating cost; and customer preference,
among others.
“As a general rule, 90% of all mining filtration applications can be
accomplished in a filter press but, that doesn’t mean that they are the best
choice,” says Mr Neumann. “There are many drivers of filtration equipment
selection. Purpose and end goals, such as residual moisture content, chemical
content and recovery and filtration rate (kg/h/m2 of filtration area) are often
primary considerations.”
Mr Bosgraaf says: “Each alternative technology listed has an application within
the mining process where it is most applicable. However, pressure filtration
with filter presses is the most broadly applied due to the high driving force in
the filtration cycle, the ability to purge additional moisture from the filter
cake, and their robust and reliable operation. The filter press provides the
driest cake product at extremely high throughput rates.”
Mr Kearney agrees: “The filter press is the workhorse of the industry, and
generally speaking they are extremely reliable, less maintenance intensive, and
more cost effective than other options.”
Outotec says that in base mineral concentrate dewatering, a recent survey of
Australian producers showed over three quarters of sites utilised pressure
filtration, with the remainder using vacuum filtration. In contrast, coal and
iron-ore producers predominantly use vacuum filtration technologies.
CAPEX and OPEX must also be carefully considered in the selection of equipment.
In many instances vacuum disc, horizontal belt and even drum filters are a more
economical choice when compared to filter presses.
Mr Neumann explains that for CAPEX evaluation, recent studies at a magnetic iron
tailings application indicated that the lowest CAPEX options were vacuum disc
filtration and horizontal filter presses. These were followed by horizontal
belt, ceramic, hyperbaric and vertical filters with CAPEX factors of 2.9, 3.6,
5.3 and 9.8, respectively. The overall results of the study selected the
horizontal belt filter as the best overall option when OPEX evaluations were
thrown into the mix.
Vendors:
MM spoke to a selection of global vendors to see what their filter press ranges
have to offer.
Metso: Metso’s range of VPA vertical plate pressure filters are available in
three different sizes: VPA 10 series, with up to 40 chambers (15t/h); VPA 15
series, up to 60 chambers (>70t/h); and VPA 20 series, up to 54 chambers
(>250t/h). The firm is also developing a larger model.
The chamber depth varies from 30-50mm, and the machines operate using 7-10bar of
pressure. Metso filters use membrane compression for cake formation before air
is introduced, resulting in improved filtration. The filter has a simple
mechanical construction, with few moving parts. Metso’s main markets for VPA
filters are Europe, Australia, India and South America.
It has recently installed three VPA1540 filters at Boliden’s Aitik mine in
Sweden for filtration of copper concentrate, and one VPA2050 model for a mining
customer in Norway.
Metso also offers three sizes of Tube Press (75L, 100L and 200L), which are
applied for dewatering fine concentrates. These are hydraulically driven and
operate at up to 100bar pressure. Shane Sullivan, regional product manager, Tube
Presses, says: “The main benefit is they dewater to a far greater degree than
conventional filter presses, this then leads to considerable energy savings in
downstream drying.
“Also, using greater filtration pressures, a hard, compact filter cake is
formed, making it possible to meet the TML requirement without further
dewatering.”
Recent mining installations for Tube Presses include a dewatering plant for a
customer in the US producing a <10% moisture filter cake. A customer in Sweden
also uses Tube Presses in a water purification system to remove slimes and
general waste.
For both the VPA and Tube Presses, Metso offers a fully automated control system
consisting of a PLC and a user friendly SCADA interface.
Since the acquisition of Tamfelt Group (currently Metso Fabrics) at the end of
2009, Metso has broadened its fabrics range. It supplies filter cloths for its
own filter presses with various design options.
Marko Salmela, Metso Fabrics product sales manager, says: “Our product range for
the mining and chemical industries includes fabrics for automatic pressure
filters, horizontal belt filters, chamber press filters, drum filters, Kelly
filters, belt filters and disc filters. All our filter fabrics are tailor-made
and designed for optimum productivity.”
Siemens: Siemens offers two lines of filter presses covering a range of mining
applications and throughput capacities – the J-Press and the MC Press. The
J-Press is typically used for lower volume, throughput and duty cycle
applications such as Merrill Crowe, electrowinning, electrorefining and small
mineral concentrate applications.
The J-Press line can be supplied as small throughput, manually-operated units,
or large throughput, fully-automated units.
J-Press filter presses are available in sidebar and overhead styles that can be
configured in manual, semi-automatic or fully automated versions to meet exact
requirements.
The MC Press is a fully-automated model used for high-throughput mineral
concentrate and tailings applications. It is designed for high duty cycles (up
to 175 times per day), with no operator attendance and an uptime as high as 95%.
The MC Press is available in three frame designs, with two plate sizes, all
available in a wide range of cake thickness and operating pressures. Each press
can be further customised with either recessed chamber or membrane squeeze
plates depending on individual needs.
The company recommends the D model for large applications. This incorporates a
dual movable-head system that discharges cake at twice the rate of a
conventional filter press design. The D model is available with up to 130 filter
chambers for a throughput capacity of up to 170t/h on a typical mineral
concentrate.
Siemens also offers consumables and on-site repair services. Consumables, such
as an initial set of filter cloths, are typically included.
Some recent examples of installations include three MC Presses that were
recently shipped to a mining project in Bolivia. The application is tailings
from a gold and silver mine and the presses are still in the start-up phase.
Five large J-Presses were shipped in 2011 to a gold and silver recovery project
in Chile for a large Merrill Crowe application. These presses will be started in
2013. Two Siemens J-Presses were shipped in 2011 to Chile for a molybdenum
application. One press is dewatering molybdenum concentrate and the other on a
small copper concentrate application that results from the molybdenum process.
Mr Bosgraaf says: “We are focused on process design and equipment improvements
in all our filter press lines that will offer higher throughput rates, simpler
design, easier maintenance and higher availability. The first of these many
improvements should be available in early 2013.”
FLSmidth: FLSmidth offers three pressure filtration systems: Pneumapress
filters, Shriver filter presses and EIMCO AFP filter presses. All models are
available with full automation, and through its Automation group, FLSmidth
offers a range of controls that utilise industry standard component and
programming logic.
The company is also able to provide a variety of filter cloths with many
proprietary designs. FLSmidth says that its product ranges are virtually
infinite in terms of construction and materials, and are suitable for any given
application.
FLSmidth has now begun marketing the EIMCO Colossal automatic filter press,
which is specifically designed for ultra-large tonnages (typically tailings).
The first filter is under construction and will be delivered at the end of 2012.
The filter design includes more than 2,000m2 of filtration area, 38.4m3 in
chamber volume and can process up to 9,000Mt/d of copper tailings.
Outotec: Outotec offers the Larox PF tower filter presses (fine and difficult
concentrates) and Larox CC Ceramic filters (coarser concentrates). Both models
achieve a high degree of solid/liquid separation and deliver the product
requirements for transport.
“Changing conditions, water availability and environmental legislation is
creating a requirement for tailings dewatering where a lesser degree of
separation is required, but where the scale of operations can be many times
greater than that of concentrates,” says Mr Palmer. “In response to this,
Outotec has developed some very large filters, such as the Larox FFP
(fast-operating filter presses), large Larox PF filters and Larox RB-SV
horizontal vacuum belt filters.”
Nickel producer Talvivaara uses specially designed Larox RB-SV filters at its
operation in southeast Finland. “There are several stages in the filtration
process, and as there is alkaline solvent in the slurry, cake washing must be
performed several times,” says Tapio Hyödynmaa, from Talvivaara.
Xstrata’s Cosmos nickel mine in Australia also uses a large, refurbished and
modernised second-hand Larox filter, which was retrofitted to increase the
processing stream throughput capacity.
In the past year, Outotec has released a new large Larox PF 180 series filter
with up to 256m2 of filtration area, and a new Larox Filtration Plant concept.
The company also has several products in development ready for launch next year.
Diemme Filtration: Diemme (part of the Bilfinger Berger Group) offers a range of
filter press sizes and designs for different applications. The GHT 4X4 overhead
beam pull to close-type filter press is designed specifically for mining. The
filter plates, which are hung from the upper beam, are moved automatically by a
rapid shifting device equipped with a carousel system that reduces the filter
press opening and closing sequence to the minimum. Four hydraulic cylinders
placed on the plate pack corners ensure perfect operation and limited structural
stress, even in difficult working conditions.
Mr Pezzi says that all of Diemme’s machines are fully automatic, and are able to
interface with the different control systems present in plants.
The machines are supplied complete, including cloths and all ancillary equipment
that could be required. The choice of cloths, valves, pumps and compressors is
made after testing in Diemme’s laboratory.
The company recently sold three GHT 2000 overhead beam filter presses to a
silver mine in Mexico. The machines are working well and dewatering up to
4,000t/d of dry solids.
“We also sold a GHT 2500, for iron-ore concentrate filtration in India, which
dewaters up to 200t/h of product,” says Mr Pezzi. “We are going to assemble two
GHT 2500 units in Chile that will be employed for boric-acid recovery; two GHT
2500 in Mexico for gold and silver extraction with cyanide; and three GHT 2000
filters in the US for aggregate treatment.”
Diemme’s filters are currently sold in Russia, Chile, Mexico and India, and the
company is looking to expand its reach in Canada and Brazil. In the US, Diemme
works with Phoenix Process Equipment to distribute its recessed chamber filter
presses.
Diemme is working on the evolution of some existing filter-press models for
mineral concentrate dewatering and hopes to release further details by the end
of the year.
ErtelAlsop: ErtelAlsop offers filter presses from 102mm bench/lab scale, up to
1,500mm fully automatic units. All models have uniform fill manifolds, and
feature caulk and gasketed filter cloth for leak-free operation. The option for
membrane squeeze plates for additional cake dryness and shorter cycle dewatering
is available at every size.
Mr Kearney says: “Our smaller 102mm and 203mm units are standard with stainless
steel frames, and 4.5t hand hydraulic closing systems, while our larger presses
offer automatic hydraulic closures, plate shifters, cloth washers, automated
valves and feed pumps, cake carts, conveyors and Bombay doors.”
All ErtelAlsop presses can be fully automated depending on the specific
needs/wants of the customer.
Control systems range from simple spindle closures to semi-automated
air-over-oil systems and fully automated PLC-controlled electric hydraulic
controls.
ErtelAlsop also offers filter media for its presses, which are selected based on
the operating conditions, performance required, criteria given by the client,
and/or by sample slurry processing in its laboratory. “We also offer replacement
media/cloth/paper for our presses and all other manufacture’s presses,” adds Mr
Kearney.
In addition, the company offers pre-coat and body-feed processing aids, as well
as lab services to maximise the efficiency of the cloth and process employed.
ErtelAlsop has filters installed in North, Central and South America and Europe,
with a large number of inquiries currently from South Asia. Its customers
include: First Majestic Silver Corp’s La Guitarra mine in Mexico, Endeavour
Silver’s Guanacevi mine in Mexico, and Western Mesquite Mines’ operations in the
US, among others.
© Aspermont UK (Mining Communications Ltd) Albert House, 1 Singer Street,
London, EC2A 4BQ