FAQs

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Washing

1.Why does washing aggregate create a higher value product?

DUO Washing Plants have evolved over the years to provide customers with equipment that guarantees the making of the tightest specification materials thus giving the maximum return on investment by the production of higher value products. Washing enables tighter specifications to be achieved, not possible by dry screening, this applies to Sand & Gravel but not necessarily to Crushed Rock. The washing process removes the silt and clay from the sand and due to this, less cement is required in concrete production. A growing market over recent years is Decorative Aggregate; this could not be achieved with out washing.

2.If material isn’t heavily clay bound can I not just dry screen it?

The down side to Dry Screening is that if the material is damp or sticky you will not be able to achieve clean in spec material. Your throughput will also be very low. By Rinsing rather than Dry Screening you will guarantee an acceptable throughput and also the correct specification as long as there is little or no clay in the deposit.

3.How do I manage the specification of my sand?

The Finesmaster range of Sand Plants have the facility to blend the fine and coarse sands, giving the customer optimum specification. This mechanical blending maximises the production of the higher value product. Sand is initially sized on the Rinsing Screen meaning the Sand Plant is not relying on density separation to grade material. The range includes plants with or without buckets depending on customer requirement. The addition of buckets reduce the size of Pumps and motors, thus reducing power consumption. The moisture content in the sand is generally as low as 10% - 12% once it comes off the high frequency Dewatering Screen.

4.I have very little water / no water on site. Can I still wash?

Yes, the DUO systems are designed to use minimal water. However to achieve this you would need to incorporate a DUO Water and Silt Management System.

5.How should I process heavily clay bound material?

At DUO we have a range of Powerscrub Logwashers ranging from 60 to 200 tons per hour capacity. The interlocking blade system that has been a feature of the Powerscreen design for over 10 years has a scrubbing action which is second to none in the marketplace and guarantees a clean clay free product. The "R" designed Logwashers have the facility to float out lightweight materials such as wood, paper, plastics and organics from either virgin or recycled aggregates.

6.Can I sell my quarry dust as sand?

Quarry dust can be sold as a sand but due to its low quality and high silt content it suitable for few applications. Quarry dust can be washed to remove the silt and clay content and create a natural sand substitute that is of concrete sand specification.

Our range of Powerscreen sand plants are a simple and effective way of processing quarry dust. We have installed a number of these plants, all of which have been very successful.

7.I want to start washing on a small scale but I want to have the option to expand in the future. Would Duo’s equipment allow me to do this?

The beauty of the DUO range of Washing Equipment is that it is of Modular Design. This means that as your requirements change, so can your plant. We have a range of different sized Screens, Logwashers, Sand Plants and Conveyors, which can be installed as required.

8.Do I need a lagoon to settle my silt?

No, DUO have a full range of Water and Silt Management Systems, which are tailored to customers individual needs. We have Plate Presses, Belt Presses and Centrifuges as well as Thickeners of varying sizes designed to your personal requirements.

9.I want to start washing recycled aggregates. What equipment do I need?

The first thing do is contact DUO and arrange a FREE site survey. An experienced Sales Engineer will visit your site and complete and discuss all the relevant areas such as your requirements, planning and environmental issues. We have over 30 installations in the UK which are processing Recycled Aggregates and will be able to answer any question you may have, based on our individual and company knowledge. The plant will then be "Tailor Made" to suit your needs and the physical attributes of your site. Not every one has acres of space and in the past we have installed full Processing Plants into small areas.

The plant will be made from our range of Screens, Logwashers, Sand Plants, Conveyors and Water and Silt Management equipment which incorporates Plate Presses, Belt Presses, Centrifuges, Thickeners and Flocculent Mixing and Dosing equipment. DUO will supply a dedicated Engineer to the project from start to finish who will liaise with you and your site ensuring that all Risk Assessments and Method Statements are done ensuring a safe installation. You will be supplied with a delivery and installation schedule so you can plan ahead keeping your customers informed.

Once you have chosen DUO you are part of the team; Two Working As One.

Centrifuges

1.When do you use a Centrifuge?

A centrifuge is used for dewatering sludge to produce a dry, manageable cake. Centrifuges can either be used as a mobile unit or as part of a fixed installation. They are very compact in size relative to the quantity of solids they can process and can easily be mobilised for applications such as dewatering lagoons or as part of a mobile washing plant.

2.What volume of material can they handle?

The capacity ranges from as little as 1tph right up to 30tph of dry solids. Gennaretti's ranges of centrifuge are designed specifically to handle larger quantities of silt than similar size machines from other manufacturers. This is a result of over ten years research and development in to processing silts from aggregate washing.

3.How do Centrifuges work?

Centrifuges separate solids and liquids using ‘centrifugal force'. Centrifuges contain a cylinder which rotates at a high rpm, inside the cylinder a screw rotates at a lower rpm thus creating the centrifugal force required for separation.

Thickened slurry is pumped into the centrifuge for separation. The centrifugal force pulls Less-dense materials (water) toward the axis of the centrifuge whilst more-dense particles (silt) are propelled away from the axis. The result is water suitable for reusing in the washing process and a cake that is dry enough to handle so it can be disposed of.

Gennaretti are a leading centrifuge manufacturer with over 30 years experience who offer a range of centrifuges suitable for range of different sized applications. Gennaretti are so confident it their products that they offer a 5000 hour warranty on their main wear parts.

 

4.What equipment do I need to run a centrifuge?

Centrifuges are normally supplied complete with a sludge feeding pump, flocculent mixing and dosing system and discharge screw for the silt cake. If you have an existing thickener system for the dirty water from your wash plant the pump can be connected directly to the underflow from the thickener or you can add a sludge storage tank for increased production.

It is possible to take the dirty water direct from the cyclone overflow on the wash plant subject to the volume and the size of the centrifuge. A centrifuge will usually require a generator to operate the machine and pumps and pipe work for the dirty water and centrate water.

5.Do I still need a lagoon?

A lagoon is not required when using a centrifuge as the centrate water is returned back to the washing process.

6.What is thickening and dewatering?

These are the two main stages of processing the dirty water from the washing process. When the water exits the sand plant with the silt fraction, the initial stage is to remove these solids from the water and settle the silt to create a thickened sludge.

This thickened sludge can be pumped to a lagoon or alternatively it passes to the second stage which is dewatering the sludge to produce a dry manageable cake and recycle the centrate water back into the washing process.

7.What chemicals are used and will they contaminate the silt or water sources?

Non-hazardous anionic flocculent is usually used to attract the silt particles together. This creates ‘floccs' and helps with the dewatering process. Subject to the type of material, cationic flocculent may be required to assist with the separation and dewatering process.

DUO can organise for each customers material to be analysed to determine the type of flocculent required and quantise required to achieve the optimum results.

 

8.What factors affect a Centrifuges performance?

The performance depends on the construction of the system itself and on the process parameters. The construction variables are: speed rotation, volume available and the speed extraction of the solid. The process variables are: speed feeding, solid specification, batches of chemical and working temperature.

Through our experience of installing centrifuges and with the support of Gennaretti, the manufacturers of the industry leading centrifuges that we offer, all of our installations are designed to meet the requirements of individual projects.

9.How dry is the cake produced by centrifuges?

This will very slightly depending on the characteristics of the feed material. Typically the cake will range from 35% to 45% moisture which can easily be transported. This cake will continue to dry further when left outside.

10.What can the cake from a centrifuge be used for?

The cake is impermeable to water and is therefore often used for lining lagoons, capping on landfill sites or blending in small quantities with soils. Often it is disposed of as a waste material.

Filter Presses

1.When do you use a Filter Press?

Filter presses are used for processing sludge, separating the water from the solids to produce a very dry, manageable cake.

2.How do Filter Presses work?

Filter press contain a series of polyurethane plates. Each plate has a recessed chamber and feed port and is covered by filter cloths. The plates are pressed together with hydraulic rams under high pressure.

A sludge feeding pump fills the void spaces between each plate and forces the water to filter through the cloths and exit the press.

The solids are held in the void space between each plate until the water has drained and produce a dry cake. The press automatically opens and the cakes are discharged.

3.What volume of material can a Filter Press handle?

Filter presses are sized specific to each application and the filtering time of the sludge. There are a wide range of sizes available from machines that process 50kg per hour up to large presses which discharge 20 ton per hour.

4.What is the time between batches when using a Filter Press?

This very much depends on the type of sludge being filtered. If the sludge is of a coarse nature and free draining, like granite silt or silt from stone cutting, this will filter in minutes and the press will complete several complete cycles per hour. If the sludge contains a high percentage of clay this will slow the dewatering process and will normally mean cycle times in the region of one hour.

Due to the quick opening and closing times of the presses offered by Duo the non-productive time is minimised and the processing time is maximised.

5.Do I still need a lagoon?

A lagoon is not required as the filtrate water is recycled back into the washing process.

6.What chemicals are used and will they contaminate the site or water sources?

Non-hazardous anionic flocculent is used in the thickener system. No further flocculent is required when feeding the sludge to the press.

7.How often do I need to clean or replace a Filter Press’s clothes?

This will vary depending on the type of silt, size of silt particles, type of cloth and maintenance of the cloths. When processing slow filtering sludge the cloths will need to be cleaned more often. DUO will supply the correct type of filter cloths material following trials on sample material and will guidance on the cloths specific to each application.

8.What factors affect a Filter Press’s performance?

The performance is affected by the following parameters: type of sludge, feeding pressure, feeding pump wear, type of filter cloths and wear of filter cloths, cleanliness of filter cloths, type and quantity of flocculent used. With regular maintenance the filter presses performance will remain consistent.

9.How dry is the cake produced by a Filter Press?

Filter presses will produce the driest cake possible. The moisture content will vary depending on the type of sludge and other factors. Typically the moisture content will vary from 25% to 30% moisture.

10.What can the cake from a Filter Press be used for?

Lagoon lining, landfill capping or fill material.