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The Continuous Discharge Elutriator.

What's that you might well say? It is a machine for classifying sand by rising water currents. Up until the 1950's one spoonful of sand could be classified in a laboratory tool called the Andrews elutriator. In the 1950's I worked at the Hydraulics Research Station under Director Sir Claude Inglis FRS. Sir Claude needed thousands of tonnes of very clean sand to do an experiment. His instructions to me were go and make a machine to do the job of classifying the sand so that it would conform to a chosen special specification.

Sir Claude was a World famous Scientist. I was not a scientist but an experimenter trained at the National Physical Laboratory in workshop practices and in the ways of assisting scientists.

Sir Claude had said "I will give you a day a week to think provided you spend four days a week managing the building construction section for the Station. You can take the day how you wish". His offer definitely paid off and we had a good relationship. I was qualified in building construction and we were gathering a very highly skilled team of artisans.

I worked out that I could set up some vertical glass tubes and connect them with rubber tubing such that I could have openings to put sand in and get it out again without stopping either the water flow or the feed. So the continuous discharge elutriator basis was established and the first mild steel machine made and a few pictures of the components taken.

Take a look at these few pictures

The early control method of making a change in sand concentration for discharge actuate a photo sensitve cell which in turn opened a valve on an oil pressure line shutting a rubber lined valve; thus letting the sand out of the process.

The trick had been to open the side of a vertical glass tube and introduce another smaller open ended tube into the wall of the first with an angle of about 30 degrees off vertical between them. Like a branch on a tree. The large vertical tube was stood in a continuously supplied, water filled dish and the side tube was temporarily sealed. A siphon was established in the main tube and then controlled by simple valves until atmospheric pressure was established in the middle of the set up such that water would stand in the side tube when the seal was removed.

Wet sand, when put into the static water standing in the side tube dropped down into the flow of the main tube. The fine particles and the dirt went upwards and the clean particles above a certain size downwards. By changing the rate of upward water flow one changed the size of both the downward falling and the upward moving sand fractions.

In this simple way continuous elutriation was done. The next step was to use the moving sand as a moving seal at the bottom of the machine while letting out an amount equal to the fresh sand arriving from above.

Imagine a cone point downwards full of sand as in an egg timer. Open the spout and the sand runs out fast but the top surface sinks very slowly that is how I kept the sand moving in the machine.

By measuring other water flow in the settled solids it was possible to electronically open a valve to let them out at the rate of feeding them in. Settled sand offers more resistance to water flowing through it sideways than sand which is about to settle.

This first machine was fed by a man using a shovel at about 2 tonnes per hour which was a big advance on spoonfuls one at a time.

Government held Patents were taken out and in 1963 a Licence was granted to Floatex Separations Limited for that company to build and market commercial versions of the machine. Soon it was apparent that Floatex could not design machines using the patents without help so they invited me to join them and become their production manager.

The experiments were done in 4" diameter glass tubes. The mild steel commercial machines replicated the 4" tube idea by sub dividing the very big vessels into approximately 4" tubes. By sucking the water upwards at the same rate of flow in each tube, enormous amounts of sand and water could be on the move but the upward flow could be as little 3 centimetres per second to 20 or more centimetres per second.

In the days before metrication sand smaller than five millimetres when worked upon in laboratories was always measured in parts of a millimetre. Typically a feed 100 tonnes of sand an hour in 2000 gallons or 9200 litres of water a minute all being slowed down to about 3 or 4 centimetres per second depending on what choice of size of grain of was required.

My first task with the company was to design around my own patents but also to bring about a series of machines that could be erected and worked on by fitters using only hammers, spanners, screwdrivers and welding gear. Sand and gravel workings are not research stations which is not to say research is not done there. It is.

Having discussed the normal market needs my response was to design a machine to process fifty tons and hour of building sand to the very high specification of having absolutely no silt or clay left in it. This would make possible the manufacture of extremely high quality concrete. The development was interesting in that company owner John Phipps asked me to build the machine but using his own design of discharge mechanism. I did this and all seemed to work up until the point where the product should come out and nothing happened. I telephoned John about this predicament to which he replied "that's OK. My idea was to relieve you of the worry until your half was done. Throw my part away and design the lower, discharge section". This I did.

Take a look the first commercial prototype elutriator in action at the Burghfield plant of the Amey Group alongside the Kennet and Avon Canal.

In these machines the cone holding the sand was inside another holding water so that the sand could be weighed as in Archimedes law. I weighed the sand on load cells I designed. One of these moved the controlling piston in a hydraulics control valve like you have on a bulldozer. Then water under pressure from a pump closed a valve or allowed it to open. Squeeze a piece of soft tubing with your fingers and you will get the idea.

The sand was sold as fast as it was produced and no stockpile ever built up.

John Phipps arranged a visitor's day for Directors of Sand and Gravel companies and Floatex finished the day with orders for four machines and a sale of the prototype. What one might call a good days' work. The sale of four complete plants in a year was then typical.

The first machines produced clean building sand in areas where the sand, as dug, was high in silt and clay.

Floatex next embarked on elutriators to clean sand to make asphalt and thus opened a new market at several places see the pictures of the 60 TPH and 100 TPH machines,

The Floatex machines ranged in capacity from 20 tonnes per hour to 156 measured tonnes per hour over whole working days. All the machines were designed and commissioned in a 24 month period. These machines ranged in size up to 12 feet in diameter were about 30 feet tall on towers that brought the tops to about 85 feet above ground level.

They were impressive machines with outputs of sand with consistent water content far exceeding anything that had been achieved previously.

We would use a small machine to open up the market.When the market was established that machine would be replaced with medium sized one, the first machine taken to another site to create a new market and so on.

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