23 August 1996

Stone separators novel operation old as physics

By Andrew Faulkner

SAND, stones/dirt and root crops all have different densities. Fundamental physics enables one of the materials, sand, to separate the other two.

Sound simple? Well, it is. The crop floats on the aerated sand, while stones and dirt sink through. In essence, that is how the US-built Sand Machine stone and clod separator works.

Launched in the UK by Armer Machinery last week, the first massive separator has already been bought by Greens of Soham, the vegetable and root crop grower/grader, which has been using the machine to part crop from clod since mid-July.

To date, the Greens separator has worked on daffodil bulbs, pickling onions and seed potatoes, with ware onions, beetroot, and maincrop potatoes still to come. Total annual throughput will be about 7000t, and target daily output is 100t+ an hour in maincrop potatoes.

In the Greens set-up, the Sand Machine replaces a conventional, into-store picking table. That brings an instant labour saving of 10-20 pickers who would normally be taking out stone/clod debris as the crop is transferred from tipped trailer into store.

"In a year we expect to save about £30,000 in labour costs alone," says Peter Backshall, of the Dennis Clarke Partnership (part of the Greens of Soham group).

"There are other potential savings, too. We should be able to substantially reduce our cultivations and destoning costs because the separator does such a good cleaning job. And in certain crops we may even cut out destoning all together."

Armers Tom Woollard confirms these potential savings, but advises some caution. "We would never advocate completely giving up destoning maincrop potatoes, for example. But even in that crop, there are other savings such as reduced storage and ventilation costs.

"The machines big advantage over water-based separation systems is that the crop stays dry, and you dont have the problem of getting rid of contaminated water."

So much for the benefits, but how exactly does the Sand Machine work? Probably the best way to explain is to follow the crop through the machine, which can be set up either in the field, powered by a generator, or back at the yard.

Density altered

A conventional web conveyor starts the process, dropping the mix of root, stone and clod onto a "fluidised" bed of sand. In simple terms, that means air is pumped up through a layer of sand to alter its density. The more air you pump in, the less dense the sand bed becomes. This is due to the particles becoming separated to a greater or lesser extent in respect of the volume of air passing through.

The aim is to make the sand more dense than the crop but less dense than the stones and clods, so the crop floats while the stone sinks. To set the machine up for different crops, the air flow is adjusted to alter the density of the fluidised bed.

When the crop reaches the end of the sand bed, it floats over the top of a chipper roller, onto an elevator and into store. Debris passes under the roller for disposal.

Meantime, the sand drops onto a rotating drum and is recirculated back to the beginning of the bed for further "fluidising" with air, which is warmed by three heater banks to keep the sand dry.

"This is a key detail," explains Cees Mensen, sales manager of Ploeger, the Sand Machines European distributor and Armer Machinerys supplier. "The type of sand is not important as long as it is consistent in size. Otherwise it tends to separate out," he adds.

"Average sand use is 1cu m/ day in normal conditions and 3cu m on a wetter day."

Ploeger is currently working with the Sand Machines US manufacturer, Sorting Technology Incorporated (STI), on other markets. These include the refuse sorting industry.

&#8226 STI has been building the Sand Machine for about eight years, and there are five models currently working in Europe.n


Sand machine

ModelRated output (in potatoes)Target buyerPrice (£000)

Standard125t/hourGrading groups£155-£160

and processors

2001-TLC60-70t/hourPotato growers£130-£135

with 100ha+

Stone separation the American way. Here the Idaho-built Sand Machine parts stones and clods from pickling onions before the crop is conveyed into ventilated store. Output is up to 60t/hour; 100t+/hour in potatoes. Inset: Air blown through the sand keeps these onions afloat while stones and clods sink below the surface.