Rota Val valves cut waste at limestone quarry

Seeking to cut product waste and improve efficiency, the Tarmac Ballidon site chose Rota Val heavy duty rotary valves to handle limestone powders and granules

Limestone is an extraordinary mineral.

Described as the "material of a thousand uses", Calcium Carbonate represents about a kilo of our bodyweight in the form of our bones, and calcium is the fifth most abundant element by mass in the human body.

It is used extensively in foodstuffs such as baking and brewing, dairy products and even digestion aids, and in fertilisers, glues, pesticides, toothpaste, pharmaceuticals and sugar refining.

Other uses include construction and building materials such as cement and asphalt, plus agriculture, abrasives, adhesives, cosmetics, glass manufacture, drinking water treatment, floor covering, carpets, paper, paints, rubber and shower trays.

There can be few other materials which have such a fundamental impact on modern human life, yet limestone is rarely accorded the widespread importance it deserves.

This sedimentary rock occurs in many places around the British Isles.

The deposits were formed over millions of years from a complex mix of the skeletal elements of organisms such as marine algae, phytoplankton, corals, shellfish and by precipitation under relatively high ambient temperature conditions.

The variety of formation conditions leads to many different types of limestone of commercial interest.

One of the larger UK geological areas covers most of Derbyshire in the form of the Peak District, and the area around Ballidon in particular is noted for its very pure limestone formed in a tropical lagoon some 330 million years ago.

Probably quarried from the earliest paleolithic times, the area has seen extensive lead mining under the Romans, building stone extraction in the Mediaeval period, and then the establishment of a quarry by a local butcher to employ former soldiers shortly after World War 2.

This quarry was taken over by Tilcon and then by Tarmac.

The history of Tarmac is itself interesting.

As if by chance, on a road near Denby ironworks in 1901, the county surveyor of Nottingham - Edgar Purnell Hooley noticed a barrel of tar had fallen from a dray and burst open.

To avoid a nuisance, someone from the ironworks had thoughtfully covered the sticky black mess with waste slag from nearby furnaces - and the world's first tarmacadam surface was born by accident! Hooley noticed that the patch of road, which had been unintentionally re-surfaced, was dust-free and hadn't been rutted by traffic.

So he set to work and by the following year, 1902, Hooley obtained a British patent for a method of mixing slag with tar, naming the material Tarmac.

By June 1903, Hooley formed the TarMacadam Syndicate Limited, the origin of what is now the Tarmac Group, the UK's leading supplier of building materials.

Tarmac employs around 12,500 people worldwide with a turnover of more than GBP2.1 billion.

One of the company's many UK sites, Ballidon, is named from the Saxon words meaning 'bag-shaped valley', and encloses a small village ten kilometres North of Ashbourne.

Here, there are three quarries covering more than 55 hectares which produce some 1.25 million tons of stone per year.

On average, 40% of this output goes to the industrial market with the remainder being used by the construction industry.

A new processing plant has been built on the site, where limestone is extracted, crushed and then screened into fine powders and small granules.

The new plant has replaced four smaller, less efficient plants.

Materials are blown from drying ovens into seven new silos each holding 250 tons, using Rota Val rotary valves and Rota Val plug diverter valves to control and direct the flow.

From the silos, powder and granules are discharged on to enclosed belt conveyors and then into road tankers through two loading stations.

Originally built with two rotary valves to permit controlled discharge from the silos on to the conveyors, Tarmac decided to further upgrade their facilities on the Ballidon site, and asked Rota Val to recommend suitable valves for the project.

After detailed consultation about the Tarmac requirement, Rota Val recommended five new HDG Rotary Valves.

These valves are frequently used to provide high levels of discharge control for fine powders which are subject to flooding under certain circumstances, especially when used in applications with anti-bridging systems.

Steve Bottomley, the Tarmac Ballidon Site Construction manager, commented "Tarmac has a very strict and active policy on environmental impact, and expends considerable efforts to ensure our activities are acceptable to local people.

The crushed limestone powder does not represent any danger, but obviously we place great importance on keeping our products where they belong - inside the plant and under control.

The two silos fitted with Rota Val rotary valves gave no trouble, and loaded the conveyor correctly under perfect control.

We had already standardised on other Rota Val valves in the plant and have about 30 rotary valves from 250 mm up to 450 mm, plus 18 plug diverter valves, so it was no surprise that we called in the company to make recommendations for further plant improvements".

Rota Val suggested the installation of five new HD450 Rotary Valves to provide the required degree of flow control.

The HD heavy duty valves are available in 12 standard valve sizes ranging from 50mm to 750mm to give capacities up to around 250m3/hr.

Round, rectangular or square flange options can be provided to meet a wide range of customer application requirements.

The HD series valves are designed and constructed to ensure high sealing efficiency with maximum durability.

They are the "design hub" around which all Rota Val rotary valves are based, a feature which results in competitive prices without compromising performance.

The valves may be manufactured as standard in cast iron, SG iron, aluminium and stainless steel, with alternative materials such as Monel, Inconel, titanium, and Hastelloy also available.

For the Tarmac application, valves were specified to operate effectively at up to 85C when discharging fluidised powders.

The HD series of valves may be supplied for temperature ranges from -100C to +900C, with a pressure differential of around 1barg as standard and 2barg options.

Full ATEX certification for explosion and flame containment for S2 dusts may be supplied to order.

"Our experience of Rota Val products has been very positive," concluded Steve Bottomley: "The company has a considerable body of expertise which has been helpful in solving our problems, and they offer an excellent after-sales service.

These features are most important in an industry which demands continuously reliable plant to maintain productivity".

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