Eight members of the Cleveland Institution of Engineers recently visited Cleveland Potash 's Boulby Mine (Part of ICL Fertilisers), situated near Staithes on the edge of the North Yorks Moors National Park. They were welcomed and guided underground by Mr Neil Rowley, a Newcastle University graduate mining engineer who had spent much of his career in coal mining before moving on to Cleveland Potash. The mine shafts when sunk some 35 years ago were the deepest in Europe (1.1km) and working temperatures are in the range 30 to 44C. The potash was deposited during evaporation of a prehistoric inland sea stretching from what is now northern England into northern Europe, the sites of present day mines. The potash lies above a layer of strong rock salt and because of the geological properties of the strata, the method of working involves driving long term roadways through the salt to provide access to the production districts in the mechanically weaker potash. The immediate roof strata consists of an extremely weak marl and this needs to be supported by leaving a 2m thick beam of potash in the roof. Rock bolts are used throughout the mine to add strength to this roof beam. Potash is mined on 5 days each week and salt on the remaining 2. The rock salt is sold as winter road salt without further treatment other than sizing. The potash, which is generally used in fertilizer manufacture, has to be separated from its impurities by froth flotation in a large surface processing plant, the tailings being discarded to the sea. Products are shipped from the mine by rail and exported world wide via dedicated facilities at Teesdock.
There is some water ingress into the mine but the air is very dry. There is no danger of a dust explosion, salt unlike coal being inert, but there can be incidence of explosive gases. Electrical devices, cameras, watches etc. are banned underground and the party was transported in flat bed trucks (modified Transit vans) stripped of electrical gear other than lights and horn.
A working face 7 miles out under the North Sea was visited (temperature 35C), extracted rock being moved on a bendable conveyor (FCT) to the main conveyor system and then on to be hoisted out of the mine by skips in the upcast shaft. The FCT is 'intelligent', in that it can be steered to the desired location, and when withdrawal is required, the conveyor 'remembers' its return route. The introduction of the FCT has resulted in a 50% increase in output from this production panel over the more conventional panels operated using shuttle cars.
Geological surveying and roof bolting operations were witnessed. Interesting features being the single slender pack of both resin and hardener which is inserted into the bolt hole, mixing occurring as the bolt is spun home, and the simple design of tell-tale giving an indication of subsequent roof movement.
In order to locate the best areas of potash for future mining operations, exploration holes are drilled horizontally up to 2km from the existing workings using a core drill. The extracted cores are placed on sample trays for inspection by the geologists. The sample's colour can be misleading as regards the potash content of the rock: both potash and salt can vary greatly in colour due to the presence of trace impurities, however the potash in the North of the mine tends to be reddish in colour. The taste of the mineral can be used to differentiate between salt and potash; potash has a much sharper taste than salt.
Ventilation of the mine is achieved by means of a surface fan forcing air into the downcast shaft. This is supplemented by booster fans at various locations within the mine. The intake air is kept separate from the return air by means of ventilation seals constructed from large polystyrene blocks. It has been found that these blocks are excellent for absorbing ground movement whilst still maintaining an effective seal
In case of fire preventing evacuation of the mine there is a series of emergency refuges with air supply from the mine compressed air system or, in the event of its failure, bottled air to individual masks.
The winding gear is a-typical in that both shafts have headgear supported by chimney-like structures of reinforced concrete rather than conventional steelwork. The winding engines are ground mounted drum winders. The DC motors provide both the acceleration and regenerative braking. The brake shoes on the winding drums are normally applied only as a 'parking brake' when the position of the skips needs to be held. The skips hold 23 tons of rock each, fed from the conveyor system at the bottom of the shaft, and discharging into a hopper at the top of the shaft from the bottom of each skip. Travel of the 1.1 km deep shaft takes approximately 2 minutes including loading and discharge. It is interesting to note that the Transit vans are lowered into the mine in a vertical position, slung from underneath the skips.
Winding ropes last approximately 18-24 months, and are inspected regularly in order to ensure continued safe operation. Maintenance is carried out on the shafts between 7am and 12pm with the rest of each day used to hoist rock to the surface. Storage bunkers are provided underground in order to maintain the mining operations at the face.
Communications underground are provided by telephone, tannoy and 'leaky feeder' radio systems.Health and Safety in the mine is paramount, demonstrated by the mine's excellent safety record, with all personnel equipped with full safety gear including high-visibility orange clothing. Traffic travelling inbye (to the rock face) gives way to traffic travelling outbye. Vehicle top speed is limited to 25 miles per hour (which in the dark seems quite fast). Vehicles sound their horns at junctions and sharp bends.
This was a most informative, exciting and enjoyable visit and the Institution is grateful to Cleveland Potash for allowing us to spend the day with them and to Neil Rowley for his enthusiastic and detailed explanations of the mine's equipment and operations.
