Restrictions on the opening of new quarries and the development of existing sites have meant matching production to demand has become a challenging balancing act. Companies are now looking at the often vast hidden reserves of waste that were left behind as the quarrying of more easily accessible good rock became the prime objective. As good quality rock reserves become less abundant, new methods for both cleaning and sizing have been developed to capitalise on these material assets.
The development of the heavy-duty trommel screen has been at the forefront of this recent trend, and has resulted in very high recovery rates of good, clean rock from waste overburden. This, in turn, has helped quarry companies increase their operating profits.
Industrial evolution
Trommel screens have been in use since mechanised quarrying began in the late 18th century. At this time, the screens consisted of circular steel meshes bolted together to form a single barrel, which rotated around a long central shaft set at a slight inclination. Crushed material travelling through the barrel was graded out according to the size of the apertures making up the cylinders. These screens were used extensively until the 1950s, when vibrating flat-bed versions started to appear on the market.
The development of bigger, more robust, high capacity trommels in the 1980s – largely for the production of rip rap and armoured stone – paved the way for heavy-duty trommels that were purpose-built to handle high tonnages of material and very large lump sizes. The aggressive tumbling action within the barrel breaks up unwanted clays to produce a clean product. The barrel is self-cleaning and the entire unit works without a permanent operator.
Aperture sizes in the barrel are selected to suit the individual customers’ project requirements. As well as cleaning dirty and contaminated feed materials, trommels have been used for taking out fines prior to crushing. Other duties include the high-volume production of sea/river defence rock and sized material used for gabion baskets to support road and rail cuttings.
Reducing the waste pile
Stelex Construction Equipment has been manufacturing trommels for more than 25 years from its own facilities in the UK, and as the popularity of the trommel has increased, so Stelex’s range has expanded to include static, mobile and now tracked trommels.
During an extended demonstration at a major quarrying company site in the UK, one of Stelex’s Hercules HT182M mobile trommels was used to process heavily contaminated waste material, and recover the clean stone that existed within it. The waste had amassed millions of tonnes over a number of years and was a product of previous quarry extensions along with continued areas of dirty feed material within the quarry face. The quarry was rapidly running out of space within the waste area and this would have an impact on valuable areas for further excavation work in the future.
The large quantity of material entering the tip area was recorded over a three-year period. From the recovery rates achieved during the trial it was found that the trommel could reprocess 800,000 tonnes of waste in only 20 months. This reprocessing would reduce the size of the waste pile while also creating a saleable material for the quarry operator.
Feed material in the quarry is notoriously sticky in wet conditions, and during the demonstration the weather was particularly bad, with long periods of heavy rain. The versatility of the Hercules trommel controls allowed the barrel rotation speed and reciprocating feeder rate to match the feed material and weather conditions, maximising screening efficiency and throughput. The barrel apertures on the trommel were selected to suit the customer’s project requirements and were also suitable to cope with the demanding feed material. A gabion-sized barrel section was also introduced to add a new product for the customer and generate extra sales revenue.
A survey was made of the quantity of existing overburden, large waste piles and wash plant fines, and a plan was made for the reduction of these for years to come, while also processing material at the existing quarry face. During the lengthy trial, a wide variety of material was used from the existing quarry face and differing depths of the old waste pile. Combinations of these materials were processed to achieve a range of recovery rates in different weather conditions.
The heavy-duty Hercules HT182M uses a 1.8m diameter x 8m long barrel. This barrel was divided into sections to provide fines-recovery/clean-up sections, and there was an added sizing section for the new gabion product, which was an unexpected bonus for the customer.
{{image2-a:l-w:310}}{{image3-a:l-w:310}}
The trommel was fed by an existing excavator, with the rejected material and clean stone removed from under the trommel using loading shovels. The oversize material off the end of the trommel was dropped over the existing bench and allowed to build up until removed by loading shovel when required.
During the demonstration an accurate record was made of daily weather conditions, where the feed material was located within the waste pile, the throughput of the trommel and the recovery percentage achieved. Calculations were also made on the costs for blasting, royalties, load and haul, and fuel. It was quickly apparent that significant savings could be made in many areas with the trommel in place, eg:
- Reduced fuel costs for mobile plant.
- Reduced haulage costs with less mobile plant movement, leading to less mobile plant being required on-site.
- Planned quarry extensions were suspended, giving extended life to the quarry.
- Less pressure on other crushing/screening plant within the quarry, as the stone reaching the plant was not contaminated. This led to improved reliability.
- New products produced by the trommel generated extra sales revenue.
Recovery of quality stone
A maximum recovery rate of 81 per cent was achieved, with an average of 54 per cent achieved over the whole trial.
Various locations were used within the vast waste pile to select feed material for the demonstration. Some had seen feed material sitting in the same location for many years, which made the contamination very compact, especially lower down, where finer material has washed through.
Even with heavy rain during the demonstration, the trommel coped well with the difficult feed material, and excellent recovery rates were recorded. This recovery would have been almost impossible by any other current method.
The amount of unwanted waste material returning to the waste pile after passing through the trommel was greatly reduced, and further working of the wastepile would result in more quality stone being recovered and more room on the quarry floor. As the existing waste pile could be worked on for many more months, planned quarry extensions were put on hold, resulting in further cost savings from expensive planning applications.
The clean gabion was removed from under the trommel and made available for immediate sale as a new product for the quarry. The clean oversize material off the end of the trommel was sent to the primary crusher without the additional problem caused by the contamination. This helped to prevent future breakdowns and allowed the rest of the plant to be much more efficient and reliable.
The purchase cost of the trommel was broken down into a cost per tonne and factored in the costs for royalties, blasting, processing and fuel to give a monthly value to the quarry operator. This value used the recovery rates of the trommel during the test period.
A plan was made at this stage to position the trommel in a safe area, to provide ideal access from all areas of the quarry.
The increasing requirement to reclaim and recycle a greater proportion of primary rock, and the need to clean materials containing heavy contamination, will ensure heavy-duty trommels continue to play a major role in these projects worldwide for many years to come.
In Australia, Stelex trommels are available nationwide from the Lincom Group.
 |
| TRACKED TROMMELS NOW ON LOCAL SOIL |
|
After a prolonged design process, the eagerly anticipated tracked Hercules trommel range is now available in Australia. The new tracked machines form part of the standard product range and incorporate many of the newest design features seen on existing static and mobile models. The new HT152T and HT182T models keep the “heavy duty” tag and are designed to work in almost any site location with dirty/contaminated material. The heavy duty hopper and pusher-feeder provide a regulated feed rate into the heavy duty barrel, and all speed and feed rates can be changed very easily to suit the application. The screening area on the HT152T is 1.5m x 6m long and 1.8m x 6m on the HT182T, and can produce a maximum of two product sizes plus oversize. The unique barrel design and extra manufacturing processes help to provide a long life in all conditions. Three on-board conveyors stockpile the screened material ready for collection. All conveyors, along with the feed hopper extensions, fold to allow for simple transport. A tracked trommel gives extra flexibility to the customer and can be positioned at the heart of the working area, with no permanent foundations required. |
Source: Stelex Construction Equipment/The Lincom Group