Innovations in aggregate production

Innovations abound in the aggregate production sector - Mike Woof reports

With road construction accounting for a significant percentage of aggregate production, developments in this industry are of major importance for the highway sector. Technical advances in aggregate production methods have pushed technological boundaries, resulting in cost/tonne reductions for products and improving operating efficiency for major producers.

The sophistication of available crushing and screening systems has advanced considerably. Major manufacturers such as 6934 Metso and 325 Sandvik have advanced monitoring and control technologies for their crushing and screening machines, allowing more efficient operation and continuous monitoring of systems and throughput. These firms also offer software tools as part of the process package for larger and more sophisticated plants. Meanwhile 1412 Bedrock Software has developed a comprehensive optimisation software package that can be used for crushing, screening and washing stages for large modern plants from all manufacturers. The novel 153 Aggflow modelling program and AggOpti tools are designed to help quarry operators with financial planning and forecasting of production, with an internet-based AggFlowSite version also available.

One notable factor at the recent Association of Equipment Manufacturer's (676 AEM) CONEXPO-CON/AGG exhibition in Las Vegas was the large number of new crushing and screening equipment being introduced to the market. The 1250 Astec and 1222 Terex Groups for example introduced an array of new machines for the aggregate production sector, with their machines having been developed under different brand names for an array of market needs.

Recycled aggregates

One of Metso's latest Lokotrack ST3.5 mobile screens has proven its efficiency in a demanding project involving rock-based recycled materials in Italy. In Pollenza, south of Rimini, the ST3.5 is being used to classify tricky, rock-based recycled materials. The machine has a throughput of around 1,000tonnes/working day and the customer says that the end products have a high quality. The company, 2751 Re i cal, produces aggregates needed for concrete at its quarry and broadened its operations by moving into the recycling sector a few years ago. The firm says that the compact design of the ST3.5 is a useful feature as this allows the machine to be transported quickly, easily and cheaply, increasing its versatility and utilisation.

The optional vibrating grid increases the feed rate and reduces the need to tilt the grid for cleaning, a key issue. In recycling applications, the main benefit of this optional unit is in securing the cleanliness and quality of the end products. The vibrating grid separates wood, plastic and other scrap from the feed, allowing cleaner screened end products. By choosing different grid mesh sizes, the user can also easily adjust and control the feed size to the screen. With the ST3.5 mobile screen, the upper screen deck houses a 30mm mesh size, and the lower deck 5mm. Oversized materials separated by the grid are returned to the crushing stage. The Lokotrack ST3.5 used by Re i cal also features separate remote radio controls for the mobile screen and the vibrating grid, allowing total steering and adjustment of the screening process easily from the excavator or front-end-loader cabins.In terms of outright innovation, most of the new models introduced showed progressive development over earlier models. However the huge array of new machines showed that this sector is one which manufacturers consider of future economic benefit.

Compact and highly mobile crushing and screening equipment has become significantly more popular in the market in the last 20 years, particularly track-mounted machines. These versatile units can be used in a range of applications for recycling duties as well as quarry production and are favoured by contractors because they can be easily transported to site and commissioned. Whereas only a small number of manufacturers built tracked crushing and screening machines in the past, these are now offered by a wide and diverse group of firms. Some users have replaced the traditional static plants in quarries with units that are mobile and in a number of operations the primary crushers are tracked to the face after blasting to process rock, with conveyors then transporting the sized material to stockpiles or for tertiary crushing. At other sites, primary, secondary and tertiary crushing stages are all carried out with mobile machines, while mobile screening is used for material grading and separation. For recycling duties, mobile units are used on demolition jobs or located temporarily on construction projects to process material. The recycled materials can be used for fill on road projects for instance, where it meets specification requirements.

Mobile crushers have widened the choices available to operations, allowing smaller quarries to contract out the crushing and screening process or to bring in rental machines for set periods to carry out the work. The size and throughput capacity of mobile machines has grown enormously, while the quality of the material produced has similarly improved. Customers can now select from a wide choice of jaw crushers, impact crushers, vertical shaft impactors, cone crushers and gyratory crushers for an array of applications, with a diverse range of screening equipment also available. Some machines even combine primary crushing and screening stages in a single unit, with either pre-screeners or recirculating feeds.

Several companies have moved into this market in the recent past, either by acquisition or by developing their own models in-house, and there is such a proliferation of models available it does beg the question of how many can remain profitable. A number of smaller companies have been bought by larger firms, adding to their portfolio, with 161 Atlas Copco for example buying the Austrian company Hartl, one of the pioneers of the mobile crusher.

Efficient screening

Canadian firm 2708 Major Wire Industries has developed a durable double weave, woven wire product for use in high impact aggregate processing applications. This features two, side-by-side, lighter-gauge wires and is said to outperform heavy-gauge, single-strand woven wire and perforated punch plate in high-impact duties. Impact from large aggregate material can cause screen breakage even with heavy gauge single wire. Woven with two smaller-diameter wires, the firm's new screen media has lower wear points than larger, single-strand wire. Each wire helps preserve the life of the other as it wears. The combined tensile strength of the two finer-gauge wires withstands up to four times greater impact providing up to 40% better wear life than for a single, heavy-gauge wire, meaning the product is well-suited for scalping applications and top screen decks. The double-weave product also weighs less and is more flexible than single heavy-gauge screen media, making it easier and safer to install. Compared to perforated punch plate, Double-Weave has far more open area, increases throughput up to 50% without costly screen deck alterations and costs up to 25% less than conventional products, according to the firm.

In addition to the added wear life provided by its two wires, Double-Weave provides up to an extra 40% greater wear life because it is manufactured with the firm's highly abrasion-resistant, high carbon and high manganese content wire. This is said to offer a combination of ductility, hardness and tensile strength. However the Double Weave product is also available in stainless steel, which helps eliminate blinding in sticky material applications. Producers can choose opening sizes from 31.8-12 mm) and wire diameters from 6.2-12.7mm for heavy, medium and light-duty applications.