Crushers have been used in concrete and demolition (C&D) recycling applications for over 20 years. Recycling applications use fixed plant, jaw crushers, cone crushers, screens, etc, to recycle C&D materials into products that can be reused in construction and domestic applications. From fixed plants, mobile (tracked and wheeled) crushing systems were developed for crushing applications. The tracked machines first appeared in the 1980s and their development has been ongoing. It is predicted that as recycling legislation develops, the amount of C&D material needed for recycling will increase exponentially. Therefore, development of productive, efficient crushers will continue.
This paper identifies the key component systems of a crusher used in C&D recycling applications that affect the machine?s overall operational efficiency. From the key components and desirable features, future development paths for crusher efficiency improvements are extrapolated. The improvements are centred on machine efficiency, operating cost reductions and environmental factors.
DESIRABLE FEATURES OF A C&D RECYCLING CRUSHER
There are many desirable features that a tracked crusher needs to operate in a C&D recycling application. The features, when working together, enhance the crusher?s ability to process the C&D materials faster and economically. Figure 1 is a view of a typical impact crusher, indicating the locations of the desirable features.
The desirable features have been broken down into various sections and explained in more detail below.
The crusher is the system?s key component, reducing the raw material to the size required. It must perform at peak efficiency. A typical C&D application will use an impact crusher with either three or four impact (or blow) bars and two or three impact toggles. The toggles can be adjusted via hydraulics to set the gap between the toggle and the blow bars, otherwise known as the gap setting, which determines the end material size. The blow bars are mounted on a rotating drum or rotor that spins at 400 to 600 rpm. The key features that need to be optimised within the crusher are the material flow path or crusher geometry, the blow bar material properties and the wear plate material properties. Manufacturers are continuously improving the crusher geometry and materials to increase wear resistance and reduce blockages. Increased wear resistance translates into less maintenance and reduced costs. Reductions in blockages mean less operator intervention and downtime, which results in increased machine availability and increased outputs.
There are two areas of screening on a typical C&D impact crusher: the in-feed (raw material screening) and the out-feed (product screening). The in-feed screen (see Figure 2 on page 28) removes all material that is below the grade required for the final product. This material can either be removed via a side conveyor or rejoin the crushed material via a chute, by-passing the crusher. The out-feed screen removes all material greater in size than the required product grade and either returns it to the feed hopper for further crushing or stockpiles it to the side. The importance of a screen is its efficiency at separating the material into graded products. Numerous factors affecting the screen efficiency include spacing, angle, motion, screen materials and size. More efficient screens will ensure materials are directed to the correct conveyor or chute for the processing of the material.
Material flow path
The design of the material flow path (Figure 3) through a tracked crusher is vital to the machine?s efficient operation. Conveyors, feeders, chutes, guides, rollers, etc, need to be designed to move the materials around the machine in the most efficient way to avoid the blockages and choke points that will reduce machine productivity through downtime or reduced feed rates.
These include screen mesh, sealing rubbers, feeder liners, punch plates, crusher liners, blow bars and chute liners. It is important these items are matched to the materials being processed. When matched and maintained correctly, the machine availability is maximised and high productivity results.
There are several power plant options, depending on the crusher?s design and configuration (Figure 4). Typically, there are hydraulic powered machines and electrically driven machines. Hydraulic powered machines drive the crusher, conveyor motors and feed chute motors, while an electric machine drives the conveyors and feed chute motors. An electric machine and the newer hydraulic machines can drive the crusher with a ?direct? coupling from the motor. Many machines offer a choice of diesel power supplies and/or an optional external electrical supply. There are advantages and disadvantages to powering all modes of machine. The choice of power and machine configuration can affect the running costs in terms of fuel, oil and electricity supply and maintenance costs and needs to be evaluated carefully at the time of acquisition.
The advanced machines currently being built include state of the art control systems (Figure 5). These will include a PLC controller, engine management system, electrical and hydraulic controls and an advanced Graphic User Interface (eg a touch screen). The components of the control system monitor the crusher?s various operating systems and adjust parameters to maximise running efficiency. For example, the control system will monitor crusher, conveyor and chute loads and adjust engine power to compensate. This will ensure the engine is running at its most efficient for the work required, thus minimising fuel usage and reducing fuel costs.
PROJECTED ADVANCES IN C&D CRUSHERS
The following improvements will be made to tracked crushers over the next three to five years.
This will be the main driver for tracked crusher improvements. Improved efficiency directly translates into lower operating costs and higher machine availability, resulting in higher productivity. To achieve higher efficiency, there will be improvements in material flow path design, internal crusher design and screening technology. These will combine to reduce choke points, bottlenecks and blockages in the material path, resulting in fewer stoppages for blockage clearing, fewer feeder ?slow downs?, higher outputs and better final product grading. The other major area of efficiency improvement will be more efficient power plants which will burn less fuel for similar or higher power output. Coupled with the latest control and engine management systems, the fuel savings will be significant.
Governments are legislating for more fuel efficient and environmentally friendly machinery. This means that engines will need to burn fuel more effectively and reduce greenhouse gas emissions. Engine manufacturers are working on new engines to satisfy these demands. Greases and oils will also need to be environmentally conforming. Another factor is the end of useful life on the machine. In Europe, there is legislation for disposal of the machines at the end of their life, therefore machine components will need to be recyclable.
There will be a continuous drive to reduce operating costs on all machinery. Improvements to crushers will focus on operating cost reductions. Some of the more prevalent improvements will be wear part life extensions, lower fuel and consumable usage and more effective control systems. There is current development in wear part life improvements, involving new wear resistant materials for high wearing parts such as blow bars, hopper/chute liners, crusher liners and conveyor belts. Blow bars are the primary focus of development as they are the fastest wearing parts on an impact crusher. Typically, a set of blow bars should process in excess of 20,000 tonnes of product before requiring replacement. Higher tonnages are possible, depending upon the abrasiveness of the feed material and blow bar materials. Currently, manganese, Martensitic and chrome steel bars are commonly used, with ceramic insert bars becoming more popular. Development with other wear resistant materials is in progress.
REDUCED COST, MORE PRODUCTIVITY
It is difficult to quantify the benefits of the above improvements, but it can be envisaged that an overall productivity increase will be in the order of 10 to 20 per cent. Operating cost reductions would reduce by a similar percentage, giving an overall improvement in machine efficiency of 20 to 30 per cent.
Andrew Gotley is the national development manager for Wirtgen Australia Pty Ltd.