All In The Wash

Log-washer specification in mineral-washing applications

By Kevin Vallelly, product development manager, CDE Global

As the requirement to process increasingly difficult materials grows, the specification of log-washers has become more prevalent. With the exploration of deposits containing higher levels of clay contamination than has previously been the case, log-washers allow operators to consider the processing of materials that would otherwise not be viable.

As well as applications in the processing of materials such as crushed rock to produce construction aggregates, the increase in construction and demolition waste recycling applications, particularly in the UK market, has resulted in an increase in the specification of log-washers. The high level of contamination within construction and demolition waste material is the influencing factor, with a requirement to remove a large proportion of lightweight contaminants in addition to clay necessitating the introduction of this type of attrition technology.

There are also applications for log-washers in the processing of a variety of mineral ores, and CDE’s specific experience in this area lies in the processing of iron ore. In this application attrition is required to remove the fine particles of alumina and silica from the ore to increase efficiencies in the steel production process. The net effect of removing the silica and alumina contamination in iron ore washing is to increase the Fe value while also eliminating contamination which can build up as a waste product in the kilns (alumina) or take very high energy consumption to burn off (silica).

KEY ISSUES

The function of a log-washer is to break down insoluble or ‘plastic’ clays through a high-attrition scrubbing process to, ultimately, produce a clean final product. In determining which log-washer offers the best option for a particular application, there are a number of issues to be considered, including:

1. Design of the shafts and paddles.
2. Shell design.
3. Drive assembly.

Design of the shafts and paddles

There are many important issues relating to the shaft and paddle design that have a great impact on the effectiveness of the machine. The first consideration is to ensure maximum distance between the shafts to reduce impact from the feed material. The feed point should deliver material to the centre of the log-washer to prevent material impact on the shafts. As well as minimizing wear, this also maximizes the attrition applied to the feed material, thus ensuring maximum removal of fines and unwanted contaminants from the final product.

The configuration of the paddles on a log-washer is also important, as various set-ups are employed. CDE’s Rotomax unit, for example, utilizes a spiral configuration, which is considered to be the most efficient choice for a number of reasons:

• It allows the paddles to inter-mesh and ensures that a greater level of attrition is applied to the material, compared with a standard set-up.
• It offers superior protection against plant failure by eliminating intermittent shock loading on the shafts, bearings and gearbox. Rather than shocks occurring as the paddles and material connect at regular intervals, the impact of material on the paddles is more evenly distributed.

It is also important to consider the design and construction of the paddles themselves in order to avoid unnecessary wear. The material used to construct the paddles is largely dependent on the material to be processed but because, more often than not, either hard rock or mineral ores of a highly abrasive nature will be processed through a log-washer, CDE have opted for a high cast chrome iron (600 Brinell), which is the same specification as that used on impact crushers. A variety of other options are available, but this selection is based on trying to ensure the longest possible processing life, while also offering the lowest cost per tonne of material processed.

Using high cast chrome iron paddles may require a higher capital investment relative to other less-durable materials, but there is extensive evidence that high-specification paddles offer the lowest cost per ton of material processed by a considerable margin. This is not only the result of a reduced requirement to replace worn paddles than would otherwise be the case, but also the reduction in plant downtime that is enjoyed as a result of less time spent on plant maintenance.

Following installation of a Rotomax log-washer, Paddy Lynch (Dundalk) Ltd, of Co. Louth, Ireland, first replaced the paddles on their unit after they had processed between 240,000 and 280,000 tonnes of sand and gravel over a three-year period at a cost of less than 4p per tonne. Site manager Gerry Malone commented: ‘The paddle specification on the Rotomax is far in excess of others on the market. We were initially concerned with what seemed like a high cost for the paddles, but the experience we have had shows them to be the most cost-effective option available, in my opinion.’

Although the above example is an application where the feed material is not particularly abrasive, on hard rock or construction and demolition waste recycling applications, the Rotomax paddles will still comfortably process in excess of 100,000 tonnes of material before they need to be replaced. This compares with approximately 20,000 tonnes for some of the lower-grade materials that may be used in such machines.

The size of the paddles on any log-washer will influence the effectiveness of the system – larger paddles are best, as they ensure maximum transfer of energy to the material within the unit. This ensures that attrition is maximized as it is the material-on-material impact that generates the attrition required to effectively clean the material.

However, the paddles are only as good as the protection they are given to prevent them from becoming loose or being exposed to unnecessary wear during plant operation. The Rotomax log-washer has multi-stage paddle protection, which begins with the paddle bracket. This is constructed from 20mm plate and the paddles are connected to it by means of a countersunk high-tensile bolt connection, which minimizes the risk of the paddle becoming disconnected. Additional protection is provided by a high chrome wear shoe fixed on the leading face of each bracket, which further protects gussets and bolts from wear.

Shell design

Ensuring maximum attrition through the highest level of material-on-material impact is carried through to the design of the shell of the log-washer. This should allow material to form its own bed within the unit, thereby minimizing wear and maximizing plant life.

Management and control of the water level within the log-washer is another critical factor in ensuring optimum performance, and by integrating the water connection and control valve the operator is able to effectively regulate the water supply to ensure both effective cleaning of material and efficient removal of fines and unwanted contaminants from the feed material.

The design of the outlet weir for waste water containing fines and lightweight contaminants also needs to be carefully considered. Any oversight in this area can be a cause of unnecessary plant downtime, because if the discharge chutes are not able to remove waste water efficiently they can become blocked.

By including a large outlet weir at the rear of the Rotomax, in combination with steep discharge chutes, waste water, fines and lightweights are efficiently removed. This not only has a positive impact on plant production, but also maximizes the efficient recovery of fines and removal of lightweight contaminants.

Drive assembly

Key to the success of the drive assembly is the motor and gearbox set-up. CDE consider a single motor and gearbox to drive the dual shafts within the log-washer to be the most efficient configuration. This not only reduces the number of wearing parts, but also allows for reliable synchronization of the shafts.

CDE have opted for a planetary gearbox with a hydraulic clutch to allow for start-up under full load. The ability to provide gradual acceleration during start-up ensures that the drive is protected against overloads and also minimizes the time spent on maintenance.

When a log-washer without a hydraulic clutch has been shut down it will need to be cleaned out before it can be re-started, which not only adds considerable time to the maintenance regime, but also creates waste material that has to be re-processed.

Also, by using a belt drive CDE have allowed for simple adjustments to be made to the rev/min to increase or decrease cycle time, depending on the specific nature of the material being processed. The company’s experience in tackling heavily clay-bound materials has shown that, when dealing with such deposits, there is an inevitable variation in the level of contamination to be dealt with as different sections of the deposit are extracted. It is often the case that clay contamination is present in large seams and there may be some areas where the clay content is considerably lower than others.

By decreasing the rev/min of the machine, it is possible to increase the retention time when processing very heavily contaminated material to ensure effective removal. Equally, the rev/min can be increased when the contamination level is lower. This functionality is further enhanced by the ability to adjust the operating angle of the log-washer to take account of variations in feed composition.

Bearing protection

Given the potential for increased maintenance and spares costs as a result of bearing contamination, it is also important to consider the level of protection given to the bearings. A variety of simple design adjustments can be made to ensure that this protection is provided, the most obvious being the location of the bearing housings.

These should be located above the water level to reduce the risk of contamination from the sand and waste water discharged at the rear of the log-washer. On the Rotomax there is a dual-seal bearing arrangement and a temperature sensor that monitors the back bearing temperature. This acts as an early warning indicator for bearing failure and represents another example of the proactive steps that can be taken to reduce the time required for plant maintenance.

LOG-WASHER SPECIFICATION

Given the likely increasing specification of log-washers in the years ahead, it is essential that a full ‘cost of ownership’ calculation is carried out alongside any capital investment comparison, to ensure specification of the most efficient unit possible.

The design and construction of the shafts and paddles within any log-washer are critical to the machine’s success and should be carefully evaluated prior to any decision. CDE can assist in this process through the provision of extensive return on investment calculators.

Plant maintenance time and the cost of spares is critical to the operational cost of any equipment – log-washers in particular, due to the potential for high wear if the correct components are not included at the outset.

The Rotomax has proven to be a unit with the flexibility to cope with the differing demands of a wide range of materials, including sand and gravel, crushed rock, construction and demolition waste, iron ore and other mineral ores.

For further information, search the Internet for: CDE Rotomax