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Plant automation ? Who benefits?

Automation in quarrying is sometimes regarded at first sight as a necessary – but not always universally welcomed – step taken by producers to reduce costs and maintain a competitive edge over other producers.

While this is often true in itself, Cantech’s experience over more than 30 years of automating quarry processes has shown there is far more to be gained, and more widely, than just that – providing the automation is appropriately matched to the task.

This article discusses what “good” automation can achieve, and for whom, and examines how well designed control systems deliver some of these benefits.

Having confidence in the quality of a quarry product is vital, and not just to the producers. Production, technical and sales staff, and significantly their customers too, all benefit if there are fewer complaints to investigate and less material is questioned/rejected.

A reputation for a consistently good end product can put a supplier in a stronger position, both when bidding for prestige contracts and when negotiating supply terms such as price, frequency of testing required, etc.

Examples that have been achieved include more consistent composition of nominal size aggregates in crushing/screening applications, cleaner sand from a well controlled washing plant and tight control of bitumen content in asphalt, cement/admixtures in concrete and pigments in coloured products.

While almost all operations strive to maximise production, this seldom equates to making the plant run at its nominal maximum capacity for much or indeed any of the time. {{image2-A:R-w:250}}

Instead, maximum tonnage is often achieved by ensuring the process operates at the most appropriate rate, given the nature of the quarried material at the time, eg not overfeeding one section of a plant relative to another and thereby avoiding blockages, spillage, tripping of items on overload, etc, minimising the downtime as well as often reducing wear and tear on the plant, while not underfeeding the plant for long periods. 

This can be most notably achieved on crushing/screening and sand/gravel processing plants, where the grading/cleanliness of the incoming material may vary significantly during the day, making it vital to monitor feed rates, crusher/screen motor currents, surge bin levels, etc and dynamically control the input feed rate in response.

This high production rate combined with reliability in the expected output from the plant again benefits both producers and their customers who can have confidence that ordered loads will arrive with less susceptibility to breakdowns and interruptions.

It can also reduce staff frustration at having to repair/clean up after such events and limit friction between management and staff over associated blame.

The past 10 years or so have seen an increasing drive to move the point of control away from the plant itself, primarily for one or both of two reasons: operator health and safety and centralised control.

Operator health, safety and well-being
Providing control from a clean control room or weighbridge office away from the plant removes the operator from the dust, noise, vibration and heavy vehicle movements typically encountered in conventional on-plant control rooms.

One notable example involved remote operation of a rock-breaking process at the intake to a primary crusher, which previously required an operator to be stationed all day in a small cabin immediately alongside but is now conducted, with camera assistance, from a clean weighbridge office at the entrance to the site.

Furthermore, this often means the plant operator shares an office with others, eg weighbridge clerks. Not only can they work closely but this provides a greater element of human interaction for the operator, rather than being somewhat isolated, as can be the case in on-plant controls.

The benefit of this contact to an operator’s wellbeing should not be downplayed – the volume of radio chatter operators have with drivers of mobile plant and incoming trucks on a typical quarry operation bears this out. 

Centralised control of more than one plant
The sophistication of monitoring and control that a comprehensive system provides often means the operator only needs to intervene occasionally when prompted by the system. He/she can then be available to control other processes, particularly if that process is controlled by a similar system.

One illustration combines control of a crushing/screening plant from the same office as an aggregate blending/load-out system, in which drivers report to the office to be allocated a load of either single size or blended aggregates, the details of which are entered by the operator.

The drivers are then given a small remote handset (like a rugged version of a TV remote control) and proceed to the load-out plant, where they press the handset. The system recognises their code, recalls the details of the load as previously entered and loads the truck with the required material.

There is relatively little operator involvement in the load, other than to enter the initial details, ie it is quite reasonable for the same operator to manage both the crushing/screening and load-out plants without being overworked, once having received initial training in the operation of both.

Interestingly, this combination of tasks not only allows economies in staffing costs for the quarry company but is often regarded as an increase in the skill level of those operators, and hence of their value to their employer, improving their sense of self-worth. {{image3-A:L-w:250}}

Experience has shown that most quarry operators learn the operation of Cantech’s systems reasonably quickly, meaning it is practical to train several people. There is less dependence on the skill of any one operator, allowing the workload to be spread to cover holidays and busy periods without anyone needing to work excessive hours. 

This more efficient and flexible use of staff is further enhanced by developments such as wireless displays for dump trucks and loading shovels, which typically include indication of the levels in the bins, allowing the driver to attend to other matters on the site until the bins fill up enough to need emptying, or positioning of radial conveyors and/or shuttles to fill bins/wagons on a concrete plant or railhead, and even to initiate the batching of ready mix from a loading shovel or forklift on a blocks/pre-cast operation. 

These facilities both reduce the workload of the plant operators – making it more practical to combine operations, as described above for the crushing and aggregate load-out systems – and again result in a far safer operation: drivers stay in their cabs rather than risking falls getting in or out or more serious injury from passing vehicles.

In batching processes such as mixing of asphalt, concrete and coloured mortars there can also be potential for savings in the use of the critical (and usually expensive) ingredient material.

This has been frequently seen with asphalt production. A key element governing the quality of asphalt is the bitumen content of the end product, which is determined not just by the achieved weight of the bitumen itself but also that of the aggregates and filler.

(A bitumen content of five per cent would result from weighing 100kg bitumen and 1900kg aggregate, but if the aggregate underweighs and only achieves 1840kg – which can happen if the plant is struggling to heat the incoming aggregate and thus one or more of the hot aggregate bins runs empty during weighing – then even with exactly 100kg of bitumen the proportion of bitumen in the mix is 100/1940, or 5.15 per cent.) 

While it would be wholly unreasonable to expect an operator to note for every batch the achieved weight of aggregate (1840kg in the case above) and recalculate the weight of bitumen required to give a five per cent content, this is exactly what Cantech’s AP-2000 asphalt batching system does, batch in, batch out.

It stops the bitumen weighing slightly less than its nominal target and waits for the aggregate weighing to finish, measures the achieved aggregate weight and, providing that is within an acceptable tolerance of its target, recalculates the target weight for the bitumen – in this case the target corresponding to 1840kg of aggregate is 96.8kg – and tops the bitumen up to this revised target weight.

Moreover, it then works out that it only achieved an overall batch weight of 1936.8kg instead of 2000kg, and adjusts the target weight of all subsequent batches in the load slightly to ensure the correct overall target load weight is still achieved.

This tight control of bitumen content not only ensures a consistent high quality of final product – with the benefits described earlier – but also allows potential for a significant material cost saving to the producer.

Without this dynamic adjustment of the bitumen weight it may be necessary to set a slightly higher target bitumen percentage, to allow for those cases where the aggregate might overweigh and thus the achieved bitumen content falls below its nominal target percentage.

However, ensuring much tighter control of the final bitumen content means no such overtargeting is necessary. A reduction in target bitumen content of just 0.1 per cent would result in a saving of 100 tonnes of bitumen – ie tens of thousands of dollars
– on an annual output of 100,000 tonnes of coated product.

As discussed, the Cantech family of control systems, specifically designed for quarrying processes, give rise to a number of different benefits, covering health/safety and improved satisfaction for both customers and staff, in addition to valuable financial gains to producers.

Cantech continues with the enhancement of these systems and development of new applications, and is always receptive to new customer requirements. 


Source: Cantech (UK)

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