In a system as complex as a mineral processing plant, each component can impact the overall performance of the operation-for better or for worse.

It was in recognition of this fact that Metso Outotec, one of the world's largest mining equipment, technology and services companies, developed its "whole process" philosophy. The philosophy holds that each piece of equipment should be run in a way that optimises the piece of equipment that comes after it, rather than being managed in isolation.

"If the focus is limited to one piece of equipment, you may actually make the plant throughput and efficiency worse, potentially leading to other equipment overloading and needing additional equipment to compensate," explained Martin Smith, General Manager, Screen Sales & Support (Asia-Pacific) at Metso Outotec.

The whole process philosophy is especially important when managing the crushing circuit, one of the biggest energy consumers in any plant. This is where optimisation of screening comes into play.

"There are two ways of looking at it: there's screening prior to crushing, and then screening after crushing. We want to make sure that we've got the whole process balanced. We're not just looking at getting the crusher working at 100% or a screen working at 100% efficiency. We're trying to optimise both, so you're getting both of them up at the highest efficiency possible."

Smith recalled a visit to an iron ore operation in Western Australia that was having trouble with a double-deck banana screen with a capacity of 3,000 tonnes per hour.

"I looked at it and could immediately tell that the screen was overloaded. The Operator said, ‘No, it's running at 3,000 tonnes per hour', and I said, ‘I'm sorry, but the screen is overloaded, and I can tell it's overloaded by looking at the bed depth'," he said.

"When calculating screen capacity, we always look at the bed depth because that reveals the amount of material you're carrying over on the screen. If you get your calculations right, you get equal bed depths on the top and bottom decks. In this instance, the top deck was very high and the bottom deck was very low.

"After an hour's discussion with the site manager, we eventually discovered that they had changed the front end of the process. Previously, they were digging from a couple of different pits, blending it and feeding via one stockpile into the plant, and that's what the screen was seeing and had been sized to. [But] later they began feeding the screen directly from one pit at a time with no blending. This led to a couple of scenarios, when feeding from one pit the top deck was overloaded and when feeding from the other pit the bottom deck became overloaded. The solution was quite simple: they just needed to revert back to blending, which they did, which restored the plant's capacity and the plant throughput went straight back up and the crushers became balanced again, exactly as they needed to be."

He summarised, "It really is about looking at the problem from the start to the finish, and sometimes you have to show the customer how the changes they thought would improve the operation have actually affected pieces of equipment later in the process."

The whole process philosophy does present its challenges, including how to manage conflicting KPIs. One common example seen by Smith involves the use of gear box exciters as drive units for large screens.

"What generally happens is that you get to the point where you need more force output for the load you're carrying on the screen, so people try to redesign the screen to get more force output out a larger exciter, which means they then need a larger exciter beam, and for a larger exciter beam they need thicker (added doubler plates) side plates and sometimes larger cross beams," he said.

"These exciters were designed to have at least a four-year life, but the industry has got to the point where it becomes a struggle get even one year out of them. It's a vicious circle where everyone's trying to get more tonnes through, which means your screen has to be bigger, but then that shortens your maintenance schedule.

While problems will always arise, Metso Outotec believes most issues can be avoided by conducting comprehensive material characterisation prior to equipment selection and plant design.

"It's important to understand the customer's challenges with materials and targets. It's very easy to select the wrong equipment, which will lead to increased bottlenecks and perhaps machine failures or increased breakages," Smith said.

He added that customers should always consider the impact of plant layout and equipment interactions on the overall plant performance.

"People often forget or simply do not know how to design and integrate equipment into a processing plant. Screen and crusher material feed presentation alone can make or break a plant. Mechanical parts, process and post commissioning audits are often required to ensure the plant runs according to the targets."

For its part, Metso Outotec sees itself as not only as a provider of screens and other equipment but also as a long-term partner. It offers all its customers optional Life Cycle Services. In addition to handling all commissioning and maintenance, Metso Outotec also aims to collaborate closely with partners in order to maximise productivity levels throughout the process and respond to evolving challenges and targets.

As part of this ongoing relationship with customers, Metso Outotec's R&D team is also working hard on developing solutions to common screening issues-including the one mentioned earlier about gearbox exciters.

"We have a design of an exciter, which we're actually going to call a mechanism, because there are no gears or roller bearings in it. What we're able to do is increase the force through that mechanism. We don't have one exciter sat on the top of the machine; we have two drive lines, so we're sharing the load into the side plates, which allows us to put more force through the screen body," Smith said.

"The biggest part that interests our partners is that there are no roller bearings to maintain. There are no oil changes on the screen, you don't have to have guys climbing over the machines for servicing, it's all done remotely. Those safety and maintenance issues all go away, that's probably our biggest (development). You'll be hearing lots of things about that very shortly."