Ideally, the feed material entering the comminution process in a mine consists of nothing but ore. In real life, it also contains uncrushables known as tramp iron– teeth and bolts from loader buckets, drill bits and mill balls from ball mills.

The amount of uncrushables can be minimized by careful plant maintenance and operating practices. They can also be detected and separated, but sometimes this is difficult due to magnetic ore or specific feed arrangements.

What happens when tramp iron enters a crushing process? The crushing chamber of a cone crusher in a secondary, tertiary, quaternary or pebble application is subjected to a series of blows translating into potentially damaging forces. This results in hydraulic pressure peaks unless a system is in place to temporarily increase the crusher setting to release the pressure and let the tramp iron out.

The unwanted impacts can cause structural damage, such as fatigue failure, bearing seizures, damaged drive components and cracks in manganese liners. Beyond the cost of new liners and spare parts, even higher financial impact may result from lost production due to unplanned crusher downtime.

Mine operators are continuously challenging the availability rates targeted for mining crushers, and original equipment manufacturers are driven to find new and innovative ways to increase these rates.There is little leeway for unscheduled maintenance caused by tramp events.

The heart of Sandvik cone crushers is the Hydroset mainshaft support and positioning system. It consists of a large hydraulic piston inside a cylinder pressing the mainshaft with mantle against the concave in the topshell. The system allows automatic setting regulation during full load simply by raising or lowering the mainshaft as hydraulic oil is pumped in or out of the Hydroset cylinder. The pulsating operating pressure is monitored in Sandvik CH/CS crushers for automatic regulation of the crusher setting by means of the computerized Automatic Setting Regulation (ASRi) system. This unique Sandvik control system is in use worldwide in thousands of units.

The Hydroset concept provides additional overload protection by allowing the passage of most uncrushables through the crusher. The electric dump valve reduces damaging pressure peaks by up to 90 percent, as has been validated by extensive mine trials. Additional key benefits include safer working conditions and process improvements.

In the competing, so-called spiderless crusher concept, a bowl (i.e., topshell) with a concave interior is screwed up or down on a large thread to regulate the crusher setting against the mainshaft and mantle. The tramp release function is achieved by allowing uncrushables to lift the heavy bowl as the pressure triggers the precharged accumulators connected to hydraulic cylinders around it. This does not allow setting regulation under load, and tramp release requires lifting the large bowl. High-pressure impacts cannot be avoided. The combination of the Sandvik electric dump valve, the Hydroset system and the ASRi offers a clear advantage over these types of crushers.

Until now, tramp iron protection was realized in Sandvik CH crushers by a mechanical dump valve located in the bottom of the Hydroset cylinder. While effective as such, the pressure peak reduction is suboptimal due to its relatively slow, stepwise opening sequence. Difficult access for maintenance is another downside.

The operating sequence of the electric dump valve is triggered when the operating pressure of the hydraulic system exceeds the set maximum value. An electric pilot valve opens by a signal from the control system and is kept open until the tramp iron has passed, opening up the main valve and consequently the dump line to the oil tank, which makes the mainshaft sink and rapidly increase the closed side setting (CSS).

The high sampling rate (200 times per second) of the programmable logic controller (PLC) control system regulating the electric dump valve enables immediate response to the first pressure peak and enables control of the opening time of the pilot valve. The mainshaft descends quickly to the maximum CSS in order to allow tramp release and prevent further pressure peaks. The mechanical dump valve with spring-loaded pilot valve responds only gradually, opening up the setting step by step after several blows inside the crushing chamber. This results in six to 10 pressure peaks before the exit of uncrushables.

The PLC control system also notifies the operator to take action to remove the uncrushables from the process stream. Tramp iron removal can be automated by transmitting the signal directly to the mine automation system.

The ASRi system automatically repositions the mainshaft after tramp removal. An event log of all the control operations is also created, making possible both preventive measures upstream and corrective measures downstream. The PLC control system is designed to work independently as well as in parallel to the ASR+ and ASRi crusher automation systems.

In previous crusher models, the dump valve was located under the crusher. Entering confined spaces should be avoided at all times in mines, where fast exit is imperative in case of an emergency. Furthermore, mechanics had to work overhead, vulnerable to any falling debris or oil spills. The new side location provides top-serviceability in both new and retrofitted crushers, and it also eliminates standstills of complete crusher lines discharging material onto a single conveyor due to a service stop under one of these crushers.

It should be noted, however, that not even the new electric dump valve is a cure-all to any tramp iron problem. Oversized uncrushables that are larger than the maximum CSS will most likely cause blockages, and it is always advisable to screen the crusher feed with a metal detector, magnet or other removal system. Relying on the electric dump valve only could be compared with reckless driving in slippery conditions with total reliance on antilock brakes.