February 2009

Grace under pressure

An innovative solution saves Xstrata Nickel from downtime troubles at its Strathcona mill

By J. Talbot

The pressure transmitter mounted on the pot head of the cyclopack monitors the performance of the hydrocyclones.

The last thing any resource industry operation can afford, especially in current market conditions, is process downtime. Unfortunately, the vast volumes of throughput and the speeds of processing seen at typical mills make the occasional glitch inevitable. This was the case at Xstrata’s Strathcona mill in Onaping, Ontario.

An abrasive problem

At Strathcona, nickel ore is ground first in a rod mill and then in a ball mill. Coarse ore is separated from fine material in a cyclopack — a circular arrangement of hydrocyclones. Underflow from the cyclopack recycles to the ball mill for further grinding while nickel ore fines in the overflow proceed to further processing.

The 70 per cent solid nickel ore slurry from the ball mill, fed into the bottom of the cyclopack, travels to the pot head, which distributes the material evenly into the hydrocyclones. The pressure at the top of the pot head is closely measured and monitored, because it is an indicator that things are running smoothly. Based on the pressure reading, operators turn individual hydrocyclones on or off. The number of working cyclones is adjusted to maintain optimum pressure, with more being deployed when the pressure rises and some being turned off when the pressure falls. Obviously, none of this optimization is possible if the pressure sensors don’t do their job.

But the pressure sensors, as their name suggests, are under a lot of pressure. “At a flow rate of 200 to 400 cubic metres per hour, the slurry is literally sandblasting this pressure measuring system,” said Brian Couling, electrical and instrumentation supervisor at Strathcona. The solids in the slurry range up to 12.5 millimetres in diameter, but most are about 1.1 millimetres across, enough to create a highly abrasive jet of material that does little good to the sensitive instrumentation. To protect the pressure sensors and extend their life, Xstrata used to cover the measuring diaphragm with a rubber pad. But this was not enough to avert one or two failures a year that caused thousands of dollars per hour in downtime losses.

A nano-scale solution

Recognizing Xstrata’s problems as an opportunity to push forward innovation, Ron Howarth, transmitter product manager for ABB Instrumentation in Canada, chose Strathcona as the testing ground for a new diaphragm coating material called Diaflex.
The coated diaphragms were specified for ABB’s 2600T series flange-mounted pressure transmitter. For the trial, Xstrata replaced the pressure sensor without adding a rubber protective pad for the sensing diaphragm. Instead, the diaphragm was coated with Diaflex — a new-generation material with superior anti-abrasion properties. A nanostructured material, Diaflex coating contains titanium and silicon of the nitrides class. Using a process called physical vapour deposition, ABB coats the sensor’s 316 stainless steel diaphragm to a thickness ranging from three to five micrometres.

Diaflex does its job even at such infinitesimal thinness because its hardness is similar to that of diamond. Despite its durability and rigidity, the coating retains a good spring rate value because of the way it is deposited on the diaphragm. This allows the diaphragm to flex as needed in the normal course of its duty cycle process temperatures ranging from 100 to 700 degrees Celsius. The result: a durable diaphragm that can withstand the harsh rigours of the cyclopack without compromising critical sensitivity and accuracy in pressure measurement.

This revolutionary ABB 264 pressure transmitter has been operating for over two years without the need for a rubber protective pad on the diaphragm. Impressed with its performance, Xstrata has converted all cyclopacks at Strathcona to ABB 264 pressure transmitters with Diaflex-coated diaphragms and is testing Diaflex-coated transmitter diaphragms in other mill applications.

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