Producers can purify these metals by chemically treating impure cast metal, or dissolved ores, in the processes of electrorefining and electrowinning. Basically, they take a tank full of hot acid, throw in an electrified metal plate or two, and wait for the metal to form 99% pure deposits. In the case of electrowinning, metal molecules from ore dissolved in the acid deposit on a single cathode. In the case of electrorefinining, metal molecules migrate from an anode, sometimes an impure metal ingot, to deposit on a cathode. After a while, the plates fill up, and must be swapped out with fresh replacements so the metal can be harvested.
Producers tend to run between 400 to 1,200 tanks in a tank house. Typically the process takes seven days. If each tank requires swapping out a couple of racks of plates per week – then in the biggest tank houses there would need to be 4,800 anode movements per week, or 43 per hour on a two-shift, seven days a week schedule.
This demanding job requires heavy-duty cranes that can keep up with the schedule every day and every week. The most important part of the crane is the below-the-hook attachment, called bails, which extract a rack of the full 150kg cathodes (which can be two-thirds deposited metal by weight) from the tanks. The bail must be electrically isolated from the rest of the crane to prevent a short-circuit between electrified bath and earth ground that touches the metal structure of the building. The bails must also be capable of fine positioning within a few millimetres – fractions of an inch – over the cathode hooks.
There are three ways that these cranes are controlled. The crane may be driven only by an operator, only by a computer (fully automated), or have a mixture of the two types of controls.
The manually-operated cranes require an operator and a spotter, according to Jeff Breitrick, manager of international sales and also the Chile operations of P&H brand licensee Morris Material Handling. “The second guy tells the operator where to position the crane. It’s a tight fit,” Breitrick says.
Gert Coppens, marketing manager at Belgium-based Fémont, describes how the bail works. “The cranes carry a beam with a double row of driven and turnable hooks. Once the crane has positioned itself above the cells, the crane bail is lowered until the hooks are in the right place to lock up the lugs. With the hooks closed, the crane bail loaded with a complete series of copper plates (or blanks) is lifted up and the crane is ready to transport its load.”
Breitrick says that typical tank-house bridge cranes have trouble keeping track of their position in long and cross travel, which means that they cannot be relied on to return to exactly the same place as they went to 100 cycles before. Float in the wheels, misalignments in the runway or supporting structures, or skew of the crane bridge on the runway can all affect the precision of positioning, he says. “If you go very slowly, you can get a precision of half an inch. The faster you go, the greater the accelerations and decelerations and the greater the error,” he says. Minor deflections of the bridge can also throw off positioning accuracy, says Eero Tuuppa, tankhouse technology director at Outokumpu, so the crane structure needs to be of a higher spec than its rated load might suggest.
These eccentricies of old manually-controlled cranes can limit their potential to upgrade to full automation. On the other hand, there is a lot of room for improvement. According to Walter Leiler, project manager of crane builder Hans Kuenz, a maximum of only 80 cranes are automated.
That leaves 400 or so that are manually-operated, according to a rough estimate from Outokumpu’s Tuuppa. Worldwide, there are not more than 200 electrorefining and electrowinning plants, Tuuppa says. Walter Leiler, project manager of crane builder Hans Kuenz estimates there are between 200 and 250. If each operates on average 2.5 cranes, the global installed base is about 500 cranes, according to Tuuppa, although local manufacturers may supply more. Coppens at Fémont estimates the annual market of new-build tank-house cranes to be about 80.
As the price of base metals rises, new tank-houses are planned and customers contemplate improving or upgrading existing cranes. Although the need for steel appears to have faded, the prices of copper and zinc have risen over the past year. The copper market is also driven by demand for other metals. “It’s not a big secret that the precious metals (silver, gold) and other remains (zinc) in the sludge – the liquid remaining after production – play a very important role in the motivation to win copper, although the amounts are not that big,” Coppens says.
The allure of high-tech controls
Although there are not many tank-house cranes, each is worth 10 industrial cranes. Even a new manually-controlled crane could be worth half a million dollars (Euro 410,000). Automatic cranes cost more; double that, according to Breitrick. Coppens disagrees. He says that fully-automated cranes cost less than $1m, based on the new-build cranes it recently supplied to the Radomiro Mine in Chuquicamata, Chile and Noranda’s CCR copper refinery in Montreal, Canada.
Tank-house cranes are made automatic to speed them up: computers may be able to calculate the most efficient paths around a tank house than a human operator could. This is one of the specialties of newcomer Outokumpu. Unlike the other three, Outokumpu does not build cranes, but buys complete cranes from Finland’s Demag distributor Algol Oy, or from another suppier or two, and then builds the control system and the grab and integrates it with the crane. Outokumpu also supplies other sorts of tankhouse equipment.
The computer controls of the most basic semi-automated tank-house cranes, Breitrick says, used to only take care of the big movements across the tank house – shifting the crane to the next cell to be worked, for example. “Typically you only needed the operator for final positioning,” he says, though he adds that the technology has moved on since then. “Often the configured grab will have a link to the tanks and locks into the side of the tank,” Breitrick says. Then, having established an absolute position, it either feels its way – with a mechanically-actuated pin – or sees its way with an optical sensor.
Hans Kuenz’s cranes have a travel position precision of 15mm (9/16 in) that relies on a position encoder, according to Walter Leiler, project manager at Hans Kuenz. When the crane and the trolley are roughly positioned, the hoist lowers a frame with guide bar to a position between two pyramids mounted on each cell. As the guide frame lowers, the pyramids push on the bar and force the frame into its final position, correct within 2mm (1/16 in).
A few rungs up the ladder of automation further removes the operator from direct control of the crane, according to Tuuppa at Outokumpu. Crane operators working in the cabin of a semi-automatic tankhouse crane do not drive each movement of the crane, but rather oversee the entire process. Instead of manoeuvring the crane to the next position, they would approve the computer’s next move by pressing a button, and the crane would perform the necessary steps to get there on its own. The operator selects the next destination from a screen, or follows a pre-programmed workflow. Tuuppa says that with this kind of crane, the operators manage the entire extraction process.
“To go fully automatic is really one step further, more challenging, when there is no back-up of an operator, or no continuous backup,” Tuupa says.
But its rewards are attractive as well, Coppens says. “When it comes to efficiency, nothing compares to an automated crane. A 2mm precision standard can hardly be obtained by a crane operator, but it can be by means of an automated solution with cameras, automatic positioning and automatic adjustment routines.”
Sparing the workers
Automated cranes also help remove the need for people to work inside the tank-house. Hundreds of tubs of hot bubbling electrified acid create a burning corrosive mist inside the tank-house. “My lungs hurt as I came out of there,” Breitrick of Morris says. Automated cranes reduce the need for people to work in there.
Some tank-house operators have begun installing hoods or covers on top of the tanks. Tuuppa of Outokumpu says that the European Union now requires that tanks in new electrowinning plants have hoods.
Although automated cranes need fewer people inside the tank-house, they probably need more people outside of it, for technical support. A computer-controlled crane with a broken computer cannot work.
The problems of maintenance can be compounded by distance. Although electrorefining often takes place in industrial areas, where crane technicians may be available, electrowinning operations are often set up near the primary mine, Tuuppa says, which is likely to be in the middle of nowhere.
Different crane builders deal with this problem in different ways. Tuuppa says that Outokumpu relies on the international service networks of its crane builder suppliers for service. Morris Material Handling, for example, sells many of its tank-house cranes out of its Santiago, Chile-based operation, which employs 60 people. Hans Kuenz can install sensors that report back to headquarters. Coppens argues that automated cranes actually make maintenance easier. “When the tank-house runs automatically, down-time of cranes and carriers can be foreseen and maintenance schedules can be adopted.”
Whatever sort of crane is working, mostly, it is the operators – rather than the crane builders – who take care of the cranes. Part of maintenance is simply cleaning. Over the course of a week, the acid mist can form a thick solid film on the cranes. “An optimal operator would do some checking every day, and wash critical parts every day. Not all plant operators are optimal,” Tuupa says.
