Going for gold

3 October 2016


Changes in gold mining processes have resulted in tower cranes being introduced in various projects around the world, with manufacturers seeing business potential.

Deliveries of new and used cranes to mining companies, primarily ones involved in gold mining, have increased over the last decade.

“Seven years ago we started to offer more tower cranes in the mining industry. Historically mining developers didn’t use tower cranes, they used other lifting solutions, but now we developed some of the biggest projects in Chile and Peru and the tower crane is getting more and more popular,” says Henning Koehler, head of tower cranes for Liebherr Chile.

As orders rise, crane manufacturers have started to invest resources in this sector, with Liebherr’s mining equipment division in Chile currently having 430 members of staff.

The first project that the group supplied to was the Los Pelambres mine in the Coquimbo region of northern Chile where a 550 ECH 20 Fr.tronic crane with a 3.5t capacity at 81.5m reach assisted with construction and now carries out maintenance at the open-pit copper mine.

Koehler explains the reasons behind the use of a tower crane in this project, “The mining developer looked at overall costs for 20 years and the tower crane was much more economical than other solutions, it didn’t need that much space or any fuel, it was low maintenance and these were the four things that convinced the customer to go with the tower crane back in 2008.”

Julio Nazer, commercial manager at Chilean crane dealer and advisory firm ETAC, says developers have come to realise the production and financial benefits from deploying a crane.

“In the confined environment of a mine, tower cranes are more suitable because of higher jib reach and limited interference with the production processes.”

ETAC, which supplies Manitowoc’s Potain crane range in South America, has also witnessed a surge in demand for this equipment for mining activities.

Nazer says the most popular cranes in mining are the 10t capacity models, but there is also high demand for models up to 40t. “Generally the tower cranes need a jib radius between 60m and 75m with a weight capacity of 2t at the tip.”

The developers decide whether to deploy new or used tower cranes depending on a mine’s reserves. Quentin Van Breda, managing director of construction equipment supplier SA French, part of Torre Industries, says that used cranes are more common in this sector.

“If there are 25 years of reserves on the mine they buy brand new. If they see older mines with only 5-6 years of reserves the preference is to buy used cranes for the application,” he explains. The low duty cycles in mining ensure that cranes have longevity.

Van Breda says: “On mines in South Africa we have supplied fully refurbished cranes that are 18-20 years old to put into the mineral processing plants and that has worked successfully for 8-10 years but in most cases the required life span was 5 years. It is because the duty cycle is so low that the crane lasts a lot longer.”

In order to accommodate for harsh weather conditions, manufacturers offer some additional features to improve user experience and productivity.

Koehler describes the conditions on some of these sites, “In Antapaccay in Peru or Sierra Gorda in Chile they are working at up to 3000m above sea level with harsh conditions and earthquakes, hot during the day, cold at night. Apart from the dusty environment, sometimes there are high wind speeds.”

He says that Liebherr’s latest Litotronic cranes have air conditioning units on the electrical cupboard to maintain temperature overnight. On some occasions the firm might also supply parts with salinity protection if mines have high water tables or operate in sea water.

Applications

Van Breda says that 90% of tower cranes used in mining are used in gold mining, where they are used for a number of applications.“I see two tower cranes used in a processing plant. One would be used in the ball mill, where the semi-crushed rock enters the plant and the crusher is charged with ground steel cast balls and they crush the rock into a powder.

Depending on the geological properties of the rock depends how long they have to mill, but new balls have to be put in usually once a day and a tower crane would be utilised to do that,” he says.

Typical loads for the ball mill are 1-3t depending on the size of the plant and the crane will sit very close to the ball mill itself, but a long reach of 45-50m may be required to bring the balls into the mill. “The height of a typical ball mill and the height of the tower crane are in the order of 30- 40m,” adds van Breda.

The second area where tower cranes are used is in the ‘carbon in leaching’ (CIL) process, where a series of chemical reactions are used to separate the gold from the source rock. In this process a number of flotation tanks is required.

Van Breda says: “Pulverised material from the ball mill gets sent into a tank mixed with water and various chemicals. The number of settling tanks depends on how much gold you have but every one of them has a stirrer. On this stirrer there is a screen and the solids and pulverised rock go to the bottom and the gold attaches itself to the screen which is removed once a day. The screen is not heavy, it is 2-3t. Most importantly, the crane in the CIL tank farm is also used for maintenance.”

He explains that the motor and gearbox driving the screens can be anything from 4t to 8t and have to be regularly lifted out and maintained. In this instance the cranes are not particularly high, perhaps 4m or 5m tall and the motor gearbox unit may add another 3m to the required lift height.

On the processing side Liebherr’s Koehler says there is a trend for moving to larger tower cranes. “In 2006 the 550 was the biggest crane in the market and now we have up to 1260HC which is the strongest tower crane in the country. Initially there were perhaps rows of 4 by 4 tanks but now we see as many as 12 by 12, so the tendency is for bigger cranes with a lot more range and a larger tip loads.” In some cases this might mean the use of two or even three tower cranes along the processing line.

Paul Wheeler is a senior lecturer at the world-famous Camborne School of Mines, part of University of Exeter, and former mining geologist for gold mines and base metals. Wheeler says a general trend for lower grades of gold ore means that additional processes need to be undertaken to maintain supply levels.

“In the industry as a whole we are seeing lower grades of gold ore going into the processing plant so we need to do more with the ore to get the same amount of gold, and that means more tanks,” he explains.

In fact the gold mining process is determined by the nature of the gold present in the rock. This can either be free milling, which is easier to remove, or refractory which is bonded more tightly.

“Free milling means that when you crush that rock it is more immediately amenable to the cyanidation process where the carbon in leach (CIL) or carbon in pulp (CIP) processes are used,” says Wheeler. “Refractory gold means that the gold is locked normally between other sulphide grains and needs more crushing and potentially re-treatment before you can use cyanide.”

In either case the first step is to crush and grind the rock because the grains of gold contained within are at a parts per million (ppm) level. “The grains are so small you are talking microns, which means that we have to break the rock apart for any process to work,” says Wheeler. Construction and then maintenance of the crushers is one of the main operations for tower cranes on-site. They are used to replace crushing tools and insert the steel balls that are often used to accelerate the pulverisation process.

In some cases the crushing alone is enough to then send the pulverised rock into the CIL or CIP process where chemical reactions using cyanide and then carbon remove the gold. But in other cases more effort is required to make the mineral more amenable to removal. “There may need to be some sort of flotation technology. With flotation you are effectively getting the crushed rock in slurry and putting it in to big tanks and bubble air through and make particles that are either hydrophobic or hydrophilic. The (hydrophobic) sulphide particles containing the gold stick to the air bubbles creating a surface froth which you skim off known as the flotation concentrate.”

For free milling gold this can then go ahead into the CIL or CIP plant, but if it is refractory gold it may need to be broken down yet further using biological oxidation or pressure oxidation, processes which also need large tanks that must also be maintained.

The CIL or CIP process then carries out the physical removal and separation of gold from the ore. “The chemistry of the process is that gold has great affinity with cyanide, so if you add this gold with large surface area (after crushing) to react with the cyanide solution the gold will then form a solution as a cyanide complex. What you can then do is add carbon into this mix and the carbon has an even greater affinity with gold which gets loaded on to the carbon from the solution.”

For CIL the cyanide and carbon are added simultaneously, while for CIP cyanide is added first and then carbon. In both processes the final outcome is a substance called loaded carbon which is then taken to an elution plant where an acid wash strips the gold from the carbon after which the smelting process can take place. “It is a big industrial chemical engineering process as we are talking about very large volumes of rock to get small quantities of gold.”

Such industrial processes mean that there is a lot of pipework surrounding the equipment and it is another reason that tower cranes are finding favour when it comes to maintaining the multiple tanks, their motors and other equipment. Mobile cranes may not be able to get close enough to the equipment in some cases and their larger footprint can be a disadvantage.

Tower cranes on these sites may not be in constant operation but their reliability is crucial. “Although it is low duty in mining, it is critical that you are on standby 24/7. It is highly critical to the mine that the crane in the ball mill or the tank farm remains operational. Older cranes particularly have to be extremely well refurbished because they can’t break down. If you supply a crane to the mining industry we are on standby 24/7,” says Van Breda. This usually means that tower cranes are purchased with a service maintenance contract ranging from 2-5 years.

ETAC’s Nazer also points to the “indispensible” nature of tower cranes on site. “Maintenance services to tower cranes are scheduled monthly, where our experts check mechanisms and the complete structure according to its usage,” he says. Effects of commodity price fluctuations

In terms of market dynamics, low commodities prices have impacted demand for tower cranes, with some markets more affected than others. “Demand was impacted because there were many projects where tenders have been put on hold particularly in Chile. We expect to see the situation improve in 2017. It slowed in 2015 and 2016 we are still in contact with many mine developers but we know that projects won’t happen this year, but they are preparing for better times and preparing the mines,” says Koehler.

Van Breda says that South African demand has remained stable but that demand drops tend to be a result of political issues rather than commodity prices.

“Gold mining is fairly buoyant, probably less so in South Africa than it should be. But then the situation is very unique in that it tends to be tied more to new labour legislation which is always demanding unrealistic pay for mine workers. Mines can’t afford to pay the wages that the unions are demanding so more and more mines are mechanising but it is not as attractive as West Africa or places like Ghana.”

As a result tenders bid by the firm recently have been outside of South Africa. “Out of the ten tenders that we have out in the mining industry one is in South Africa and nine are West Africa or North Africa and the furthest we have quoted is Kazakhstan.”

A 10t capacity Potain MC235B on site in northern Chile
Liebherr mining projects for tower cranes in South America
A 10t capacity Potain MC235B on site in northern Chile
A Liebherr 1000EC-H at the Centinela Mine Chile with capacity of 18.2t at 60m radius, 40m hook height
Three Liebherr 1250HC machines at the Sierra Gorda mine in Chile
Liebherr’s 630 EC-H 40 Litronic, which can lift 5.8t at 80m, on site at Las Bambas mine in Peru