Going for Gold: Linde's Extraction Expertise
How Linde’s oxygen-based solutions for gold extraction help increase a mine’s productivity through reduction in the use of harmful cyanide.
The notion of “striking gold” tends to conjure up images of unearthing solid lumps of the precious metal worth millions of dollars. While gold nuggets do exist, the vast majority of commercial gold occurs in tiny particles within rock, at low concentrations. The concentration is so low that it typically takes about 1 metric ton (1000kg) of ore - usually brown, iron-stained rock - to yield just a few grams of gold.
“There are two main parts of gold mining,” explains Randall Marx, commercialization expert in water-based technologies for the Americas at Linde. “The first is excavating the ore from the mine, whether underground or open-pit. The second is purification of the gold through ore-oxidation, leaching of the gold into water and gold recovery from the leach water.”
Following excavation, the ore is crushed to a particle size small enough to allow the gold to be dissolved by the extracting solution, which is known as the “lixiviant.” That’s when chemistry takes over. The gold is oxidized and dissolved as a salt. In this second, dissolution step, water, oxygen, and cyanide are added to the ore to transfer the gold from the solid phase to a water-based gold-cyanide complex. It’s at this stage that Linde’s dissolved gas management solution for gold extraction can really add value.
Incidentally, the COVID-19 pandemic saw margins for gold miners increase as the price rose by 28 percent from January 1 to August 14, 2020, alongside falling energy prices and currencies in some mining countries. “When price is high, production is high, because higher prices make gold mining viable in more locations” says Randall. “Our oxygen-based solution can not only help goldmining customers improve the financials of gold recovery but also reduce the use of a hazardous chemical, namely cyanide, in the process.”
Cyanide: Not All Glistens in Gold Extraction
Actually, oxygen has two roles in gold recovery. One is the oxidation of the gold itself to an ion that can form the water-soluble complex salt in the presence of cyanide. The other is the oxidation of other metal ions, such as those of iron and copper. Once oxidized, these metallic ions form insoluble salts, which cannot bind to cyanide. In other words, thorough oxidation of the ore reduces the overall requirement for cyanide - an expensive and hazardous chemical, which is subject to rigorous environmental oversight due to its toxicity. “There has always been a general awareness that oxygen could reduce the consumption of cyanide,” explains Randall, “but it was never approached as the focus of a suite of commercial water-based technologies.”
Oxygen is typically added to ore in a reactor such as an autoclave, which runs at high temperature and pressure in the presence of oxygen, and oxidizes most of the cyanide-consuming compounds in the ore. But Linde’s R&D team demonstrated, through laboratory and pilot studies with actual gold ore, that addition of oxygen and maintenance of high levels of dissolved oxygen in the atmospheric pressure water-based steps, could reduce the consumption of cyanide and thereby improve the profitability of the mine operations. “We realized that we could further reduce the cyanide usage by adding this polishing step,” says Randall, thus further reducing cost. But Linde’s solution doesn’t stop there.
Linde’s Gold Standard Engineering - and Expertise
“I would say what we can provide is an incremental but substantial improvement to traditional processing,” claims Randall. While the large infrastructure like tanks already exists in mines, there are small tweaks that can make a big difference to productivity when it comes to gold extraction.
With any gold-mining project, the first step is to evaluate the ore and current process in order to determine whether new oxygen technologies are likely to be beneficial. Linde’s experts then look at each step of the water-based “hydrometallurgy” process and design the best technology to introduce and optimize the oxygen levels - from optimal oxygen-dissolution equipment to technical consultation on where and how to introduce oxygen. “One thing we can do is introduce automated continuous monitoring systems to maintain optimal oxygen levels at all times, resulting in a more efficient process,” says Randall.
Although simple in their concept, the sort of advanced gas dissolution equipment that Linde can introduce to a processing facility requires a high level of engineering. Things like spargers and inline oxygenators for example must have very small holes through which the gas can pass and dissolve but also must be made of a material that won’t be worn down through abrasion with the ore. For this, special metal alloys and ceramic materials are used.
Oxygen for Pure Productivity
In one example gold-mining customer in the US, introducing oxygen led to a direct net savings in operating costs. “The introduction of our injection equipment as well as dissolved oxygen sensing and monitoring resulted not only in increased gold production but also a net saving of over 5.8 million USD over a 6-month period,” explains Randall. The savings was linked to reductions in cyanide usage. In addition, it was noted that the new spargers had minimal maintenance requirements.
Looking beyond goldmining, Randall sees promise in technology transfer into other mining industries: “In South America for example, where mining is an important part of the economy for several countries, we are looking at new ways of using gas-dissolution technologies to improve the recovery and reduce costs associated with production of other metals such as iron, copper and lithium”.
That’s not to say the applications within gold are exhausted. Linde’s dissolved gas management solution is not just important for improving productivity of gold and reducing the use of cyanide, but also for destroying cyanide after use - a detox process that also depends on oxidation.
“In goldmining, a small improvement can make a big difference,” says Randall.