Introduction
ASARCO LLC (Asarco) was organized in 1899 as the American Smelting And Refining Company. Originally a consolidation of several lead -silver smelting companies, it evolved over the years into an integrated producer of primary non-ferrous metals , including copper , lead and zinc and associated co -products. Grupo México, S.A.B. de C.V acquired Asarco in 1999. Today, Asarco is a vertically integrated producer of primary refined copper and associated co -products such as gold , silver , selenium and tellurium. Asarco operates three open-pit mines and mills, two leach -solvent extraction (SX)-electrowinning (EW) plants and a copper smelter in Arizona and a copper refinery in Texas.
The primary copper industry comprises extraction (mining), mineral processing (also referred to as beneficiation, which includes unit operations such as leaching , SX, EW and milling or concentration) and metallurgical processing (smelting and refining , which includes electrorefining ). This industry is capital, water and energy intensive with the potential for significant environmental impacts, if the environmental aspects are not properly managed. This paper introduces the concept of sustainable development , examines how it applies to the primary metals industry and illustrates the application of sustainable development and product stewardship considerations in copper hydrometallurgy , using examples of practices at Asarco.
How Metal Mining is Compatible with Sustainable Development
The classic Brundtland Commission definition of sustainable development is “development that meets the needs of the present without compromising the ability of future generations to meet their own needs” [1]. A report by the International Council on Mining and Metals (ICMM) discusses other related concepts such as ecoefficiency and product stewardship and their interrelation. “Product Stewardship ,” which is also referred to as “Material Stewardship,” is defined by ICMM as “an over-arching model required to ensure the optimal and appropriate production and use of minerals and metals in society” [2]. The two characteristics that distinguish mining from non-extractive industries are a finite mine life and the risk that exploration efforts do not result in a viable mining project.
Continued exploration concurrent with production, research and development (R&D), and implementation of new technologies can further the sustainability of the mining industry . Mining companies have robust exploration programs that add to the probable and proven resource base.
As demand for metals grows, so do proven and probable resources. This is due, in part, to the discovery of new resources through robust exploration programs undertaken by mining companies and, in part, due to advances in technology that allow the economic processing of lower grade ores and even wastes. Examples of technological advances in primary non-ferrous metals production include: froth flotation for the concentration of non-ferrous metal sulfide ores; Solvent Extraction -Electrowinning (SX-EW) for leaching of copper ores and cyanide heap leaching for gold ores.
Another important property of metals is their inherent recyclability. Many metals once produced are capable of being recycled, smelted time and again to their original elemental form and refined to demanding specifications. Furthermore, recycling can conserve energy since recycling processes are much less energy intensive than the primary metals production processes.
Mining is sustainable when it is conducted in a manner that balances economic, environmental and social considerations, i.e., by paying attention to the “triple bottom-line.” Sustainable mining practices are those that promote this balance. These practices begin with exploration, continue with mine development, mineral and metallurgical facility design, process development, through operations and until reclamation and mineral and metallurgical facility closure is completed and post-mining land uses are implemented.
Sustainable Development Applied to Copper Hydrometallurgy
In applying sustainability development and related concepts to copper hydrometallurgy , we need to examine opportunities for conservation of water, resource and energy , control effluents and properly manage wastes. In doing this, it is important to recognize some of the unique characteristics of mining. Most important among these is that the location of a mine is based on geological occurrence of the mineral and from which the metal of interest and associated co -products can be extracted profitably. It also means that the composition of an orebody, i.e., valuable minerals associated with the product and co -products, as well as impurity minerals , referred to as “gangue,” is dictated by location and geology.
The hydrometallurgical copper production route produces electrowon cathodes, which are 99.9% pure copper . Good examples of resource conservation practices include the use of acidic scrubber solutions from the Hayden smelter in the leaching operation at the Ray mine, for its water and acid values. Used lead anodes from electrowinning and lead flakes from the tankhouse operation are sent for lead recovery recovery to a lead recycler, who also generally supplies new anodes.
A major challenge in primary copper production is the requirement to produce high-purity refined copper (in the form of electrorefined cathodes which are 99.9% pure copper ) from feedstock containing a wide range of impurities that can adversely affect product quality. The most critical impurities are bismuth , arsenic and antimony which can cause grain boundary cracking in wire drawing [9]. Metal impurities present in copper ores including lead , bismuth , arsenic and antimony report to copper anodes. In the electrorefining process, these impurities dissolve in the electrolyte and need to be maintained at acceptable levels, so as not to adversely impact refined copper quality. Innovative processes for electrolyte impurity control at the Asarco’s Amarillo Copper (ACR) are discussed later in the paper.
Water Conservation
Renovation of the Pumping Station at Hayden and Installation of a New Booster Pump Station at Ray
The only source of fresh water for Asarco’s Ray Operations (Mine, Mill, Leach SX-EW) is the Hayden well fields located some 22 miles away and is delivered by an aging pipeline. The pumping station at Hayden originally consisted of two 2000 HP pumping groups, one as back-up, running at constant speed. In this system, water was delivered such that it was either on, operating at 350 psi, or off. This meant that when the system was on, Ray had to use this water limiting the amount of reclaim water that could be used. In addition, when the pumps were turned on to full speed, it caused water hammering in the aging pipeline leading to two pipeline breaks in 2012, which resulted in the washing of historic tailings into the Gila river and that had to be remediated.
Booster pumping station at Ray Mine
Besides saving water, there were fewer pipeline breaks because the pipeline could be operated at lower pressures due to the installation of the new booster pumping station at Ray. More importantly, the project resulted in a reduction in electricity usage estimated at 5,786,595 kWh annually at the time of installation. In addition to the rebate from Salt River Project (SRP) of $636,525, the annual savings at a cost of 0.0551 per kWh amounts to about $318,841.
Fresh water usage is influenced by various factors, mainly loss by evaporation and availability of reclaim water. A new 4 million-gallon fresh water storage tank was added in 2015 to provide 24 h of runtime in the event of a disruption in fresh water delivery. In addition, 14 miles of aging pipeline has been replaced. The renovated pumping system has significantly improved control of the water system leading to increased use of reclaimed water. It has also significantly reduced pipeline breaks and associated environmental concerns associated with a break.
Trends in fresh water consumption and power use
2012 | 2013 | 2014 | 2015 | 2016 | 2017 | |
|---|---|---|---|---|---|---|
Water delivered to ray (Million Gallons) | 1458.0 | 1683.0 | 1654.0 | 1538.4 | 1591.4 | 1687.0 |
Million kWh | 10.859 | 8.040 | 6.587 | 5.163 | 4.583 | 3.724 |
Implementation of Green Chemistry in Electrolyte Purification at the Amarillo Copper Refinery
Anastas and Eghbali [10] define green chemistry as “the design of chemical products and processes to reduce or eliminate the generation of hazardous substances.” In applying this construct to chemical systems, the authors state that “it is better to prevent waste than to treat or cleanup waste after it is formed,” and recommend that “the design of chemical reaction systems that do not require intensive energy use is highly desirable.”
Different technologies have been used at ACR to control impurities in the electrolyte , including removal of a bleed stream and controlled arsenic addition. The conventional practice at ACR was to continuously bleed certain amount of copper electrolyte from the tankhouse and treat it in liberator cells using insoluble lead anodes and copper starter sheets as cathodes in a two-step process. In the first step, copper is deposited at the cathode and can be sold or used to make rod or cake. In the second step, some of the arsenic , antimony and bismuth is co -deposited with the copper , as foul cathode and the remainder precipitated as a sludge called copper removal residue. Both these secondary materials are returned to the Hayden smelter for copper recovery . A disadvantage of this method is that some of these impurities are returned to ACR in the copper anodes and constitute a re-circulating load.
Molecular Recognition Technology
In 2009, ACR implemented Molecular Recognition Technology (MRT ) for controlling the bismuth levels in the electrolyte to make a saleable bismuth sulfate/bisulfate product [11, 12]. In MRT specially designed ligands such as organic chelating agents or macrocycles, chemically bonded by a tether to solid supports such as silica gel or polymers called SuperLig® are used to get highly selective metal separations. Such MRT products are packed into fixed-bed columns that, in commercial operation, are built in skid-mounted modular form, and are fully automated for continuous operation. Feed solution is passed through the column and the target metal is removed from the solution in a highly selective solid phase extraction (SPE) process.
Schematic flow diagram of MRT process
In the actual operation part of the eluent, ~2.2 BV, is sent to the Bi precipitation tank and the remainder is recycled back into the eluent tank. After precipitation is triggered by dropping the temperature below 42°C, the product is sent to a filter press to remove the bismuth sulfate product. On a periodic basis, a 6 M HCl wash is performed to remove Pb and Sb that have built up on the SuperLig® 83 product. Pb and Sb cannot be removed effectively with 9 M H2SO4.
The MRT process has proven to be a successful method for controlling Bi levels in the electrolyte at ACR. Main benefits of the process include: (1) The SuperLig® 83 product has high selectivity for Bi and has a high capacity. (2) There is no pick up or loss of Cu during the loading phase. (3) Due to rapid reaction kinetics, very high feed flow rates are possible for Bi loading. (4) The Bi product in the eluent solution is very concentrated and readily precipitates to form a high purity bismuth bisulfate precipitate that can be collected by filtration . (5) The bismuth bisulfate has a significant market value that makes a sizeable contribution to reduction of operating costs of impurity control . (6) Recovery of Bi for sale avoids its wasteful discharge into the environment . (7) The number of BV used to remove Bi from SuperLig® 83 is low making it possible to have a smaller purification plant than would be the case when using other methods for Bi removal .
Acid Purification Technology
In 2012, ACR installed an acid purification unit (APU) technology to recover copper and nickel in the electrolyte and recycle acid and sulfuric acid to the tankhouse.
The use of liberator cells is a common practice at copper refineries to control metal impurities in the electrolyte and to recover the remaining copper coming from different acidic streams (anodic Cu slimes leach liquor , Cu rod rinse). During this electrowinning process, a decopperized electrolyte is produced and if the concentration of some metals (mainly tellurium) is high enough that is not suitable to be recycle back to the tankhouse, it needs to be discarded as a waste. Some refineries can send it to the heap leach if a mine site is close enough and the cost for transportation is reasonable, but by doing this, valuable metal (nickel , tellurium and selenium ) are never recovered.
In order to recover these metals together with sulfuric acid , ACR installed an APU, where after the electrowinning process, the decopperized electrolyte solution is sent to this APU®, where sulfuric acid and arsenic are absorbed into the resin and then desorbed using water which is then returned to the tankhouse to be reused as acid make up and to increase arsenic concentration in the electrolyte , and a byproduct aqueous stream, high in nickel , is generated, that can be further processed to produce a nickel carbonate co -product. Adequate arsenic levels are necessary for controlling the levels of antimony and bismuth in the electrolyte .
There are two steps in the APU process, the upstroke and the down-stroke. During the upstroke, filtered decopperized electrolyte is pumped into the bottom of the resin bed. Acid is adsorbed by the resin particles and the remaining de-acidified metallic salt solution, called byproduct stream, is collected from the top of the bed and sent to a holding tank. Next, during the down-stroke, water is pumped into the top of the bed, desorbing the purified acid plus the arsenic from the resin so that a purified acid product is collected from the bottom of the bed and send to a holding tank for return to the tankhouse.
This process eliminates the production and disposal of black acid and the use of evaporators with their associated high energy consumption. More than 50% of the arsenic is returned with the recovered sulfuric acid , making it feasible to modify the operating conditions on the final liberation stage to increase the concentrations of Cu and As remaining in solution and to minimize the amount of As leaving the plant, as a sludge to be recycled back to the smelter.
Material/Product Stewardship at the Amarillo Copper Refinery
Replacement of Wooden Pallets with Plastic Pallets for Copper Rod
Plastic pallet for copper rod at ACR
Copper rod on plastic pallet at ACR
Another notable consideration is that plastic materials used for the pallets are typically not recyclable. For most plastics to be recycled, they need to be heated to a molten state and then injected into a mold that is then quickly cooled to solidify the plastic. When plastic is heated to a molten state, it can release toxic gases into the workplace, requiring engineering controls or the use of personal protective equipment. In addition, every time plastic is heated up, it loses some of its properties, reducing its useful life.
However, the pallet supplier for ACR can make the plastic bond together by compressing it at high pressure, with no additional heat other than that generated by friction in the process. Since the pallets are not heated up as in traditional plastic production processes, they can be used multiple times without appreciable degradation of their properties.
Conclusions
This paper illustrates how sustainable development and product stewardship considerations can be applied to copper hydrometallurgy , using examples from operations at Asarco. It is hoped that the variety of examples provided can serve as a roadmap for application of these concepts to the primary metals industry.