© The Minerals, Metals & Materials Society 2018
Boyd R. Davis, Michael S. Moats, Shijie Wang, Dean Gregurek, Joël Kapusta, Thomas P. Battle, Mark E. Schlesinger, Gerardo Raul Alvear Flores, Evgueni Jak, Graeme Goodall, Michael L. Free, Edouard Asselin, Alexandre Chagnes, David Dreisinger, Matthew Jeffrey, Jaeheon Lee, Graeme Miller, Jochen Petersen, Virginia S. T. Ciminelli, Qian Xu, Ronald Molnar, Jeff Adams, Wenying Liu, Niels Verbaan, John Goode, Ian M. London, Gisele Azimi, Alex Forstner, Ronel Kappes and Tarun Bhambhani (eds.)Extraction 2018The Minerals, Metals & Materials Serieshttps://doi.org/10.1007/978-3-319-95022-8_96

Minimizing the Hydro in Hydrometallurgy

G. T. Lapidus1  
(1)
Departamento de Ingeniería de Procesos e Hidráulica, Universidad Autónoma Metropolitana-Iztapalapa, San Rafael Atlixco no. 186, Col. Vicentina. México, 09340 Mexico, D.F, Mexico
 
 
G. T. Lapidus
Email: gtll@xanum.uam.mx

Abstract

One of Society’s major concerns with hydrometallurgical processing is the large amount of water that is quintessential to its nature. This is especially troubling in arid areas, where the population struggles to procure potable water for their daily needs. For that reason, it has become our responsibility to conceive, design and implement processing schemes that not only use as little water as possible, but recycle streams within the plant itself and minimize waste that exits its perimeter. With this in mind, the conference will center on the importance of selecting the appropriate chemical systems and integrating material balances to avoid reagent overkill and to detect impurity accumulation. The three “R’s” (reuse, recycle and recovery ) of aqueous solutions within the processes are emphasized. Examples of processes for the recovery of lead , copper , precious and PG metals from minerals and waste materials will be presented.

Keywords

Hydrometallurgical processesSolution recyclingWater minimization

Introduction

In the XXI century, fresh water has become one of the most scarce and treasured resources on Earth. Many industrial processes have been demonized, and rightly so, because of their excessive expenditure or contamination of aqueous sources. It is no wonder that the mining and recycling industries have been the target of numerous protests by the general population. In the past, hydrometallurgical processing schemes many times have not taken into consideration the economic and social implications of fresh water scarcity or the use of alternative non-potable water. Mining operations in arid regions, such as in the Atacama Desert in Chile , have historically suffered this problem. For that reason, traditional hydrometallurgical alternatives to smelting have been losing ground in recent years.

Therefore, new and innovative hydrometallurgy requires holistic approaches, where solution reuse or recycle within the process is prioritized. The selection of the appropriate chemistry is paramount; this implies that appreciable pH alterations and significant reagent additions in the recovery stages should not be considered. Furthermore, the proposed strategy will probably lead to minimal reagent expenditures and waste stream generation. Additionally, the utilization of other aqueous “resources”, such as treated or seawater, should be contemplated, when possible. These measures will take the pressure off the potable water system and should not disturb the normal activities of the neighboring populations and ecosystems.

In the following sections, the basic process characteristics and methods for decreasing water consumption will be proposed. Several integrated processes, investigated by the Hydrometallurgy research group at the Universidad Autónoma Metropolitana (HG-UAM), will be presented and discussed. These are only examples that illustrate the principles that support these designs.

Basic Aspects

There are specific techniques that can be employed in the design of any given hydrometallurgical process to decrease fresh water usage. The most obvious is to increase the solid to liquid ratio in the leaching stage, which not only saves water and reagents, but also raises the metallic ion concentration for a more efficient recovery . However, in many circumstances, solubility limitations may restrict its application. Yet, even when this measure is possible, others may be combined to enhance resource economics.

Solution Re-use, Recycle or Recovery (RRR)

This is the most important principal, when correctly applied, to save reagents, as well as water. The first case implies that the solutions are relatively “clean”, once the desired metallic values have been removed, and are in a state relatively close to that of the fresh solution. This condition can be met when the recovery methods do not significantly modify the leaching solution; therefore, the pH, reagent concentrations and impurity levels remain basically the same. Furthermore, when economically feasible, oxidizing and reducing agents that do not leave contaminating products should be employed (ozone, peroxide, hydrazine, electrical current). For metal recovery , solvent extraction and electrodeposition should be preferred over precipitation .

When solution re-use is not possible, solution recycle can be considered; this involves a purification or restitution step, after the chosen metal ions are removed and before recirculation back to the leaching stage. Many times, solvent extraction or electrorecovery/purification to lower the impurity level may be advantageous. In some cases, raw material pretreatments may be sufficient to minimize the problem of impurity build-up, opening opportunities for solution recycling .

If neither of the above techniques are plausible, water and other reagents can be recovered or concentrated. Distillation has been used in the past, but is highly energy intensive, so alternative methods should be considered. Operations that involve the use of membranes, such as reverse osmosis [1], and ion exchange resins are becoming more cost-effective. The former is especially applicable for wash waters with low levels of total dissolved solids (TDS), while the later can be used to remove highly charged ions, such as phosphates. When the solution value is high (reagents and water), the economy of the entire process may depend on its degree of recoverability.

Reagent Minimization

The basic principal employed is “less garbage in means less garbage out”, which can be translated as less fresh reagent feed promotes a decrease in waste generation. This is a direct consequence of solution reuse or recycle but can be used in synergy with more selective leaching and recovery . Increased selectivity in the use of reagents (oxidants, reducing agents and complexing ligands) should lead to fewer secondary reactions and their resultant products, which must be removed from the aqueous circuit.

In some cases, chemical oxidants and reductants can be replaced by applied potential, which can be more closely controlled. In the case of leaching , this concept may be applied to semi-conducting or conducting materials or through the use of a charge-carrying species as mediators, such as ferrous or ferric ions.

Integral Process Conceptualization

Finally, to optimize any process, the proposed integrated processing scheme should be analyzed. It is necessary to map out the trajectory and fate of each chemical species, including water, throughout the entire flow diagram; detailed material balances are mandatory. In this manner, impurity build-up can be anticipated and resolved either by purging or preferably by auxiliary intermediate or pretreatment stages.

Different Process Proposals

Lead

Lead is globally the most recycled metal [2]. Lead from primary and secondary sources is processed by large-scale smelting operations. However, as times goes on, the environmental restrictions for smelting are becoming more stringent and the concern about the its carbon footprint is growing. Furthermore, as the ore deposit quality decreases, less selective high temperature processes are losing applicability. For that reason, several hydrometallurgical alternatives have been considered as possible replacements for processing galena (PbS) concentrates and for recycling used lead -acid batteries (ULAB).

The process proposed by the HG-UAM is based on citrate chemistry at pH 6–7, where lead is selectively leached (Fig. 1) [3]. The material (PbS-containing ore or ULAB) is leached, first with a clean reductant (ULAB), followed by a clean oxidant. After the solid-liquid separation , the solution is electrowon to produce metallic lead and then recycled. If sulfate ions are present in the pregnant leaching solution, one of two alternatives can be implemented: (1) purge can be taken to decrease the its concentration (PbS) or (2) a previous desulfurization stage can be introduced (ULAB).
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Fig. 1

Process flow diagram for the production of lead metal from galena (PbS) concentrates or used lead acid batteries (ULAB)

The advantages that this scheme over others, that are under consideration, is that it operates at room temperature , does not expel toxic fumes and does not require a membrane electrolytic cell to separate the anodic and cathodic compartments. Furthermore, since the citrate ion is a natural buffer, no pH control is required.

Copper

Copper is obtained mainly from flotation and smelting operations, since over 70% of copper ores are composed of refractory minerals , such as chalcopyrite and enargite . However, over time, liberation sizes have been diminishing to a degree where mineral phase separation is not economically feasible and other metal values are being lost to slag . For that reason, for the last half century, there has been a constant drive to find a hydrometallurgical route that is competitive with smelting . Many, many processes based on leaching have been proposed; however, for most, temperatures near or above the normal boiling point of the solution are necessary due to passivation phenomena. At those conditions, leaching is relatively non-selective, a situation which leads to excess oxidant utilization and impurity build-up.

On the other hand, two other routes have been explored in the HG-UAM: reductive leaching for chalcopyrite concentrates [4–6] and oxidative leaching in mixed solvents [7] (Figs. 2). In both cases, chalcopyrite transformation is selective and copper leaching is carried out at ambient temperature . However, the problem of iron or arsenic removal is a constant. For the reductive process (Fig. 2a), the iron must be eliminated from the acidic solution (pH ~ 1.8); this still posed a challenge for recycling the leaching solution. Despite this drawback, subsequent copper oxidation and recovery are straightforward. For the oxidative route (Fig. 2b), the iron problem subsists, although the pH is slightly higher, favoring its removal by solvent extraction .
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Fig. 2

Process flow diagrams for copper leaching and recovery : a Reductive/oxidative leaching route and b oxidative leaching route with aqueous polar organic solution

The advantage of these processes resides in their selectivity and relatively mild operating conditions, although there are still drawbacks that must be confronted and resolved before these processes are feasible.

Copper can also be economically recovered from e-waste. Furthermore, it must be removed before the precious metals are leached, since most lixiviants preferentially extract copper over silver and gold . The conventional methods use relatively concentrated inorganic acids at slightly elevated temperatures (up to 80 °C). However, carboxylic acids have proven to be more selective and the leach is performed at ambient temperature with less acidic pH values. Copper can be electrowon efficiently from these solutions, which are directly recycled back to the leaching stage [8]. The advantages from an environmental standpoint are enormous, especially compared to smelting .

Precious Metals

Gold and silver are extracted from their mineral sources mainly by cyanidation . However, some ores and concentrates present various degrees of refractoriness caused by the presence of cyanicides, occlusion within different phases (arsenopyrite , carbonaceous materials, among others). One of the proposed alternative chemistries for treating these materials is based on the thiosulfate ion. With the exception of the treatment of oxidized ores, the chemistry of the leaching solution is extremely complex, and the thiosulfate ion is sensitive to the solution ORP. Therefore, the leaching solution itself is very valuable (elevated reagent concentrations: S2O32−, Cu2+, NH3, additives ) and small changes in the solution conditions (pH, ORP, temperature , etc.) can result in excessive thiosulfate degradation. For those reasons, cementation is favored for precious metal recovery .

Due to its complexity and value, the entrained solution should be recovered from the solid residue, as much as possible (Fig. 3) [9]. Washing of the solid residue is imperative for two reasons: to eliminate contamination of the tailings dam and to retrieve reagents. However, the wash waters are usually dilute and should not be directly combined with the recirculating solution. For concentrating the reagents in the wash waters without a significant change in the solution conditions, one suggestion is the use of reverse osmosis.
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Fig. 3

Process flow diagram for thiosulfate leaching of precious metals [9]

Platinum Group Metals

Platinum and palladium are commonly obtained as a very valuable secondary product from base metal ores [10]. However, there are many unexploited sources, such as in magnetite matrices, where these metals can be extracted as the primary product, under specific conditions. Platinum and palladium require highly oxidizing conditions and complexation to solubilize to any degree. Under these conditions, many other metals can be leached; however, by controlling the solution pH, a high degree of selectivity is possible.

The proposed process involves leaching with ozone in a concentrated brine at pH 4–5, liquid/solid separation and cementation with zinc, as shown in Fig. 4 [11]. Once the Pt, Pd and Au are cemented, the solution is reusable without further treatment. The advantages of working at pH 4–5 is that a minimal amount of iron is leached, and chlorine gas is not generated. On the materials tested, the impurity build-up has been small and only a minimal purge is required.
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Fig. 4

Process flow diagram for platinum , palladium and gold from magnetite concentrates

Conclusions and Implications

Some basic principles for water-saving hydrometallurgical process design have been presented and illustrated. The concepts introduced are extremely general, but they represent a change in philosophy by conceiving the integrated processes, instead of concentrating on first optimizing a single stage. Hopefully, this change of approach will lead to better and more sustainable processes in the future.