Using Partial Desalination Treatment To Improve the Recovery of

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Using partial desalination treatment to improve the recovery of copper and molybdenum minerals in Chilean mining industry Constanza Cruz, Arturo Reyes, Ricardo I. Jeldres, Luis Alberto Cisternas, and Andrzej Kraslawski Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00821 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 5, 2019

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Industrial & Engineering Chemistry Research

Using partial desalination treatment to improve the recovery of copper and molybdenum minerals in Chilean mining industry Constanza Cruz a,b,c*, Arturo Reyes b,d,f, Ricardo I. Jeldres b,c , Luis A. Cisternas b,c and Andrzej Kraslawski a,e a

LUT School of Engineering Science, LUT University, FI-53851, Lappeenranta, Finland

b

Centro de Investigación Científico Tecnológico para la Minería, CICITEM, 1240000,

Antofagasta, Chile c

Departamento de Ingeniería Química y Procesos de Minerales, Universidad de

Antofagasta, 1240000, Antofagasta, Chile d

Departamento de Ingeniería en Minas, Universidad de Antofagasta, 1240000,

Antofagasta, Chile e

Department of Process and Environmental Engineering, Lodz University of Technology,

90-924, Lodz, Poland f

Instituto Antofagasta, Universidad de Antofagasta, 1240000, Antofagasta, Chile

*

Corresponding author:

E-mail address: [email protected]; [email protected]. (C.Cruz) 1 ACS Paragon Plus Environment

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Abstract Limited water resources and greenhouse gases emissions from fossil fuels used in electricity production are problems experienced in Chile. To address water shortage, many copper mining companies have started to use seawater or desalinated water in their operations. However, seawater exerts negative impact in the mineral processing operation. It reduces the recovery of copper and molybdenum minerals in the flotation of copper-molybdenum sulphide ores due to the presence of calcium and magnesium ions. This study aims to propose a partial desalination treatment for calcium and magnesium species removal to improve the recovery of valuable minerals in the flotation of copper-molybdenum sulphide ores. The proposed method uses carbon dioxide gas and sodium hydroxide to promote the removal of calcium and magnesium ions from seawater. As a result, the partial desalination treatment can remove 60.5% of calcium and 98.3% of magnesium species. In addition, it helped in reaching high recovery of molybdenum (81.1%) and copper (93.4%) as well as depressed pyrite (0.95% of iron) in the flotation of coppermolybdenum sulphide ores. Therefore, partial desalination treatment could provide an appropriate water quality required in the froth flotation process, and it could reduce carbon dioxide come from greenhouse gases emissions.

Keywords: Mining industry; Copper and molybdenum sulphide ores; Carbon dioxide; Seawater flotation; partial desalination treatment.

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1. Introduction Copper (Cu) and molybdenum (Mo) are two of the most commonly used metals in our daily life, mainly due to their physical and chemical properties. In recent years, the demand for these metals has grown drastically, which is reflected in the continuous increase in annual world production that amounted to 20.1 million metric tons of Cu and 279 thousand metric tons of Mo in 2016. 1 Nowadays, both metals are essential for the transition to fossil-free power that is a key element for achieving sustainable development worldwide. 2 In fact, the transition to a low-carbon economy will increase demand for metals up to 10 times by 2050. 3,4 Therefore, mining, mineral processing, and metal production are key activities to provide Cu and Mo in the coming decades. The major countries producers of Cu and Mo are Chile, Peru, China, and the U.S. 1 Among these countries, Chile is one of the most important mining countries in the world due to its global dominance in the production of Cu and Mo and ore reserves of these minerals. In 2016, Chile produced around one-third and one-fifth of global production of Cu and Mo respectively, which represented 5.5 million metric tons and 55.6 thousand metric tons. 1 Cu and Mo are commonly associated with chalcopyrite (CuFeS2), molybdenite (MoS2) minerals and pyrite (FeS2), this latter as a non-valuable mineral in complex sulphide ores.

5

Nowadays, Cu-Mo sulphide ores are typically processed using a pyrometallurgical method, which comprises four operations: 1) mining, i.e., ore extraction; 2) mineral processing, i.e., beneficiation of Cu and Mo minerals; 3) smelting, i.e., separation of metals from their minerals; and 4) refining, i.e., purifying final products. 6 In mineral processing operation, Mo is commonly recovered as a by-product from Cu-Mo sulphide ores in the copper mining companies. However, mineral processing operation requires large amounts of water and energy input leading to environmental, technical, and economic challenges. One of the main reason is that Cu and Mo production takes place primarily in norther Chile, the driest place on Earth, where the weather is arid with sparse or no rainfall during the year. 7 In this area, electricity is supplied mostly by thermal power stations using fossil fuel, which generates environmental problems related to the production of carbon dioxide (CO2 (g)) that contributes to greenhouse gases (GHG) emission. 8 Besides, conventional water resources - for which mining companies, other companies, and local communities compete - are limited. To address the water shortage, the mining industries have started to use seawater leading to face some operational problems due to the new water quality. 9,10

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In mineral processing operation, the froth flotation process is used for recovering Cu and Mo minerals from Cu-Mo sulphide ores. This process requires large water input to obtain Cu and Mo concentrate and water quality is a relevant factor in achieving high enrichment of valuable minerals and high performance of the process. 9 Several studies have demonstrated that the salinity (NaCl) of seawater is not detrimental in the froth flotation process; however, seawater contains other elements interfering with the flotation of Cu-Mo sulphide ores.

11-13

In particular, calcium

(Ca2+) and magnesium (Mg2+) ions from seawater strongly reduce the recovery of Mo and slightly the recovery of Cu in the flotation of Cu-Mo sulphide ores when the process is carried out at alkaline conditions (pH>9.5). This condition is needed to depress non-valuable minerals, mainly pyrite, and is carried out using commonly lime (CaO) to raise the pH of the mineral pulps. However, the amount of lime consumed in seawater is about ten times higher than in freshwater involving significant operational costs (13). This is due to the buffering effect of seawater produced by couples of carbonate/bicarbonate ions (HCO3- / CO3-2) and boric acid/borate ions (B(OH)3 / B(OH)4-). 14 Detrimental effect on the recovery of Cu and Mo takes place because the precipitation of calcium and magnesium colloids under alkaline condition onto the surfaces of valuable minerals leads to the formation of a hydrophilic coating that reduces their floatability and, consequently, their recovery.

13, 15, 16

Beside, a recent study showed that Ca2+ and Mg2+ ions at

highly alkaline conditions also significantly affect the floatability of chalcopyrite if clay is present in the ores. The reason is that Ca2+ and Mg2+ cations may act as a bridge between the mineral surfaces, generating aggregates that stimulate the chalcopyrite depression. 17 To ensure supplies of water of adequate quality to mining operations and enhance the performance of the froth flotation process, some copper mining companies have installed their desalination plants, mainly based on reverse osmosis (RO). 9, 18 However, the desalination process is energy-intensive and involve high economical costs since water is usually pumped into the mountains as mining sites are located at long distances and at high altitudes from the coastline in Chile. 19 This situation increases the CO2 (g) emissions due to the combustion of fossil fuels used for electricity production in Chile. Therefore, the use of industrial waste such as CO2 (g) could be an attractive alternative to improve sustainability in the copper mining companies. Considering the scientific background discussed above, it is evident that divalent cations present in seawater cause problems in the flotation of Cu-Mo sulphide ores; however, salinity is not a major concern. Therefore, desalinated water from the RO plant is not necessarily used 4 ACS Paragon Plus Environment

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exclusively in the froth flotation process. Partial desalination treatment is seen as an alternative to reduce the concentration of Ca2+ and Mg2+ ions from seawater in order to improve the recovery of valuable minerals in froth flotation.

9

In this context, Castro

20

and Jeldres et al.

21

proposed a

partial desalination treatment using a mixture of lime and sodium carbonate (Na2CO3) to remove seawater hardness for the froth flotation process. However, there are still few efforts to use seawater with partial or selective desalination to facilitate the use of seawater in the processing of Cu-Mo sulphide ores. The present investigation proposes a partial desalination treatment using CO2 (g) and sodium hydroxide (NaOH) for calcium and magnesium removal aiming to improve the recovery of valuable minerals in the flotation of Cu-Mo sulphide ores. 2. Water and electricity use in mineral processing Mineral processing operation includes three processes for beneficiation of Cu and Mo minerals (see Figure 1): 1) comminution, aimed at ore particle size reduction; 2) froth flotation, to obtain Cu and Mo concentrate; 3) dewatering, to recover and to recycle water from Cu and Mo concentrate, and tailings. As mentioned before, water and electricity are strategic inputs in mineral processing operation as shown in Figure 1. Different water sources such as surface water, groundwater, acquired water, desalinated water and seawater are used in the mineral processing operation, but its main sources are desalinated water and seawater. 22 Desalinated water is supplied by RO plants, while electricity is delivered mainly from thermoelectric-power plants that use fossil fuels (see Figure 1). In 2018, thermoelectric-power plants represented 53.9% of the total installed electricity capacity in Chile and coal was the primary fossil fuel used 23 emitting large amounts of GHG to the environment, as represented by the thick yellow line in Figure 1. Besides, as mentioned above, thermoelectric-power plants supply electricity to RO plants and water pumping stations to transport water from the coast to the mining operations. Due to this situation, electricity consumption has grown at a rate of 20% per year since 2013 increasing negative environmental impacts. 24 Another environmental risk is the discharge of brine from RO plant into the ocean (see Figure 1) which disturbs the ecological balance in the marine ecosystem with high salinity and temperature and presence of dissolved chemicals. 25 The comminution process in mineral processing is energy-intensive and a thick red line represents it in Figure 1. Moreover, the comminution process needs water (thin blue line in Figure 1) to generate a mineral pulp for the next process of froth flotation. Nowadays, the declining ore 5 ACS Paragon Plus Environment


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grade has led to increased energy consumption in the comminution process since more ores must be treated to produce the same amount of metal. 4, 26 In fact, the average copper grade of sulphide ores in Chile has declined from 1.18% to 0.90% over the period 2003-2012 27, which has led to the consumption of large amounts of water and energy in the mining sector. In this context, comminution contributes a lot to the total GHG emissions for metal production.

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On the other

hand, the froth flotation process is water-intensive (thick blue line in Figure 1) and it requires the most significant amount of water in the copper mining industry. Over the period 2009-2015, froth flotation process has consumed about 70-72 % of the total water in the Chilean mining industry. 22

It also needs electricity (thin red line in Figure 1) but its demand for electricity is lower than that

of the comminution process. Moreover, as shown in Figure 1, large amounts of water recuperated from the dewatering process are re-circulated to the froth flotation process. However, the scarcity of water resources and the launching of new mining projects in Chile have led to further increase in the use of seawater or desalinated water. INDUSTRIAL SYSTEMS FOR THE SUPPLY OF ELECTRICITY AND WATER

seawater

fossil fuel sources

GHG

Thermoelectricpower plant

electricity

Reverse Osmosis plant

desalinated water

electricity

Cu - Mo concentrate Cu-Mo sulphide ores

Comminution

milled ore

brine

Froth Flotation

tailings

Dewatering

water

Cu - Mo concentrate tailings

MINERAL PROCESSING OF COPPER – MOLYBDENUM SULPHIDE ORES intensive flux of electricity intensive flux of water intensive flux of GHG

Figure 1. Links of mineral processing operation with industrial water and electricity supply systems.

3. Experimental section This section presents partial desalination treatment, which is then used in the froth flotation process to evaluate the Cu, Mo, and iron (Fe) recovery of the Cu-Mo sulphide ores. 6 ACS Paragon Plus Environment

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3.1

Partial desalination treatment: calcium and magnesium removal with CO2 (g) and NaOH

3.1.1 Materials We used seawater sample from San Jorge Bay in the coast of Antofagasta region located in the Atacama Desert in the north of Chile. The sample composition is shown in Table 1. The main reagents used to conduct the tests were a solution of NaOH 10 [M] and CO2 (g). The solution of NaOH 10 [M] was prepared using NaOH pellets (Merck Millipore) and pure CO2 (g) was used as the CO2 (g) source. Deionized water was used to prepare the solution, as well as to wash all the materials and experimental units. Experimental units used are shown in Figure 2. A CO2 (g) steel cylinder (1) is connected with a decompressor (2) and an automatic rotameter (3) (Alicat Scientific, 1-100 ml/min). These apparatus ensure stable and sensible control of the volumetric flow of CO2 (g). The rotameter is also connected to a tubing which disperses the CO2 (g) into the three flask reactor (4). It has the capacity of 2 L and is equipped with a thermometer (5) to measure the temperature of the solution. It is located above a magnetic stirrer (6) with a respective magnetic stirring bar (7) to keep the solution homogeneous throughout the test. There is also an automatic potentiometric titrimeter (8) (Shott Titroline 700) connected with pH electrode (9) (Sentix 41). It was used to control the flux injection of alkalizing reagent (NaOH) and to measure the pH values of the solution in the reactor.

Solute Mg2+ Na+ K+ Ca2+ ClF-

Table 1. Seawater composition at San Jorge Bay (Chile). Value (mg/L) Solute Value (mg/L) 1,413 HCO3 137 11,100 PO43

Using Partial Desalination Treatment To Improve the Recovery of

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