Recovery of Acid

9.4.1 Introduction

The AMD is inherently acidic due to high concentrations of sulphuric acid (Simate and Ndlovu, 2014). Two predominant theories, (1) Arrhenius theory, and (2) Bronsted-Lowry theory, give two interrelated definitions of an acid and a base (Razzaq and Khudair, 2018). An Arrhenius acid is any species that increases the concentration of H+ in aqueous solution. An Arrhenius base is any species that increases the concentration of OH in aqueous solution. In the Bronsted-Lowry definition, acids are proton donors, and bases are proton acceptors.

The predominantly acidic nature of AMD has made it to be extremely corrosive and polluting in nature (Agrawal and Sahu, 2009). The hazards associated with acidic pH in AMD on the environment and health have already been discussed in Chapter 5. Therefore, there is a need to develop techniques to recover acid from AMD and/or minimise its environmental and health effects. Actually, over the past half a century, meticulous efforts have been aimed at remediating AMD through acid removal so as to reduce the impact of the acidic water on the environment and produce water suitable for reuse (Johnson and Hallberg, 2005; Simate and Ndlovu, 2014; Nleya et al., 2016; Nleya, 2016). Indeed, apart from the production of reusable water and saleable metals, the recovery of sulphuric acid from AMD would also be used to offset its treatment costs (Simate and Ndlovu, 2014). Table 9.8 is a summary of the methods used to recover sulphuric acid from various waste- water solutions including AMD showing the recoveries, advantages, and disadvantages of the processes (Nleya et al., 2016). Furthermore, a study by Nleya et al. (2016) critically evaluated the technical and economic feasibilities of the processes in Table 9.8 for application to AMD, and the results of the study are given in Table 9.9. Based on the technical and economic feasibility results in Table 9.9, the freeze crystallisation and acid retardation processes are expected to be the most suitable technologies for acid recovery from wastewater solutions.

9.4.2 Selected Typical Studies of Recovery of Acid from Acid Mine Drainage

Several studies on processes shown in Table 9.8 have been conducted in the past and their fundamentals are well documented (Etter and Langill, 2006; Kim, 2006; Ozdemir et al., 2006; Tjus et al., 2006; Agrawal and Sahu, 2009). This section only gives typical studies of recovery of acid from AMD only.

A study by Nleya (2016) focused on the removal of toxic heavy metals as well as the recovery of acid using low-cost adsorbents and acid retardation process, respectively. In the first part of the study, the adsorption efficiencies of zeolite and bentonite were found to be less than 50% for most metal ions, which were lower compared to the 90% efficiency obtained with cassava peel biomass. The second aspect of the study involved testing the feasibility of using a process known as acid retardation to recover sulphuric acid from metal-barren AMD. Acid retardation can be defined as a process which employs ion-exchange resins to selectively adsorb acids from solution while dissolved metal salts are rejected (Nleya, 2016). The term retardation emanates from the fact that the preferential adsorption of the acid causes

TABLE 9.8

Summary of Methods Used for Sulphuric Acid Recovery

Method

Solution

Content

h2so4

Recovery

Advantages

Disadvantages

Rectification

H2S04, nitro compounds

R = 98.3%

Recovery of high-purity acid

High energy consumption High operating cost

Diffusion

dialysis

H,S04, A1 H2S04, Fe, V H2S04, Ni H2S04, rare earth sulphates

R = 82-90% R = 84%

R = 80%

R = 70-80%

High acid recovery Low pay back period

Strong salt rejection

Not efficient at low acid concentration

Electrodialysis

H,S04, Ni H2S04, Fe H,S04, Cu, Sb, As H2S04, Fe, Na

R = 80-90%

R = 90%

R = Up to 99%

Clean acid product Reduced solid waste for disposal

High operating cost

Membrane fouling

Acid

retardation

H,S04, Fe H,S04, Fe H2S04/ Ni

R = 74-96% R = 96%

R = 70-95%

Low operating cost High acid recovery Small equipment size and space

Increases product volume

High consumption of fresh water Dilute acid product

Crystallisation

H2S04/ Fe

Low cost

Reduced waste for disposal

Risk of scale formation in crystalliser.

Increased energy consumption

Solvent

extraction

H,S04, Cu H2S04, Fe, Mn H2S04, Zn

E = 75-79% E = 90%

E = 90%

Can manage great volumes of solutions with high content of toxic solutes Clean acid product Only physical separation High throughput with compact equipment

Chemicals used are hazardous Pre-treatment is required to remove impurities Difficulties in stripping from Cyanex 923 Co-extraction of Fe and Zn

R = recovery, E = percentage extraction Source: Nleya et al., 2016.

the movement of the acid on the resin bed to be retarded (slowed down), relative to the movement of the salts, resulting in the separation of the two entities (Sheedy, 1998; Sheedy and Parujen, 2012). The process is reversible and the acid can be recovered by water elution. In the study by Nleya (2016), sulphuric acid recovery from the metal barren solution was evaluated using Dowex MSA-1 ion-exchange resins. A column with inside diameter of 1.0 cm and 30 cm height was used in the tests. Sufficient resins were placed into the column to the required height. The AMD solution was fed in an upward

TABLE 9.9

A Summary of Capital and Operating Expenditures of Proposed Process Routes

Cost (USS)

Rectification

Diffusion

Dialysis

Electrodialysis

Solvent

Extraction

Crystallisation

Acid

Retardation

Fixed

capital

cost

1 193 165

381 780

1 523 478

442 956

407 268

307 973

Total

estimated

CAPEX

1 372 140

439 047

1752 000

509 399

468 368

354 169

Total

estimated

annual

ОРЕХ

427 789

247 752

266 809

125 124

154 798

124 810

Estimated

annual

revenue

=9800

=9800

=9800

=9800

=10 320

=9800

Source: Nleya et al., 2016.

flow direction and samples were collected from the top of the column at different time intervals, and analysed for metal, sulphate, and acid content. The results of the study showed that sulphuric acid can be recovered by the resins via the acid retardation process and could subsequently be upgraded to near market values of up to 70% sulphuric acid using an evaporator. Water of reusable quality could also be obtained during the acid upgrade process. Ideally, the process was stopped at approximately 70% acid (when approximately 98% of the water was evaporated) concentration, because almost no sulphuric acid could be detected in the water vapour up to an acid concentration of 70% (Nleya, 2016). An economic evaluation of the proposed process also showed that it was possible to obtain revenue from sulphuric acid which could be used to offset some of the operational costs in AMD remediation processes.

Marti-Calatayud et al. (2013) studied the recovery of sulphuric acid from AMD using an electrodialysis cell with three compartments. The ion- exchange membranes used in the study were heterogeneous HDX membranes (provided by Hidrodex®). The anion-exchange membrane (AEM, HDX 200) contained quaternary amine groups that were attached to the membrane matrix. The cation-exchange membrane (CEM, HDX 100) was charged with sulphonic acid groups and had a similar morphology to that of HDX 200. Both membranes had remarkably high ion-exchange capacities, which were 1.8 and 2.0 mmol/g for the AEM and the CEM, respectively (Buzzi et al., 2013). The structure of both membranes was reinforced with two nylon fabrics with the function of increasing their mechanical stability. The composition of AMD varied substantially depending on the source from which samples were collected. However, the AMD solution with the

TABLE 9.10

Composition of the Original Source of Acid Mine Drainage and the Synthetic Solutions Used in the Electrodialysis Experiments

Solution

Fe(III) (mol L 9

Nad) (mol L 9

SOA (mol L 9

pH

Acid mine drainage source

0.037

0.017

0.082

2.48

Synthetic solution: 0.02-M Fe,(S04)3 + 0.01-M Na,SQ4

0.040

0.020

0.070

1.68

Source: Marti-Calatayud et al., 2013.

highest concentration of sulphates was selected as a basis for the study, since the principal aim of the work was the recovery of sulphuric acid from AMD. Synthetic solutions with a composition approximate to that of the original AMD solution were prepared by mixing 0.02-M Fe2(S04)3 and 0.01-M Na2S04. Distilled water was used to prepare the synthetic solutions. The content of the most concentrated species in the original AMD source is summarised in Table 9.10, together with the concentrations and pH value of the synthetic solutions.

The results of the study conducted by Marti-Calatayud et al. (2013) showed that the recovery of sulphuric acid from AMD can be achieved by means of an electrodialysis cell. Significant increases in sulphuric acid concentration were obtained with the proposed scheme consisting of a three-compartment electrodialysis cell with cation-exchange membrane and anion-exchange membrane. An effective recovery of sulphuric acid free from Fe(III) species was obtained in the anodic compartment as a result of the co-ion exclusion mechanism in the membranes. The difference in the pH and pS042' values between the membrane phase and the external electrolyte promoted the dissociation of complex species inside the membranes. This phenomenon impeded the transport of Fe(III) and sulphates in the form of complex ions towards the anodic and cathodic compartments, respectively. The current efficiency values of the anion-exchange membrane at different current densities were approximately constant with time. However, the increase in the recovery of acid decreased as the current increased. This result is explained by the shift in the equilibrium at the membrane/solution interface as more S042‘ ions cross the anionic membrane and by the enhancement of the dissociation of water when the limiting current density is exceeded. The main limitation of the process was related to an abrupt increase in the cell voltage due to the formation of precipitates at the surface of the cation-exchange membrane.

In a study by Kesieme and Aral (2015) that has already been discussed under the recovery of water from AMD (Section 9.3.2.2), the researchers studied the potential and opportunities for DCMD to concentrate H2S04 and recover fresh water from acidic process solutions. The study was also aimed at identifying how membrane distillation can work in combination with solvent extraction in the mineral processing industry for acid recovery. After recovering water through the membrane distillation unit, the remaining concentrated acidic solution (see Table 9.7) was processed using solvent extraction so as to recover sulphuric acid. The organic system consisting of 50% TEHA and 10% ShellSol A150 (a 100% aromatic diluent) in octanol was used in the solvent extraction system. In the solvent extraction tests, the organic system was mixed with concentrated acidic solution at an A/О ratio of 1:2 and a temperature of 22°C. The loaded organic solution was stripped twice at О/A ratios of 2:1 and 1:5 at 60°C. The raffinate and the loaded strip liquors were titrated to determine acid concentrations for extraction and mass balance calculations. The results indicated that over 80% of H2S04 was extracted in the solvent extraction system in a single contact from the waste solution (i.e., the concentrated solution from the membrane distillation) containing 245 g/L H2S04 and metals with various concentrations. After three stages of successive extraction, nearly 99% of acid was extracted, leaving only

2.4 g/L H2S04 in the raffinate. The extracted acid was easily stripped from the loaded organic solution using water at 60°C. After scrubbing the loaded organic solution at an О/A ratio of 10 and 22°C, 98-100% of entrained metals were removed in a single contact with only 4.5% acid lost in the loaded scrub liquor.

Three different types of NF membranes, namely, (1) a poly(piperazinamide) active layer (NF270), (2) a double active layer (poly(piperazinamide)/ proprietary polyamide) (Desal DL), and (3) a sulphonated polyfethersu lphone) active layer (HydraCoRe 70pHT), were evaluated by Lopez et al. (2019a) for the recovery of sulphuric acid from acidic mine waters and simultaneously increasing the concentration of valuable elements for further valorisation after the removal of iron. The NF270 and Desal DLmembranes had the same top active layer based on a semi-aromatic poly(piperazineamide). However, Desal DL incorporates an additional proprietary second layer that approaches a tight UF and that was made of a material comparable to a polyamide. Both NF270 and Desal DL membranes possessed ionogenic amine (R-NH2) and carboxylic (R-COOH) groups, which were responsible for the membrane charge. The IEPs for the two membranes were 2.5 and 4.0, respectively. The HydraCoRe 70pHT incorporated a sulphonated polyethersulphone as the active layer on a standard thin film composite membrane structure with poly- sulphone and polyester on the backside. The membrane charge was caused by the presence of sulphonic groups (R-S03H). Experiments were performed with a synthetic solution simulating the supernatant of a pre-treated acidic mine water from La Poderosa Mine at the Iberian Belt (Huelva, Spain). The synthetic solution was prepared by dissolving appropriate amounts of the metal-sulphate, nitrate, chloride and oxide salts in sulphuric acid.

The results of the study by Lopez et al. (2019a) showed that NF technology offers a good chance to recover acids in the permeate and, at the same time, to concentrate metals in the retentate when treating acidic mine waters. Among the different membranes tested, NF270 showed the best performance, as it yielded negative H‘ rejections (i.e., permeate was more acidic than the feed solution), high metal rejections (>98%), and higher trans-membrane fluxes. The separation was highly influenced by the feed composition and the membrane chemistry of the active layer. Indeed, the membrane chemistry of the active layer (nature and acid-base membrane properties) and structure (single/double layer) were found to be strong parameters in the membrane- separation performance. The effect of the composition of active layer was observed when the same solution was filtered with different kinds of membranes. The composition of the active layer mainly influenced the solvent transport across the membrane and the superficial charge of the membrane. For example, polyamide membranes (NF270 and Desal DL) exhibited a positively charged surface leading to low anion rejections and high metal rejections. However, sulphonated polyethersulphone membrane (HydraCoRe 70pHT) was expected to exhibit a negative surface charge, leading to lower cation rejections and higher anion rejections than the other two membranes. Moreover, the fact that NF270 and Desal DL reached negative rejections of H+, it made them suitable NF membranes to remove acidity from the feed solution. It must be noted that an increase in the pH in the feed solution, due to the negative rejections of H+, could lead to a decrease in operational costs if any alkaline reagent is added downstream.

When the solution-electro-diffusion model coupled with reactive transport was applied to the study by Lopez et al. (2019a), it was able to fit ions rejections properly by determining the membrane permeance values for each ion. Permeance values for different species of a given element were in agreement with the dielectric exclusion phenomenon. When the values of each element are compared, they could give information of the membrane charge. For instance, NF270 membrane permeances for Fe species followed the trend: Fe(S04)2~ > FeSO/ > FeHS042+ > Fe3-, while for HydraCoRe 70pHT followed FeS04+ > Fe(S04)22- > FeHS042+ > Fe3*. The fact that Fe(S04)2- was the most permeable ion of Fe species for NF270 suggested that the membrane presents a negative charge at pH 1. As for the HydraCoRe 70pHT membrane, the fastest ion was FeS04+ due to a negatively charged surface. Moreover, membrane permeance values were found to depend not only on the salt composition, but also on the total concentration (ionic strength).

Lopez et al. (2019b) investigated the performance of a semi-aromatic poly(piperazine amide) NF membrane (NF270) in the treatment of streams generated from the off-gases treatment step of copper metallurgical industry. The streams contained a mixture of H2S04/HC1/H,As04 and metallic species (Fe, Cu, Zn, Ni, Co, Cd) and alkaline metals (Na, K, Ca, Mg). The membrane performance was evaluated in terms of acid recovery and metal ions rejection taking into account their aqueous speciation in strong acid media. Transport of acids and metallic species through the membrane was evaluated under three different total acidity scenarios with pH values from 0.2 to 0.7 and modelled according to solution-electro-diffusion model coupled with reactive transport to determine the membrane permeances to species. The transport behaviour implications of both fully dissociated strong acids (H2S04 and HC1) and weak acids (H3As04) with non-dissociated forms in the working conditions (0.2 < pH < 0.7) were critically evaluated in detail. The experiments were carried out in a cross-flow experimental set-up with flat-sheet membranes (0.014 m2) placed in a test cell (GE SEPA™ CF II) with a spacer-filled feed channel. The set-up had a needle and a by-pass valve which allowed the variation of the cross-flow velocity and the trans-membrane pressure. The feed solution was kept in a thermostatic 30 L tank at a constant temperature (25 ± 2°C) and was pumped into the membrane cell by a high-pressure diaphragm pump. The two outputs of the cell (permeate and concentrate) were recycled back to the tank to keep the same composition in the feed solution.

The experimental data of the study by Lopez et al. (2019b) demonstrated that it was possible to recover strong acids (H2S04/ HC1) from hydrometallur- gical streams by using a semi-aromatic polyfpiperazine amide) membrane. The NF270 exhibited a positive surface charge at pH < 1.0, which favoured the transport of anions, while impeded the transport of metallic species that were present as cations. With the different solutions tested, the membrane exhibited negative chloride rejections and moderate sulphate rejections. Design of processes using more than one NF stage may allow the recovery of up to 90% of the total content of the strong acids. In addition, the membrane favoured the transport of non-metallic species (As) due to their presence as a non-charged species (H3As04) and not limited by the dielectric and Donnan exclusion. As a result, the levels of As are likely to limit the application of the technology. However, a pre-treatment stage if applied, such as using a reducing agent (e.g., H2S or S2032-) to obtain As(III) and then precipitate As as As2S3(s) or as a mixture of S(s) and As203(s), will maximise the recovery of the total content of the strong acids. The solution-electro-diffusion model coupled with reactive transport fitted the experimental ions rejections properly, and the calculated membrane permeances could be used to design stages in full-scale applications.

A comprehensive study by Nleya et al. (2016), which was briefly mentioned in Section 9.4.1, proposed a number of flowsheets in which acid could be recovered from AMD. These flowsheets are integrated to pre-existing technologies (see Table 9.8) that are used to recover various acids from other industrial waste streams. A first possible flow diagram for the recovery of sulphuric acid from AMD through rectification is shown in Figure 9.9. Rectification also known as slow distillation is a promising process for the recovery of high-purity sulphuric acid from waste acid solutions (Song et al.,

2013). The rectification process which works by separating mixtures based on differences in volatilities of components in a boiling liquid mixture can concentrate and purify products in one step (Qian et al., 2011). It can be seen from Figure 9.9 that large quantities of heat energy are required for both pre-heating and rectification processes. In the rectification unit, the water

FIGURE 9.9

Proposed flow diagram for the recovery of sulphuric acid from acid mine drainage using the rectification process. (From Nleya et al., 2016.)

component is vaporised while sulphuric acid remains in solution and is recovered separately. Nleya et al. (2016) argued that when the rectification process is considered in the context of AMD, it might not be a suitable recovery process because the sulphuric acid content in AMD might be too low for any substantial economic benefits.

Figure 9.10 and Figure 9.11 are proposed flow diagrams for the recovery of acid from AMD using diffusion dialysis and electrodialysis, respectively. The concept of electrodialysis has been extensively discussed in Section 9.З.2.1. Diffusion dialysis makes use of a series of anion-exchange membranes to selectively attract the anion in the acid while electrodialysis uses an electric field to allow ions of one electrical charge to enter and pass through

FIGURE 9.10

Proposed flow diagram for the recovery of sulphuric acid from acid mine drainage using diffusion dialysis method. (From Nleya et al., 2016.)

FIGURE 9.12

Proposed flow diagram for the recovery of sulphuric acid from acid mine drainage using solvent extraction method. (From Nleya et al., 2016.)

(perm-selectivity) (Nleya et al., 2016). In both flow diagrams (Figure 9.10 and Figure 9.11), the pre-filtered AMD solution is passed through a membrane unit where a clean acid and an acid barren water product can be obtained. In general, it can be seen that most of the membrane-separation processes are environmentally attractive. Apart from significantly reducing the solid waste, high-purity acid product can be obtained in most cases.

Figure 9.12 shows the proposed flowsheet for the recovery of sulphuric acid from AMD using solvent extraction method (Nleya et al., 2016). Solvent extraction process has long been used in the recycling and/or recovery of waste acids (Gottliebsen et al., 2000; Agrawal et al., 2008; Haghshenas et al., 2009; Shin et al., 2009), and is it a promising technology that can be extended to the recovery of sulphuric acid from AMD (Nleya et al., 2016). It is a clean and proven technology (Gottliebsen et al., 2000). The organic extractants tested have good selectivity for the acid; hence high recoveries can be expected (Gottliebsen et al., 2000; Agrawal et al., 2008).

Another promising technique for sulphuric acid recovery from AMD is freeze crystallisation. In addition to the acid, AMD also contains high quantities of ferrous ions which can also be recovered as crystals, purified and marketed as a commodity using the freeze crystallisation technique. A proposed flow diagram for the recovery of sulphuric acid via freeze crystallisation is shown in Figure 9.13. In the flow diagram, pre-filtered AMD solution is chilled in a heat exchanger unit using the cold sulphuric acid product. After the solution is pre-chilled, it enters the reactor where it is agitated and

FIGURE 9.14

Proposed acid retardation set-up for sulphuric acid recovery from acid mine drainage. (From Nleya et al., 2016.)

chilled further until the ferrous sulphate heptahydrate crystals are formed. The settled crystalline solution is pumped to the centrifuge, where the crystalline product and the sulphuric acid solution are separated. The cooled sulphuric acid product is pumped back to the primary heat exchanger where it cools the incoming AMD solution and then collected for storage.

In addition to a study by Nleya (2016) that tested the feasibility of using acid retardation technique to recover sulphuric acid from metal-barren AMD, Nleya et al. (2016) proposed a flowsheet for the application of acid retardation process in the recovery of sulphuric acid from AMD as shown in Figure 9.14. In the proposed flow diagram a dilute stream of acid is obtained in the acid retardation process which can then be concentrated using a vacuum evaporator. Water of a quality suitable for recycling back to the system can also be obtained. A vacuum evaporator has the advantage of producing a large separation factor in the sulphuric acid/water system, and that it reduces the boiling point of the mixture which, subsequently, minimises the cost of heating (Nleya et al., 2016).

Simate and Ndlovu (2014) proposed an integrated process shown in Figure 9.15 that starts off with an AMD fuel cell where iron would be

precipitated as iron oxide at the anode. The water that contains most of the heavy metals would then be pumped to the adsorption circuit where the CH-collector (e.g., amino bisphosphonate adsorbent) would adsorb heavy metals directly from the wastewater (Turhanen and Vepsalainen, 2013). The resulting water that is barren of metals is then pumped to the electrodialysis circuit where a more pure form of sulphuric acid and other residual metals are recovered (Marti-Calatayud et al., 2013). The electrodialysis process efficiency is expected to be high as metals which might cause membrane fouling would have been removed in the fuel cell and by the CH-collector. The products from this integrated process are expected to be iron oxide which could be sold as a pigment for paint (Hedin, 2003), cosmetics, and possibly other uses. The electricity produced from the fuel cell can act as a power source for the electrodialysis. The sulphuric acid also produced could be used in the leaching of metal ores.

 
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