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Electrochemical Recovery of Ammonia and Sulphuric Acid from an Effluent of a Complex Hydrometallurgical Leaching Process
1B. Ash, 2M. Behera
1M. P. C. (A) College, Baripada,
2Satyasai Engineering College, Sricona, Balasore.
Email: [email protected] 1, [email protected] 2. [Received: 10th Jan.2017; Accepted: 18th Jan.2017]
Abstract- Hydrometallurgical route of metal extraction followed alkali or acid leaching path way.
The SO2-ammonical ammonium sulphate leach liquor of manganese nodule bearing iron, manganese, copper, zinc, nickel and cobalt was subjected to aeration/oxygenation to precipitate out iron and manganese leaving the other metal ions.
After extraction of copper, zinc, cobalt and nickel by solvent extraction method followed by electro winning from the Fe- and Mn- free leach liquor, large quantities of ammonium sulphate remain in the raffinate. By electrochemical splitting technique, it is possible to recover ammonia and sulphuric acid from the raffinate which can be reused in the process.
Simultaneously the effect of chloride as additive during electrochemical splitting of ammonium sulphate is presented in this paper.
Index Terms-Ammonium sulphate; Electrochemical splitting; Chloride addition; Manganese nodule.
I. INTRODUCTION
The enigmatic potato-sized lumps of mixed manganese and iron oxides with other metals are popularly known as manganese nodules (poly metallic nodules). They were first discovered lying on the bed of Pacific Ocean. Also nodule deposits have been found in the North and South Atlantic and the Indian Ocean as well as in the South Pacific. The polymetallic nodules of Indian Ocean, which contain valuable metals like nickel 1%, cobalt 0.1% and copper 1% apart from manganese 20% and iron 10%. Several industrial consortia have been engaged in research and development (R
& D) into the various aspects of metals recovery from deep-sea manganese nodules. These are quite unlike any other terrestrial ores; both with respect to their physical characteristic and to their mineralogical and chemical compositions and therefore new processes are required to extract
them. One of the processes developed and used in industries for extraction of Cu, Ni, and Co from deep sea manganese nodules is NH3–SO2 route [1, 2]. The flow sheet consists of leaching, Mn- removal, iron removal, ammonia removal, Cu-Zn- Co-Ni sulphide precipitation-dissolution and separation-recovery by solvent extraction and electro winning of metal values (SX-EW). As per the flow sheet the effluent stream after Cu-Zn-Co- Ni sulphide precipitation contains ammonium sulphate as the major component. Since in this process the main consumables/reagents are SO2
and ammonia, during the process these get converted to give ammonium sulphate. It is extremely important to convert ammonium sulphate back to the ammonia and sulphuric acid for recycle and providing better economics to the process.
The literature reports on the recovery of ammonia by lime boiling method [3]. Some industries are also using same lime boiling method to recover ammonia in their R & D activities. Though lime boiling process is a fruit full method, it is energy and labour intensive. A closed autoclave system with constant stirring is necessary for better recovery of ammonia. To find out an alternate method to recover ammonia from ammonium sulphate, the present work has been under taken. In this investigation, electro chemical rout is followed to split ammonium sulphate into ammonia and sulphuric acid. There is an increased interest in the recycling of sodium sulphate by electrochemical splitting into caustic soda and sulphuric acid by means of ion exchange membranes, which enable the recycling of caustic soda solution and sulphuric acid. N. C. Rout et al. [4] have discussed about the effect of foreign metal ions such as copper, nickel, manganese during electrochemical splitting of ammonium sulphate. They have concluded that the
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presence of these metallic impurities reduces the current efficiency and enhance the energy consumption but increase in ammonium sulphate concentration in the catholyte increases the current efficiency and reduces the energy consumption.
Thus cations have negative effect on the electrochemical splitting of ammonium sulphate. In this context, the present paper also highlights the effect of chloride (anion) on recovery of ammonia from ammonium sulphate using electrochemical technique.
The overall reaction during the process will be:
(NH4)2SO4+2H2OH2SO4+2NH4OH …(1) Is separated into two steps:
(step: 1A): Splitting of water into H+ and OH- ions.
(step: 1B): Separation of NH+ ions and SO4-2
ions.
Step: 1A is accomplished by electrolysis of water.
At the anode:
H2O2H++1/2O2+2e- E0 = +1.23V ...(2) At the cathode:
2H2O+2e-H2+2OH- E0 = -0.83V …(3) With reference to step 1B the electrolytic ion exchange membrane poly ethylene is used. This membrane is permeable to hydrated cations only.
The weak H2SO4 is fed to the anode compartment where the concentration increased partially. The migration of sulphate ions into the cathode compartment is largely suppressed, thus, pure ammonium hydroxide is obtained.
Analytical grade reagents are used for the current investigation. The parameters varied in the present study are current density (50-200 A/m2), reaction temperature (30-60 0C), analyte concentration 10- 50 gpl sulphuric acid and chloride concentration in catholyte (0-2 gpl).
II. EXPERIMENTAL
The electrolysis is typically performed in a two space cell in which anode and cathode spaces have been separated by a porous membrane or diaphragm. A 250 ml corning beaker was used as an electrolytic cell and polyethylene material of micro porous size was used as diaphragm. The anode was an alloy of Pb(92%)-Sb(8%) substrate, an insoluble material of 0.3 cm thick having dimensions 10 cm length and 5 cm width. The cathode was stainless steel sheet having the dimensions 10 cm length and 5 cm width. Fresh
electrodes were used for each experiment. The electrodes were carefully polished with fine (600 grade) emery paper, washed under running tap water, scrubbed with filter paper, rinsed with distilled water, and finally air dried. The electrodes were placed into the cell and connected to the circuit for ammonia production. The electrolysis was carried out by applying DC voltage from a regulated power supply unit. For temperature variation experiments, the electrolytic experiments were conducted inside a hot water bath maintaining required temperature. All the reagents used in the present study were of analytical reagent grade from MERCK India ltd. The catholyte was prepared from ammonium sulphate dissolved in distilled water. The chloride ion was introduced into catholyte from sodium chloride. The analyte was a solution of sulphuric acid. The sulphate content in the catholyte and was measured by back titration method using barium chloride as replacing agents.
Ammonia content was estimated from the sulphate content of the catholyte. The cell voltage during electrolysis was noted at regular intervals of time.
From cell voltage, duration and amount of ammonia displaced, the current efficiency and the energy consumption were calculated.
III. RESULTS AND DISCUSSION
In general case, reduction occurs at cathode and oxidation occurs at anode. At cathodic chamber reduction may occur to NH4
+ ion or H+ ion, and at anodic chamber oxidation may occur to OH- ion to produce oxygen. J. Jorrissen et al. [5] have reported that the electrochemical splitting of sodium sulphate produces caustic soda and sulphuric acid.
3.1 Electrolysis of ammonium sulphate:
One experiment was conducted by taking 100 ml of 10 gpl sulphuric acid (0.98 g of sulphate in anolyte) as anolyte and 100 ml of 200 gpl ammonium sulphate (14.53 g of sulphate in catholyte) as catholyte. 200 A/m2 current density was applied to electrolyze the above solution. The experiment was run for a period of 3h. In each hour the sulphate and pH change in electrolyte was measured. The results with respect to SO4
-2 content in anolyte (g), pH of anolyte, SO4
-2 content in catholyte (g), pH of catholyte are given in Table-1.
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Table-1 Experimental results for electrolysis of ammonium sulphate:
Time SO4-2 content in anolyte (g)
pH of anolyte
SO4-2 content in catholyte (g)
pH of catholyte
0h 0.98 0.87 14.53 4.86
1h 1.82 0.77 13.68 6.20
2h 2.55 0.68 12.95 7.58
3h 3.36 0.58 12.19 8.89
It was observed that as time passes, the sulphate content in the catholyte decreased and nearly same quantity of sulphate content increased in anolyte. It suggested that some sulphate ion migrated from catholyte to anolyte by the passage of electricity.
The pH of the anolyte decreased from 0.87 to 0.58, which suggests that sulphuric acid has been produced at anolyte with evolution of oxygen gas.
In catholyte the pH got increased from 4.87 to 8.89 i.e. the alkalinity in the catholyte increased. This is due to formation of a weak base ammonium hydroxide in the catholyte. During this experiment the current efficiency was around 35.91%, which is very less and needs improvements. The energy consumption was observed to be 14.95 kWhr/kg of ammonia.
3.2 Effect of current density on splitting of ammonium sulphate, in presence of 1 gpl chloride ion:
A set of experiments was conducted varying applied current density from 50- 200 A/m2, while keeping other parameters such as catholyte composition; 100 ml of 200 gpl ammonium sulphate (14.53 g of sulphate in catholyte) with 1 gpl chloride ion, anolyte composition; 100 ml of 10 gpl sulphuric acid and duration of electrolysis 3h.
All the experiments were conducted at room temperature (~ 300C). The results with respect to loss of sulphate content in catholyte, splitting of ammonia in catholyte and average cell voltage is given in Table-2. It was observed that as the applied current density increased the loss of sulphate content in catholyte decreased and the splitting of ammonia increased. The average cell voltage varied in between 2.9 to 3.9. But as the applied current density increased to 300 A/m2, the average cell voltage becomes 4.5 V. The effect of current density on current efficiency and energy consumption is shown in Fig-1. It was observed that as applied current density increased from 50 to 200 A/m2, the current efficiency increased from 17.39% to 47.80% and the energy consumption decreased from 25.01 to 12.10 kWh/kg. Thus
higher current density is a favorable condition for electrolytic splitting of ammonium sulphate in the presence of 1 gpl chloride. Beyond 200 A/m2 current density leads to rapid increase in cell voltage, results in higher energy consumption (not presented in figure). Thus the rest of the experiments were carried at 200 A/m2 current density.
Table-2 Effect of applied current density on splitting of ammonium sulphate:
Sl.
No.
Applied current density (A/m2)
Loss of sulphate from catholyte (g)
Splitting of amount of ammonia in catholyte (g)
Averag e cell voltage (V)
1 50 0.23 0.08 2.9
2 100 0.71 0.26 3.1
3 150 1.57 0.56 3.8
4 200 2.44 0.87 3.9
Figure-1, Effect of current density on current efficiency and energy consumption.
3.3 Effect of reaction temperature on splitting of ammonium sulphate, in presence of 1 gpl chloride ion:
A series of experiments were carried out varying reaction temperature from 300C to 600 C, keeping rest of the parameters constant such as applied current density 200 A/m2, catholyte composition;
100 ml of 200 gpl ammonium sulphate (14.53 g of sulphate in catholyte) with 1 gpl chloride ion, anolyte composition; 100 ml of 10 gpl sulphuric acid and duration of electrolysis 3h. The results with respect to loss of sulphate from catholyte, splitting of amount ammonia and average cell voltage is given in Table-3. It was observed that as reaction temperature increased from 30 to 600C, the rate of splitting of ammonia increased which conforms from the data of loss of sulphate from catholyte. The reason may be due to increase in kinetic energy of the ions resulting higher effective collision frequency. The Effect of reaction
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temperature on current efficiency and energy consumption is given in Fig-2. As the temperature increased from 30 to 600C, the current efficiency increased from 47.10 to 95.47 %. The energy consumption decreased from 12.10 to 6.79 when the reaction temperature became 500C from 300C, but it decreased marginally when the temperature was increased to 600C. Beyond 600C a greater energy is needed to maintain the bath temperature, and maintenance of diaphragm will also become crucial. Hence 600C bath temperature may be a suitable condition for electrochemical splitting of ammonium sulphate in presence of chloride.
Table-3, Effect of reaction temperature on splitting of ammonium sulphate:
Sl.
No.
Reaction temperatu re 0C
Loss of sulphate from catholyte (g)
Splitting of amount of
ammonia in catholyte (g)
Averag e cell voltage (V)
1 30 2.44 0.87 3.9
2 40 3.38 1.20 3.9
3 50 4.37 1.55 3.9
4 60 4.91 1.74 3.9
Figure-2: Effect of reaction temperature on current efficiency and energy consumption.
3.4 Effect of anolyte concentration on splitting of ammonium sulphate, in presence of 1 gpl chloride ion:
A test series was performed in which the effect of the ammonium sulphate concentration of a solution fed into the anode space on current efficiency, energy consumption and on the loss of sulphate ion from the cathode space through diaphragm material used in the test was tested. The rest of the parameters such as catholyte composition; 100 ml of 200 gpl ammonium sulphate (14.53 g of sulphate in catholyte) with 1 gpl chloride ion and duration of electrolysis 3h were kept constant. All the experiments were conducted at room
temperature (~ 300C) and 200 A/m2 applied current density. The results with respect to loss of sulphate content in catholyte, splitting of ammonia in catholyte and average cell voltage is given in Table-4.
Table-4 : Effect of anolyte concentration on splitting of ammonium sulphate
Sl.
No .
Anolyte concentratio n gpl
Loss of sulphate from catholyt e (g)
Splitting of amount of ammoni a in catholyte (g)
Averag e cell voltage (V)
1 10 2.44 0.87 3.9
2 20 4.37 1.55 3.9
3 30 4.48 1.59 3.9
4 40 4.57 1.62 3.9
5 50 4.68 1.66 3.9
It was observed that as the sulphuric acid concentration at anolyte increased from 10 to 20 gpl, the loss of sulphate ion concentration increased from 2.44 to 4.37 g. On further increase in anolyte concentration to 50 gpl a marginal increase in loss of sulphate ion in catholyte was observed. The splitting of amount of ammonia in catholyte was considerably increased from 0.87 to 1.55 g as the anolyte concentration increased from 10 to 20 gpl sulphuric acid and beyond that a marginal increase was observed. Thus 10-20 gpl sulphuric acid in the anolyte during electrochemical splitting of ammonium sulphate in the presence of 1 gpl chloride in catholyte may be a suitable condition.
Again as the time will pass, the concentration of sulphuric acid will increase to enhance the splitting process.
The effect of anolyte concentration on current efficiency and energy consumption is given in Fig- 3. The current efficiency increased from 47.80% to 91.20% as anolyte concentration increased from 10 to 50gpl. The energy consumption decreased from 12.10 to 6.34 kWh/kg, as anolyte concentration increased from 10-50 gpl.
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Figure 3: Effect of anolyte concentration on current efficiency and energy consumption.
3.5 Effect of chloride concentration on splitting of ammonium sulphate
On the basis of the above results, a set of experiments performed in which the additive i.e.
chloride concentration was varied from 0 to 2 gpl in the catholyte, to know its effectiveness on the splitting rate, current efficiency and energy consumption. The rest of the parameters such as applied current density 200 A/m2, ammonium sulphate concentration in catholyte 200 gpl (100ml) (14.53 g of sulphate in catholyte), anolyte composition; 100 ml of 10 gpl sulphuric acid and duration of electrolysis 3h. All the experiments were conducted at room temperature. The results with respect to loss of sulphate from catholyte, splitting of amount ammonia and average cell voltage is given in Table-5. The chloride concentration at catholyte increased the loss of sulphate from catholyte and increase in splitting of ammonia. The average cell voltage was 3.9 V for all the experiments. The effect of chloride concentration at catholyte on current efficiency and energy consumption is given in Fig-4. It was observed that as chloride concentration in catholyte increased from 0-2 gpl, the current efficiency increased from 35.91 to 54.6%. The energy consumption decreased from 14.95 – 10.72 kWh/kg.
Table-5 :Effect of chloride concentration at catholyte on splitting of ammonium sulphate Sl.
No .
Chloride concentratio n (gpl)
Loss of sulphat e from catholy te (g)
Splitting of amount of ammonia in catholyte (g)
Averag e cell voltage (V)
1 0 2.34 0.82 3.9
2 0.5 2.38 0.85 3.9
3 1.0 2.44 0.87 3.9
4 1.5 2.59 0.92 3.9
5 2.0 2.79 0.99 3.9
Figure-4: Effect of chloride concentration on current efficiency and energy consumption.
IV. CONCLUSION
The electrochemical rout can provide an excellent green path for recovery of ammonia and sulphuric acid from waste ammonium sulphate in industries, especially in metallurgical industries. The presence of chloride in the catholyte during electrochemical splitting of ammonium sulphate enhances the rate of the reaction and increase the current efficiency.
At higher temperature (~600C) the current efficiency and energy consumption becomes 95.47% and 6.05 kWhr/kg respectively. Thus this electrochemical process for splitting of ammonium sulphate has the potential for commercialization.
The increase in anolyte concentration increases the current efficiency up to 91.20 %, and the acid is formed during the electrolytic process at anolyte.
The applied current density needs to be at and around 200 /m2, during electrochemical splitting of ammonium sulphate in the presence of 1 gpl chloride at catholyte. Again the energy consumption can further be reduced by using gas diffusion electrodes known in principle from fuel cell technology.
V.ACKNOWLEDGEMENT
The authors wish to thank Prof. Dr. (Mrs.) Banaja Mohanty, Principal, M. P. C. (Autonomous) College, Takatpur, Baripad, and Prof. S. N.
Sarangi, Head, Department of Chemistry, M. P. C.
(Autonomous) College, Takatpur, Baripa, for encouragement and moral support.
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International Society and Polar Engineers, pp. 165, 1997.
[3] T. A. Regan, An investigation into ammonia recovery by lime boiling (UG Thesis), University of Queensland, Brisbane, Australia, 1999.
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