REMOVAL OF MANGANESE AND IRON FROM COAL MINING WASTEWATER BY ACTIVATED LIME MILK AT A SCALE OF 50 LITERS/BATCH

REMOVAL OF MANGANESE AND IRON FROM COAL MINING WASTEWATER BY ACTIVATED LIME MILK AT A SCALE OF 50 LITERS/BATCH

Hoang Van Duc

Ph.D, Institute for Technology of Radioactive and Rare elements,

Vietnam, Ha Noi

Nguyen Van Tung

M.S., Institute for Technology of Radioactive and Rare elements,

Vietnam, Ha Noi

Tran Ngoc Ha

Ph.D, Institute for Technology of Radioactive and Rare elements,

Vietnam, Ha Noi

Hoang Thi Tuyen

B.S., Institute for Technology of Radioactive and Rare elements,

Vietnam, Ha Noi

Hoang Nhuan

Ph.D, Institute for Technology of Radioactive and Rare elements,

Vietnam, Ha Noi

Acknowledgments: This research was financially supported by the Ministry project of VINATOM (code ĐTCB.11/20/VCNXH)

ABSTRACT

The report presents the research results of Fe and Mn ions treatment in underground coal mining wastewater with activated lime milk. Activated lime milk is prepared from lime lumps and water, sifted through a 325 mesh sieve, diluted to a concentration of 10% and has high activity. The object of the study is the wastewater solution of underground coal mining from Nui Nhen coal mine, Quang Ninh province. The obtained results show that activated lime milk is highly effective in simultaneously treating Fe and Mn in wastewater solution, meeting discharge standards according to QCVN40/2011 column B. The use of lime milk reduces both the amount of chemicals consumed and the volume of sludge generated by 60% and 75% comparing to the current practice of using lime powder. In addition, the use of activated lime milk also simplifies the treatment procedure and minimizes the overall cost of the wastewater treatment process.

Keywords: Activated lime milk, treatment of Fe and Mn, coal mining wastewater.

INTRODUCTION

Wastewater in underground coal mining activities contains a large amount of heavy metals (Fe, Mn…). Especially, Mn(II) ions are highly soluble in water when pH ≤ 7, so they are quite widely distributed in underground wastewater sources because this waste source has pH = 2÷6 [1,5]. Manganese(III) and manganese(IV) are insoluble so they are easily separated from the aqueous phase. However, the ability to oxidize manganese (II) ions to Mn(III) or Mn(IV) is more difficult and much slower than the oxidation of iron (II) to iron (III) (that is also present in wastewater from underground mining operations). The oxidation process for manganese treatment is competed by the oxidation of iron ions, when both ions are present in the wastewater solution [5]. This competitive process prevents the oxidation of manganese(II) to higher valence manganese ions. Therefore, manganese remains in the wastewater and causes the wastewater to have a manganese content that always exceeds the allowable limit if there is no effective and appropriate treatment method.

To remove manganese and other heavy metals in wastewater, a number of treatment technologies have been applied such as chemical precipitation, ultrafiltration, adsorption, ion exchange, reverse osmosis and electrolysis [5,12]. Chemical precipitation is a commonly used method in industrial processes, due to its low cost and relatively uncomplicated operation [5]. The hydroxide precipitation technique is the most widely used chemical precipitation technique due to its simplicity, low cost, and ease of pH control. Although most metals are precipitated as hydroxides, other methods such as sulfide and carbonate precipitation are also used [8]. Lime and limestone are the most commonly used substances to treat acidic wastewater containing heavy metals due to their availability in most countries and their low cost [13,19]. The main costs of treatment with hydroxide precipitation are the cost of chemicals and the cost of treating the obtained precipitate.

The reaction equations that occur when alkaline solution (OH^-) is added at the neutralization step without aeration:

Mⁿ⁺ + nOH^-  = M(OH)_n                                                         (1)

Fe²⁺ +2OH^- = Fe(OH)₂                                                       (2)

Adjust the pH of the solution at the neutralization tank to a preset value of about pH = 8. When the pH of the solution is reached, switch to the aeration step for oxidation, at this time Mn²⁺ will form Mn(OH)₃ according to the reaction (3) and Mn will be removed from the solution as a solid:

Mn²⁺ + OH^- + 1/2O₂ + H₂O= Mn(OH)_{3 (r)}                                     (3)

At this time, a part of Fe(OH)₂ from equation (3) will convert to Fe(OH)₃ and release H⁺ according to equation (4):

Fe²⁺ + O₂ + H₂O  = Fe(OH)₃ + 8H⁺                                        (4)

When the treatment is carried on in way neutralization process first, the aeration later will produce Fe(OH)₂ in solid state. The reaction occurring in the oxidation step will follow reaction (3) and (4). At this time, the oxidation is competitive oxidation of solid Fe(OH)₂ and Mn²⁺ ions. Because the oxidation ability of Fe(OH)₂ in solid state is difficult leads to the chance of oxidizing Mn(II) to Mn(III) increase.

Coal mining is one of the key sectors of the mining industry in Vietnam. However, besides the socio-economic benefits, this activity generates waste and wastewater that have a direct negative impact on human health, landscape and ecosystem change. The total mine water volume in Quang Ninh coal basin is about 25–30 million m³/year. Pollution parameters for treatment are: pH; TSS (Total Suspended Solid); Fe; Mn. There are no high concentrations of heavy metals such as Cu, Hg, As, Pb, Zn in the mine waters. In comparison with the Vietnam National Environmental Standard (QCVN40/2011), the maximum concentration of pollution parameters is much higher: TSS is 1.5 – 20 times higher, Fe is 3–35 times higher, Mn is 2–5 times higher. VINACOMIN uses traditional active coal mine water treatment methods, mostly a combination of physical and chemical methods. The treatment technology of coal mine drainage is shown in Fig.1 [20]. This treatment technology uses lime powder as a neutralizing agent and air oxygen as an oxidizing agent to treat Mn and Fe. However, this technology has the disadvantages of using a large amount of chemicals, creating a large amount of sludge, not being able to thoroughly treat Mn at high concentrations, and having high costs. Therefore, this study was carried out to overcome the above disadvantages by using activated lime milk as an alternative to powdered lime.

Figure 1. Coal mine drainage treatment technology at Vietnam [20]

Experimental

Materials

Wastewater source is taken from Nui Nhen coal mine wastewater treatment plant in Quang Ninh province, Vietnam. The concentrations of Fe and Mn in the initial wastewater solution are 84.03 ppm and 5.55 ppm respectively.

Lime is taken from Yen Bai province, Vietnam with a CaO content > 98.5% used as a raw material for the synthesis of activated lime milk.

Some other chemicals are also used such as PAC (poly Aluminum Chloride), PAM (polyacrylamide cationic) to enhance the settling capacity of sludge.

Lime powder is employ. It is using in Nui Nhen coal mine wastewater treatment plant.

Methods

The overall process for preparing activated lime milk and treating Fe and Mn in Nui Nhen coal mine wastewater solution is shown in Fig.2.

Figure 2. Process for treatment of Fe and Mn in coal mining wastewater by activated lime milk at a scale of 50 liters/batch

Fe and Mn content in the initial and post-treatment wastewater are analyzed by inductive plasma emission spectroscopy (ICP-OES) on the ICP-OES HORIBA Ultima-2 device to evaluate the treatment efficiency Fe and Mn.

Particle size distribution of activated lime milk is measured by laser scattering. The surface morphology of lime milk particles is evaluated by scanning electron microscope SEM/EDX JEOL JSM-IT 100; the viscosity of lime milk is measured by Brookfield viscometer;  solution conductivity is measured by Conductivity Meter, S230. The concentration of lime milk solution is determined by chemical titration.

The precipitated residue is determined by the phase composition by Ronghen diffraction (XRD) method.

RESULTS AND DISCUSSION

Preparation of activated lime milk

Activated lime milk was prepared from lime lumps. The main parameters as below:

• Lime lump size: 3-5cm;
• Lime/water ratio by weight: 1/3.5 – 4.0;
• Lime slaked water temperature: 80°C;
• Reaction time: 2 hours;
• Aging time: 12 hours;
• Sieve: 325 mesh;
• Concentration of lime milk: 10%.

Characteristices of activated lime milk product were showed in Table 1.

Table 1.

Some properties of activated lime milk

The preparation of activated lime milk has a recovery efficiency of 85.3%. The recovery efficiency was calculated as the ratio of the mass of CaO after passing through the sieve and the initial amount of lime. The properties of lime milk are shown in Table 1. The high viscosity of lime milk solution (mPa.S) proves that the lime milk particles are small and have relatively uniform particle sizes. The high conductivity of 7752 µS/cm indicates that the dissociation capacity of Ca(OH)₂ is high or the activity of lime milk is high.

The results of particle size distribution (Fig. 3) shows that the size of lime milk particles was mainly from 1 to 4 µm, and are widely distributed at about 3 µm. The SEM image in Fig.4 shows that there is a link between the lime particles and the average size of lime milk on the image surface is about 3.5 µm. Due to the small size, narrow particle size distribution, the dispersion of lime milk is large, the activity is high and the sedimentation rate is slow, lead to the lime milk solution is not deposited during storage. Thus, lime milk prepared by the above process having strong activity increases the dissociation ability when is used to neutralize underground coal mining wastewater containing Fe and Mn.

Treatment wastewater coal mining by activated lime milk

The experimental steps are described as below:

• Raising the pH of the wastewater solution to the desired value with activated lime milk until precipitate of Fe(OH)₂ appears.
• Air from the blower is aerated to oxidize Fe(OH)₂, Mn²⁺, the aeration time is preset.
• Coagulation, flocculation: using chemicals such as PAC, PAM.
• Suspended sediments form large flocs and settle to the bottom.

Parameters in the wastewater treatment process as below:

• Wastewater volume: 50 liters/batch;
• Neutralization pH: 8;
• Air flow: 5 liters/min;
• Aeration time: 30 minutes.

Table 2.

Summary of Fe and Mn treatment results with activated lime milk and lime powder

The results of Table 2 showed that, when Fe and Mn were treated with activated lime milk (with pH = 8), the Fe and Mn content after treatment were 0.12 ppm and 0.51 ppm, respectively, meeting the discharge standards according to QCVN40/2011 column B. The volume of lime milk used was 6.47g and the volume of sludge generated was 16.28g. However, the settling rate of sludge was very slow when coagulants (PAC) and sedimentation aid (PAM) are not used.

When using ordinary lime powder, the Fe and Mn content after treatment were 0.73 ppm and 1.22 ppm, respectively. For the case of Fe, the solution after treatment was met the discharge requirements according to QCVN40/2011 column B. However, for the case of Mn, the concentration of Mn in the solution after treatment was still higher than that of QCVN40/11 column B. In practice, to treat Mn, all wastewater treatment plants use the Mn filter sand system. The mass of lime powder used to achieve the pH = 8 of wastewater solution was 10.53 g and the generates sludge volume of 21.18 g. The generated sludge has a faster settling rate than using activated lime milk when PAC and PAM are not used. However, in practice, all wastewater treatment plants use PAC and PAM to increase the efficiency of sludge settling and filtration. Thus, activated lime milk has higher Fe and Mn removal efficiency than ordinary lime powder.

The results in Table 2 also showed that the use of activated lime milk also significantly reduces the amount of lime milk used (about 60% compared to lime powder) and the amount of sludge generated was also reduced (about 75% compared to lime powder), which reduces the cost of sludge treatment.

When using activated lime milk, Fe precipitation occurs more thoroughly. When lime powder is used, because of the low quality of lime powder and the large size of particles, the lime powder particles are easily encapsulated by the formation of iron hydroxide precipitates (Figure 3) during the neutralization process. This increases the consumption lime powder during Fe and Mn treatment.

Figure 5. XRD diffraction results of the sludge sample

The XRD diffraction in Fig.5 of the sewage sludge sample treated with activated lime milk showed that Fe and Mn in the waste solution precipitated in the form of Fe(OH)₃ and MnOOH, in addition to that other impurities in the solution are also co-precipitate.

Addtion, when activated lime milk used, the reaction is complete in the neutralization tank leads to effective Mn treatment and low sludge production. This saves a considerable amount of chemicals and the Mn filter sand system is not required.

CONCLUSIONS

Activated lime milk was prepared from quicklime lumps and water at the following conditions: slaked water temperature: 80^°C; mass ratio of quicklime/water: 1/3.5 - 1/4.0; reaction time: 2 hours; and the lime milk aging time: about 12 hours. The aged lime milk is then passed through a 325 mesh sieve and adjusted with water to obtain concentration of about 10%. Wastewater samples is collected at the centralized wastewater treatment plant of Nui Nhen coal mine, Quang Ninh Province, Viet Nam. The concentrations of Fe and Mn in the initial wastewater solution are 84.03 ppm and 5.55 ppm respectively. The obtained results show that: at condition of neutralization pH ≥ 8, the treated wastewater has concentrations of Fe and Mn as 0.12 ppm and 0.51 ppm respectively, which meet the discharge standards according to QCVN40/2011 column B. To evaluate the overall effectiveness of the solution (in economic terms) it is necessary to conduct tests on a larger scale before applying it in practice.

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