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Test results of a newly proposed neutralization process to reduce and utilize the sludge
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Minerals Engineering 21 (2008) 310–316 This article is also available online at: www.elsevier.com/locate/mineng
Test results of a newly proposed neutralization process to reduce and utilize the sludge Nobuyuki Masuda a,*, Koichi Hashimoto a, Hideo Asano a, Eiji Matsushima b, Shoichi Yamaguchi b a
Japan Oil, Gas and Metals National Corporation, Muza Kawasaki Central Tower, Omiya-cho 1310, Saiwai-ku Kawasaki 212-8554, Japan b Dowa Techno Engineering Co., Ltd., 31-10 Chikko Sakaemachi, Okayama 702-8053, Japan Received 30 July 2007; accepted 11 October 2007 Available online 8 February 2008
Abstract Sludge from AMD is difficult to handle and dispose, because of the contained toxic substances, such as arsenic, cadmium, etc. To reduce the sludge, the bacterial oxidation and two step neutralization process was proposed and tested by a bench scale plant. The test was carried out by treating the AMD from the abandoned Horobetsu sulfur mine in Japan, which contains about 300 mg/L ferrous ion and 10 mg/L arsenic with pH 1.8. In this process, ferrous ion is oxidized by bacteria, and the AMD neutralizes to pH 3.5 by calcium carbonate in the first step, and neutralized up to pH 7.0 by carbon hydroxide in the second step. The results of the test include, Fe and As contained sludge was generated at the first step and lower As contained sludge was generated at the second step; the sludge volume was reduced by 30% and the second step’s sludge possibly be used as a industrial material. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Acid rock drainage; pH control; Bacteria; Environment; Pollution
1. Introduction Acid mine drainage (AMD) treatment plants are generally located in the mountain areas in Japan. Therefore, appropriate and sufficient spaces can scarcely be obtained in their vicinity to store the heavy metals containing sludge. The cost of the sludge disposal as of toxic waste is a heavy burden on the AMD treatment and is one of the serious problems for AMD treatment of the abandoned mines. Nearly half of the total cost of AMD treatment operation in Japan is spent on sulfur and/or iron sulfide mines, which account for the largest number of the abandoned mines. An optimum treatment system is urgently required to reduce the sludge generation and disposal cost. AMD of abandoned Horobetsu sulfur mine, which is located in the northern part of Japan, has the typical water *
Corresponding author. Tel.: +81 44 520 8562; fax: +81 44 520 8730. E-mail address: [email protected] (N. Masuda).
0892-6875/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2007.10.014
quality of sulfur and iron sulfide mine; low pH (1.8–1.9) with iron (about 350 mg/L) and arsenic (about 10 mg/L). Average outflow is about 4 m3/min. Existing AMD treatment plant neutralizes the AMD to pH 7.5 by calcium carbonate and calcium hydroxide. Disposal cost of the generating sludge on the treatment cost is a heavy burden. As disposal spaces are not sufficient for future operation in long term, sludge reducing technology is strongly required. Fig. 1 is the flow chart of existing Horobetsu AMD treatment plant. Unit price of calcium hydroxide is about 2.5 times higher than that of calcium carbonate in the Horobetsu region in Japan. Therefore, calcium carbonate is preferred cost wise for neutralization. High Fe(II) and low pH AMD is treated by oxidizing Fe(II) to Fe(III) to reduce the consumption of neutralizing reagent and generating easy dewatering sludge. The higher pH precedes the faster oxidation. The existing Horobetsu plant is designed under these concepts.
N. Masuda et al. / Minerals Engineering 21 (2008) 310–316
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Fig. 1. Conceptual flow chart of existing Horobetsu AMD treatment plant.
However, in the existing process, pH is controlled up to 6–6.6 by calcium carbonate dosing, which has lower reactivity than calcium hydroxide. Therefore, the consumption of the calcium carbonate is much more than the necessary amount compared to the amount calculated by the acidity of AMD. As a result, generated sludge contains a lot of unreacted calcium carbonate. It was observed by an investigation that the average of calcium carbonate content in solid components of the dewatered sludge in Horobetsu is approximately 35%. It is suggested that appropriate amount of calcium carbonate dosing and effective reaction are necessary measures (Metal Mining Agency of Japan, 2003). 2. Theory of the proposed process Fig. 2 shows a conceptual flow chart of the proposed ‘bacterial oxidation and two step neutralization process’. In this process, almost 100% of Fe(II) ion can be oxidized by utilizing bacterial oxidation in the comparatively low pH AMD. Contained iron can be removed at pH 3.5 by calcium carbonate with appropriate amount of dosing on the acidity of AMD. It is well known that precipitated iron in the condition of around pH 3–4 absorb arsenic contained in the AMD. Therefore bacterial oxidation, which proceed the Fe(II) oxidation, is effective to restrain arsenic solution from the sludge. Further more, the sludge generated from the neutralization part, which is comparatively low arsenic content, can possibly be used as a industrial material, such as a raw materials of cement production, construction materials, etc. Utilizing bacteria is identified as Acidithiobacillus ferrooxidans, which commonly exists in the Matsuo AMD and
oxidize ferrous iron (Fe(II)) to ferric iron (Fe(III)). Ironoxidizing bacteria is most active in the condition of about pH 2.5 and brings out high oxidizing ability (Imai, 1984; Shioda, 1989). It is necessary to use carrier beds for bacteria to keep the necessary number for oxidation in the AMD flow with maintaining the bacteria’s vitality. In case of treating the AMD in the vicinity of pH 2.5, which is the most preferable pH condition for bacteria vitality, the sludge circulation is effective to keep the bacteria number and vitality. The sludge can be worked as carrier beds. However, in case of the AMD lower than pH 2.0, iron solubility increases and the sludge resolves in the AMD. The sludge cannot be used for bacteria carrier any more. In this case, diatomite or other materials can be used for the bacteria carrier and it is reused by recycling. In this study, the ‘bacterial oxidation and two step neutralization process’ was proposed and tested. This process enables the bacteria to maintain vitality and to neutralize sufficiently by recycling the sludge generated in the first step process. The points of the theory can be described as – living conditions for bacteria were maintained even when the sludge was recycled in the low pH (less than pH 2.0) AMD. Because the sludge, which consists of iron hydroxide, basic-iron-sulfate, etc. consumes sulfate ion along with its resolving process and raise the pH. Furthermore the bacterial oxidation process raises the pH as well. – pH control condition is set up at pH 3.5, because ferric iron solubility decreases drastically at around pH 3.5 and the most soluble arsenic is absorbed by the iron
Fig. 2. Conceptual flow chart of the proposed process.
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sludge at around this pH point. As a result, aluminum dominant sludge, which has low content of arsenic, can be obtained after the neutralization to pH 7.0 by calcium hydrate (slaked lime). Reactions in the iron-oxidizing tanks and pH control tank are described as following three chemical formulas. Generation of basic-iron-sulfate: Fe(OH)3 + H2 SO4 ! Fe(OH)SO4 + H2 O
ð1Þ
Oxidation of Fe(II) by bacteria: 4FeSO4 + O2 + 2H2 SO4 ! 2Fe2 (SO4 )3 + 2H2 O
A
ð2Þ
Precipitation of Fe: þ Fe2 ðSO4 Þ3 þ 2H2 O ! 2FeðOHÞSO4 þ SO2 4 þ 2H
Proposed ‘bacterial oxidation and two step neutralization process’ is designed as follows: – To maintain the vitality of iron-oxidizing bacteria, increment circumstance and living circumstance are separated. – In the increment circumstance, the reaction is limited to the iron oxidation, and there is no dosage of the reagent for pH control. Calcium carbonate or other neutralizer, which react faster than iron oxidation, is never dosed in this step.
B C
D A - Oxidation Tank : 400mmϕcolumn and corn bottom x 4 units (100 liter each) B - Bacteria Recovery Tank : 1000mmϕcolumn and corn bottom, with lake (560 liter) C - Neutralization Tank : 440mmϕ(30 liter) D - Solid Liquid Separation Tank (Thickener) : 1000mmϕcolumn and corn bottom with lake (560 liter)
Fig. 3. Picture of the experimental plant and specification.
Calcium carbonate dissolver
Polymer flocculent mixing tank
AMD reservoir tank
Slaked lime dissolver
Flocculation tank No.1
Bacteria recovery tank
No.2
Flocculation reaction tank
Air
Oxidation tank
Iron precipitation tank
pH2. 5
pH 3.5
Sludge recycling for bacteria carrier (Fe, As)
ð3Þ
Neutralization tank (pH7) Recycle of neutralized sludge (from thickener) (Al dominated sludge)
Fig. 4. Flow chart of the experimental plant.
Solid liquid separation (thickener) Treated water
N. Masuda et al. / Minerals Engineering 21 (2008) 310–316
– After sufficient iron oxidation, pH is controlled to 3.5 by calcium carbonate dosage. – To proceed the increment of bacteria by recycling a part of the sludge generated at pH 3.5. The generated sludge at this stage is dominated by iron with arsenic. – At the second stage, AMD is neutralized to pH 7.0 by slake lime dosage. The generated sludge at this stage is dominated by aluminum with low arsenic content.
Table 1 Conditions of the continuous treatment tests for proposed process AMD Oxidizing process
Neutralizing process
Treatment volume (L/min) Recycled volume of bacteria sludge (L/min) Controlled pH by calcium carbonate Controlled pH by slaked lime (neutralization) Recycled sludge (L/min)
Duration
3 0.3 Approximately 3.5 Approximately 7 0.3 21 days
3. Experiment 3.1. Experimental procedure 3.1.1. Process and test conditions Proposed process was tested on the pilot plant facilities. Continuous operations were carried out for about 3 weeks. Stability was confirmed and the sludge quality was analyzed (Japan Oil, Gas and Metals National Corporation, 2007). Fig. 3 shows a picture of the experimental plant and its general specifications, and Fig. 4 shows the flow chart of the experimental plant. Table 1 shows the conditions of the continuous treatment tests for proposed process. 3.1.2. Quality of the AMD Quality of the AMD raw water is shown in Figs. 5 and 6. The average quality is pH 1.79, Fe2+ 322 g/L, total Fe 374 mg/L, As 10 mg/L, Al 50 mg/L, and SO2 4 2484 mg/L.
Fig. 7. Trend of oxidation ratio of Fe(II). Fig. 5. Iron and pH changes in AMD.
Fig. 6. Al, As, SO2 4 changes in AMD.
313
Fig. 8. pH trend in the oxidation tank.
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3.2. Results
Table 3 Average grain size of the sludge (unit: lm)
3.2.1. Oxidation ratio of Fe(II) Fig. 7 shows the trend of oxidation ratio of Fe(II) throughout the test period. Oxidation ratio of Fe(II) was calculated from the amount of total Fe and Fe(III) in the treated water. In the oxidation tank No. 1, oxidation was unstable at the beginning, and then stabilized to more than 99%. Retention time at the oxidation tank No. 1 was 30 min. In the oxidation tank No. 2, oxidation ratio was kept in more than 99% at any time. Retention time at the oxidation tank No. 2 was 60 min.
Sampling point
3.2.2. The pH in the oxidation tanks Fig. 8 shows the pH trend in the oxidation tank during the test period. It indicates that in the oxidation tank No. 1, pH was fluctuated in the vicinity of 2.3, and in the oxidation tank No. 2. ‘pH’ was fluctuated in the vicinity of 2.4. As the result, it was successful to keep the pH in the vicinity of 2.5, which is well suited to keep bacteria vitality and oxidation ability. 3.2.3. Consumption of the neutralizer Table 2 shows the average amount of dosed reagents and 8.3Ax of the raw AMD during the test period. Calcium carbonate was dosed with 40 g/L milk density and slaked lime was dosed with 10 g/L milk density. Consumption of the equivalent volume of neutralization reagents to the 8.3Ax of the raw AMD was 0.98. 8.3Ax is the alkali consumption to make acid water to pH 8.3. 3.2.4. Analysis on the generated sludge Average grain size of the sludge was shown in Table 3. Average grain size of the recycled bacteria sludge fluctuated in the vicinity of 8–9 lm. Diluted sulfuric acid addition did not affect the grain size. Average grain size of the sludge from the solid liquid separation tank was fluctuated between 6 and 7 lm except 15 lm at the beginning of the test period. Composition ratio of the sludge was estimated from the sludge components. Table 4 is the composition ratio of the bacteria oxidation process sludge, and Table 5 is the composition of the neutralization process sludge. More than 90% of the recycled bacteria sludge is iron compounds, a little less than 3% is aluminum compound,
Table 2 Average amount of dosed reagents and the 8.3Ax of the raw AMD 8.3Ax of AMD (a) Calcium carbonate Slaked lime Total neutralization agent (b) (b)/(a)
meq mg/L meq mg/L meq meq
47.1 1948 39.0 263 7.1 46.1 0.98
Days
Oxidizing tank No. 1 Oxidizing tank No. 2 Iron precipitation tank Recycled bacteria sludge Neutralization tank Recycling neutralized sludge
5
9
13
17
21
8.7 8.9 9.2 8.8 15.8 14.9
9.2 9.4 8.7 8.9 7.7 8.7
8.4 8.5 8.7 9.5 6.4 6.7
8.6 8.3 9.2 8.3 6.4 6.8
8.6 8.1 8.6 8.1 7.0 6.7
Table 4 Estimated composition ratio of the bacteria oxidation process sludge (unit: %) Components
FeOOH Fe(OH)SO4 Al(OH)3 CaCO3 SiO2 As
Days 5
9
13
17
21
65.1 29.5 2.5 0.2 1.2 1.6
66.9 27.9 2.4 0.3 1.0 1.5
66.0 28.5 2.6 0.2 1.2 1.5
66.1 28.3 2.8 0.2 1.1 1.5
68.2 26.8 2.5 0.2 1.0 1.3
Table 5 Estimated composition ratio of the neutralization process (unit: %) Components
FeOOH Al(OH)3 CaCO3 CaSO4 2H2O SiO2 As
Days 5
9
13
17
21
13.9 50.8 2.1 26.2 6.7 0.25
13.7 42.0 3.3 35.1 5.7 0.23
12.0 37.4 3.3 41.7 5.4 0.21
11.6 36.4 1.7 44.5 5.7 0.20
13.0 35.3 1.5 45.2 4.8 0.17
and a little less than 2% is arsenic. Calcium compound is less than 0.2%. Sludge from thickener after neutralization mainly consists of aluminum compound and gypsum. About 10% of iron compounds, 2% of calcium carbonate, and 0.2% of arsenic are contained. As the test goes by, gypsum content was increased and aluminum was decreased. It was assumed that SO4 exists in the iron sludge, and the molecular formula of iron sludge was expressed FeOOH and Fe(OH)SO4. All the SO4 was used for gypsum generation and Ca, which was not used for gypsum, exist as CaCO3. Pulp density in the oxidation process is shown in Fig. 9 and pulp density of recycled neutralized sludge (thickener sludge) is shown in Fig. 10. Pulp density in oxidation process started from 200 g/L, and gradually increased to about 600 g/L. At the end of the tests, sludge density in the neutralization tank was about 15 g/L, and the recycled sludge from the thickener was about 200 g/L.
N. Masuda et al. / Minerals Engineering 21 (2008) 310–316
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Table 7 Volume of the generated sludge (dry weight) and sludge from the existing plant
Fig. 9. Pulp density in the oxidation process.
Item
Test
Existing plant data (2006)
T–Fe in AMD(mg/L) Fe ratio in the recycled bacteria sludge(%) Sludge generation from the bacteria recovery tank (kg-D S/m3) Suspended solid in the over flow of bacteria recovery tank (kg-D S/m3) Sludge extracted from bacteria recovery tank (kg-D S/m3) Al density in over flow of the bacteria recovery tank (mg/L) Al ratio in the thickener sludge (%) Fe ratio in the thickener sludge (%) Sludge generation from the neutralization (thickener) (kg-D S/m3) Total sludge generation (kg-D S/m3) New process/existing process
374 51.9 0.721
366 – –
0.061
–
0.660
–
47.2
–
12.4 – 0.381
– 24.4 1.500
1.041 0.69
1.500 –
Table 8 Consumption of the neutralizer Items 8.3Ax of AMD (a) Calcium carbonate
meq/L mg/L meq/L mg/L meq/L meq/L
Slaked lime Total of neutralizer (b) (a)/(b)
Fig. 10. Pulp density of recycled neutralized sludge (thickener sludge).
4. Discussion 4.1. pH Table 6 shows the water quality of oxidation process. Water quality on the entrance of oxidation tank No. 1 is calculated from the measured data on raw AMD and bacteria sludge recycle. The pH behavior in the oxidation tanks can be reexamined by these figures.
Test
Existing plant
47.1 1948 39.0 263 7.1 46.1 0.98
43.3 2035 40.7 320 8.7 49.7 1.15
The pH change was observed from pH 1.83 at the entrance of oxidation tank No. 1 to pH 2.39 in the oxidation tank No. 2. According to the formula (1), pH change in the process can be calculated from SO2 content. Measured data of 4 SO2 is that it was changed from 2324 mg/L in raw 4 AMD to 2058 mg/L in the treated water. Assuming that 100% of sulfuric acid is dissociated, SO2 difference of 4 266 mg/L is equivalent to the consumption of H+ 5.5 103 mol. H+ amount in AMD of pH 1.83 is consumed then the pH changes to 2.03. According to the formula (2), oxidation of Fe(II) consumes H+ and changes pH. Measured data is that Fe(II) in AMD is 287 mg/L and after the oxidization Fe(II)
Table 6 Water quality of oxidation process
Volume (L/min) pH Fe2+ (mg/L) T–Fe (mg/L) SO2þ 4 (mg/L) a b
Measured data. Calculated data.
Raw AMDa
Bacteria sludge recyclea
Entrance of oxidation tank No. 1b
Oxidation tank No. 1a
Oxidation tank No. 2a
3.0 1.79 316 374 2342
0.3 3.71 0.2 0.7 2148
3.3 1.83 287 340 2324
2.30 1.9 368 2058
2.39 0.9 308 2004
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decrease to 0.9 mg/L. 1.0 mol of H+ is required to oxidize 1.0 mol of Fe2+. Consumed H+ is 5.1 103 mol. Therefore, pH 2.03 increases to 2.38. On the other hand, soluble Fe decreased 340 mg/L to 308 mg and H+ increased, which is indicated on the formula (3). Assumed all this H+ increase contributes to pH changes, pH change is calculated from 2.38 to 2.32. Measured data on pH change are from 1.83 to 2.39. It was estimated that pH changes from 1.83 to 2.32 based on the theory. It is well indicated that the reaction of the new process is reasonable enough from a theoretical point. 4.2. Sludge volume Sludge volume from the proposed process was calculated and compared to the existing plant’s sludge data. Volume of the generated sludge (dry weight) is shown in Table 7. Sludge generation from the proposed process is about 30% less than from the existing plant. If the aluminum dominated sludge from the thickener is utilized, the volume of the waste will be reduced 25% more. 4.3. Neutralizer Table 8 shows consumption of the neutralizer in the test, such as calcium carbonate and slaked lime, and compared to the existing plant’s consumption data. This data indicate that the new process reduced 15% of consumption, because the bacterial oxidation and pH control were successful. 5. Conclusions In this study, the ‘bacterial oxidation and two step neutralization process’, which enables the bacteria to maintain vitality and to neutralize sufficiently by recycling the sludge generated in the first step process, was proposed and tested.
In the oxidation tank No. 2, oxidation ratio was kept in more than 99% at all times. Retention time at the oxidation tank No. 2 was 60 min, which is practical. It was successful to keep the pH in the vicinity of 2.5, which is well-suited to keep bacteria vitality and oxidation ability. More than 90% of the recycled bacteria sludge, which is the first step sludge, is iron. A little less than 3% is aluminum, and a little less than 2% is arsenic. Calcium is less than 0.2%. Sludge from the thickener, which is the second step, mainly consists of aluminum and gypsum. About 10% of iron, 2% of calcium carbonate, and 0.2% of arsenic are contained, but arsenic content is much smaller than in the first step sludge. The pH behavior in the oxidation tanks was reexamined. Measured data on pH change ranges from 1.83 to 2.39. It was estimated that pH changes from 1.83 to 2.32 based on the theory. It is well indicated that the reaction of the new process is reasonable enough from the theoretical point. Sludge generation from the proposed process is about 30% less than from the existing plant. If the aluminum dominated sludge from the thickener is utilized, the volume of the waste is reduced 25% less on top of 30% reduction. Test data indicates that the new process reduced 15% of consumption of neutralizer, because the bacterial oxidation and pH control were successful. References Imai, Kazumi, 1984. Independent Nourishment Bacteria. Bio-Science Series, p. 187. Japan Oil, Gas and Metals National Corporation, 2007. Report on Technology Development for Sludge Volume Reduction. Metal Mining Agency of Japan, 2003. Report on Technology Development for the Energy Rationalization of Mine Drainage Treatment. Shioda, Hideo, 1989. Microbiology with distinguished functions and it’s application. Biology and Polymer Research 4, 189–211.
Minerals Engineering 21 (2008) 310–316 This article is also available online at: www.elsevier.com/locate/mineng
Test results of a newly proposed neutralization process to reduce and utilize the sludge Nobuyuki Masuda a,*, Koichi Hashimoto a, Hideo Asano a, Eiji Matsushima b, Shoichi Yamaguchi b a
Japan Oil, Gas and Metals National Corporation, Muza Kawasaki Central Tower, Omiya-cho 1310, Saiwai-ku Kawasaki 212-8554, Japan b Dowa Techno Engineering Co., Ltd., 31-10 Chikko Sakaemachi, Okayama 702-8053, Japan Received 30 July 2007; accepted 11 October 2007 Available online 8 February 2008
Abstract Sludge from AMD is difficult to handle and dispose, because of the contained toxic substances, such as arsenic, cadmium, etc. To reduce the sludge, the bacterial oxidation and two step neutralization process was proposed and tested by a bench scale plant. The test was carried out by treating the AMD from the abandoned Horobetsu sulfur mine in Japan, which contains about 300 mg/L ferrous ion and 10 mg/L arsenic with pH 1.8. In this process, ferrous ion is oxidized by bacteria, and the AMD neutralizes to pH 3.5 by calcium carbonate in the first step, and neutralized up to pH 7.0 by carbon hydroxide in the second step. The results of the test include, Fe and As contained sludge was generated at the first step and lower As contained sludge was generated at the second step; the sludge volume was reduced by 30% and the second step’s sludge possibly be used as a industrial material. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Acid rock drainage; pH control; Bacteria; Environment; Pollution
1. Introduction Acid mine drainage (AMD) treatment plants are generally located in the mountain areas in Japan. Therefore, appropriate and sufficient spaces can scarcely be obtained in their vicinity to store the heavy metals containing sludge. The cost of the sludge disposal as of toxic waste is a heavy burden on the AMD treatment and is one of the serious problems for AMD treatment of the abandoned mines. Nearly half of the total cost of AMD treatment operation in Japan is spent on sulfur and/or iron sulfide mines, which account for the largest number of the abandoned mines. An optimum treatment system is urgently required to reduce the sludge generation and disposal cost. AMD of abandoned Horobetsu sulfur mine, which is located in the northern part of Japan, has the typical water *
Corresponding author. Tel.: +81 44 520 8562; fax: +81 44 520 8730. E-mail address: [email protected] (N. Masuda).
0892-6875/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2007.10.014
quality of sulfur and iron sulfide mine; low pH (1.8–1.9) with iron (about 350 mg/L) and arsenic (about 10 mg/L). Average outflow is about 4 m3/min. Existing AMD treatment plant neutralizes the AMD to pH 7.5 by calcium carbonate and calcium hydroxide. Disposal cost of the generating sludge on the treatment cost is a heavy burden. As disposal spaces are not sufficient for future operation in long term, sludge reducing technology is strongly required. Fig. 1 is the flow chart of existing Horobetsu AMD treatment plant. Unit price of calcium hydroxide is about 2.5 times higher than that of calcium carbonate in the Horobetsu region in Japan. Therefore, calcium carbonate is preferred cost wise for neutralization. High Fe(II) and low pH AMD is treated by oxidizing Fe(II) to Fe(III) to reduce the consumption of neutralizing reagent and generating easy dewatering sludge. The higher pH precedes the faster oxidation. The existing Horobetsu plant is designed under these concepts.
N. Masuda et al. / Minerals Engineering 21 (2008) 310–316
311
Fig. 1. Conceptual flow chart of existing Horobetsu AMD treatment plant.
However, in the existing process, pH is controlled up to 6–6.6 by calcium carbonate dosing, which has lower reactivity than calcium hydroxide. Therefore, the consumption of the calcium carbonate is much more than the necessary amount compared to the amount calculated by the acidity of AMD. As a result, generated sludge contains a lot of unreacted calcium carbonate. It was observed by an investigation that the average of calcium carbonate content in solid components of the dewatered sludge in Horobetsu is approximately 35%. It is suggested that appropriate amount of calcium carbonate dosing and effective reaction are necessary measures (Metal Mining Agency of Japan, 2003). 2. Theory of the proposed process Fig. 2 shows a conceptual flow chart of the proposed ‘bacterial oxidation and two step neutralization process’. In this process, almost 100% of Fe(II) ion can be oxidized by utilizing bacterial oxidation in the comparatively low pH AMD. Contained iron can be removed at pH 3.5 by calcium carbonate with appropriate amount of dosing on the acidity of AMD. It is well known that precipitated iron in the condition of around pH 3–4 absorb arsenic contained in the AMD. Therefore bacterial oxidation, which proceed the Fe(II) oxidation, is effective to restrain arsenic solution from the sludge. Further more, the sludge generated from the neutralization part, which is comparatively low arsenic content, can possibly be used as a industrial material, such as a raw materials of cement production, construction materials, etc. Utilizing bacteria is identified as Acidithiobacillus ferrooxidans, which commonly exists in the Matsuo AMD and
oxidize ferrous iron (Fe(II)) to ferric iron (Fe(III)). Ironoxidizing bacteria is most active in the condition of about pH 2.5 and brings out high oxidizing ability (Imai, 1984; Shioda, 1989). It is necessary to use carrier beds for bacteria to keep the necessary number for oxidation in the AMD flow with maintaining the bacteria’s vitality. In case of treating the AMD in the vicinity of pH 2.5, which is the most preferable pH condition for bacteria vitality, the sludge circulation is effective to keep the bacteria number and vitality. The sludge can be worked as carrier beds. However, in case of the AMD lower than pH 2.0, iron solubility increases and the sludge resolves in the AMD. The sludge cannot be used for bacteria carrier any more. In this case, diatomite or other materials can be used for the bacteria carrier and it is reused by recycling. In this study, the ‘bacterial oxidation and two step neutralization process’ was proposed and tested. This process enables the bacteria to maintain vitality and to neutralize sufficiently by recycling the sludge generated in the first step process. The points of the theory can be described as – living conditions for bacteria were maintained even when the sludge was recycled in the low pH (less than pH 2.0) AMD. Because the sludge, which consists of iron hydroxide, basic-iron-sulfate, etc. consumes sulfate ion along with its resolving process and raise the pH. Furthermore the bacterial oxidation process raises the pH as well. – pH control condition is set up at pH 3.5, because ferric iron solubility decreases drastically at around pH 3.5 and the most soluble arsenic is absorbed by the iron
Fig. 2. Conceptual flow chart of the proposed process.
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N. Masuda et al. / Minerals Engineering 21 (2008) 310–316
sludge at around this pH point. As a result, aluminum dominant sludge, which has low content of arsenic, can be obtained after the neutralization to pH 7.0 by calcium hydrate (slaked lime). Reactions in the iron-oxidizing tanks and pH control tank are described as following three chemical formulas. Generation of basic-iron-sulfate: Fe(OH)3 + H2 SO4 ! Fe(OH)SO4 + H2 O
ð1Þ
Oxidation of Fe(II) by bacteria: 4FeSO4 + O2 + 2H2 SO4 ! 2Fe2 (SO4 )3 + 2H2 O
A
ð2Þ
Precipitation of Fe: þ Fe2 ðSO4 Þ3 þ 2H2 O ! 2FeðOHÞSO4 þ SO2 4 þ 2H
Proposed ‘bacterial oxidation and two step neutralization process’ is designed as follows: – To maintain the vitality of iron-oxidizing bacteria, increment circumstance and living circumstance are separated. – In the increment circumstance, the reaction is limited to the iron oxidation, and there is no dosage of the reagent for pH control. Calcium carbonate or other neutralizer, which react faster than iron oxidation, is never dosed in this step.
B C
D A - Oxidation Tank : 400mmϕcolumn and corn bottom x 4 units (100 liter each) B - Bacteria Recovery Tank : 1000mmϕcolumn and corn bottom, with lake (560 liter) C - Neutralization Tank : 440mmϕ(30 liter) D - Solid Liquid Separation Tank (Thickener) : 1000mmϕcolumn and corn bottom with lake (560 liter)
Fig. 3. Picture of the experimental plant and specification.
Calcium carbonate dissolver
Polymer flocculent mixing tank
AMD reservoir tank
Slaked lime dissolver
Flocculation tank No.1
Bacteria recovery tank
No.2
Flocculation reaction tank
Air
Oxidation tank
Iron precipitation tank
pH2. 5
pH 3.5
Sludge recycling for bacteria carrier (Fe, As)
ð3Þ
Neutralization tank (pH7) Recycle of neutralized sludge (from thickener) (Al dominated sludge)
Fig. 4. Flow chart of the experimental plant.
Solid liquid separation (thickener) Treated water
N. Masuda et al. / Minerals Engineering 21 (2008) 310–316
– After sufficient iron oxidation, pH is controlled to 3.5 by calcium carbonate dosage. – To proceed the increment of bacteria by recycling a part of the sludge generated at pH 3.5. The generated sludge at this stage is dominated by iron with arsenic. – At the second stage, AMD is neutralized to pH 7.0 by slake lime dosage. The generated sludge at this stage is dominated by aluminum with low arsenic content.
Table 1 Conditions of the continuous treatment tests for proposed process AMD Oxidizing process
Neutralizing process
Treatment volume (L/min) Recycled volume of bacteria sludge (L/min) Controlled pH by calcium carbonate Controlled pH by slaked lime (neutralization) Recycled sludge (L/min)
Duration
3 0.3 Approximately 3.5 Approximately 7 0.3 21 days
3. Experiment 3.1. Experimental procedure 3.1.1. Process and test conditions Proposed process was tested on the pilot plant facilities. Continuous operations were carried out for about 3 weeks. Stability was confirmed and the sludge quality was analyzed (Japan Oil, Gas and Metals National Corporation, 2007). Fig. 3 shows a picture of the experimental plant and its general specifications, and Fig. 4 shows the flow chart of the experimental plant. Table 1 shows the conditions of the continuous treatment tests for proposed process. 3.1.2. Quality of the AMD Quality of the AMD raw water is shown in Figs. 5 and 6. The average quality is pH 1.79, Fe2+ 322 g/L, total Fe 374 mg/L, As 10 mg/L, Al 50 mg/L, and SO2 4 2484 mg/L.
Fig. 7. Trend of oxidation ratio of Fe(II). Fig. 5. Iron and pH changes in AMD.
Fig. 6. Al, As, SO2 4 changes in AMD.
313
Fig. 8. pH trend in the oxidation tank.
314
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3.2. Results
Table 3 Average grain size of the sludge (unit: lm)
3.2.1. Oxidation ratio of Fe(II) Fig. 7 shows the trend of oxidation ratio of Fe(II) throughout the test period. Oxidation ratio of Fe(II) was calculated from the amount of total Fe and Fe(III) in the treated water. In the oxidation tank No. 1, oxidation was unstable at the beginning, and then stabilized to more than 99%. Retention time at the oxidation tank No. 1 was 30 min. In the oxidation tank No. 2, oxidation ratio was kept in more than 99% at any time. Retention time at the oxidation tank No. 2 was 60 min.
Sampling point
3.2.2. The pH in the oxidation tanks Fig. 8 shows the pH trend in the oxidation tank during the test period. It indicates that in the oxidation tank No. 1, pH was fluctuated in the vicinity of 2.3, and in the oxidation tank No. 2. ‘pH’ was fluctuated in the vicinity of 2.4. As the result, it was successful to keep the pH in the vicinity of 2.5, which is well suited to keep bacteria vitality and oxidation ability. 3.2.3. Consumption of the neutralizer Table 2 shows the average amount of dosed reagents and 8.3Ax of the raw AMD during the test period. Calcium carbonate was dosed with 40 g/L milk density and slaked lime was dosed with 10 g/L milk density. Consumption of the equivalent volume of neutralization reagents to the 8.3Ax of the raw AMD was 0.98. 8.3Ax is the alkali consumption to make acid water to pH 8.3. 3.2.4. Analysis on the generated sludge Average grain size of the sludge was shown in Table 3. Average grain size of the recycled bacteria sludge fluctuated in the vicinity of 8–9 lm. Diluted sulfuric acid addition did not affect the grain size. Average grain size of the sludge from the solid liquid separation tank was fluctuated between 6 and 7 lm except 15 lm at the beginning of the test period. Composition ratio of the sludge was estimated from the sludge components. Table 4 is the composition ratio of the bacteria oxidation process sludge, and Table 5 is the composition of the neutralization process sludge. More than 90% of the recycled bacteria sludge is iron compounds, a little less than 3% is aluminum compound,
Table 2 Average amount of dosed reagents and the 8.3Ax of the raw AMD 8.3Ax of AMD (a) Calcium carbonate Slaked lime Total neutralization agent (b) (b)/(a)
meq mg/L meq mg/L meq meq
47.1 1948 39.0 263 7.1 46.1 0.98
Days
Oxidizing tank No. 1 Oxidizing tank No. 2 Iron precipitation tank Recycled bacteria sludge Neutralization tank Recycling neutralized sludge
5
9
13
17
21
8.7 8.9 9.2 8.8 15.8 14.9
9.2 9.4 8.7 8.9 7.7 8.7
8.4 8.5 8.7 9.5 6.4 6.7
8.6 8.3 9.2 8.3 6.4 6.8
8.6 8.1 8.6 8.1 7.0 6.7
Table 4 Estimated composition ratio of the bacteria oxidation process sludge (unit: %) Components
FeOOH Fe(OH)SO4 Al(OH)3 CaCO3 SiO2 As
Days 5
9
13
17
21
65.1 29.5 2.5 0.2 1.2 1.6
66.9 27.9 2.4 0.3 1.0 1.5
66.0 28.5 2.6 0.2 1.2 1.5
66.1 28.3 2.8 0.2 1.1 1.5
68.2 26.8 2.5 0.2 1.0 1.3
Table 5 Estimated composition ratio of the neutralization process (unit: %) Components
FeOOH Al(OH)3 CaCO3 CaSO4 2H2O SiO2 As
Days 5
9
13
17
21
13.9 50.8 2.1 26.2 6.7 0.25
13.7 42.0 3.3 35.1 5.7 0.23
12.0 37.4 3.3 41.7 5.4 0.21
11.6 36.4 1.7 44.5 5.7 0.20
13.0 35.3 1.5 45.2 4.8 0.17
and a little less than 2% is arsenic. Calcium compound is less than 0.2%. Sludge from thickener after neutralization mainly consists of aluminum compound and gypsum. About 10% of iron compounds, 2% of calcium carbonate, and 0.2% of arsenic are contained. As the test goes by, gypsum content was increased and aluminum was decreased. It was assumed that SO4 exists in the iron sludge, and the molecular formula of iron sludge was expressed FeOOH and Fe(OH)SO4. All the SO4 was used for gypsum generation and Ca, which was not used for gypsum, exist as CaCO3. Pulp density in the oxidation process is shown in Fig. 9 and pulp density of recycled neutralized sludge (thickener sludge) is shown in Fig. 10. Pulp density in oxidation process started from 200 g/L, and gradually increased to about 600 g/L. At the end of the tests, sludge density in the neutralization tank was about 15 g/L, and the recycled sludge from the thickener was about 200 g/L.
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315
Table 7 Volume of the generated sludge (dry weight) and sludge from the existing plant
Fig. 9. Pulp density in the oxidation process.
Item
Test
Existing plant data (2006)
T–Fe in AMD(mg/L) Fe ratio in the recycled bacteria sludge(%) Sludge generation from the bacteria recovery tank (kg-D S/m3) Suspended solid in the over flow of bacteria recovery tank (kg-D S/m3) Sludge extracted from bacteria recovery tank (kg-D S/m3) Al density in over flow of the bacteria recovery tank (mg/L) Al ratio in the thickener sludge (%) Fe ratio in the thickener sludge (%) Sludge generation from the neutralization (thickener) (kg-D S/m3) Total sludge generation (kg-D S/m3) New process/existing process
374 51.9 0.721
366 – –
0.061
–
0.660
–
47.2
–
12.4 – 0.381
– 24.4 1.500
1.041 0.69
1.500 –
Table 8 Consumption of the neutralizer Items 8.3Ax of AMD (a) Calcium carbonate
meq/L mg/L meq/L mg/L meq/L meq/L
Slaked lime Total of neutralizer (b) (a)/(b)
Fig. 10. Pulp density of recycled neutralized sludge (thickener sludge).
4. Discussion 4.1. pH Table 6 shows the water quality of oxidation process. Water quality on the entrance of oxidation tank No. 1 is calculated from the measured data on raw AMD and bacteria sludge recycle. The pH behavior in the oxidation tanks can be reexamined by these figures.
Test
Existing plant
47.1 1948 39.0 263 7.1 46.1 0.98
43.3 2035 40.7 320 8.7 49.7 1.15
The pH change was observed from pH 1.83 at the entrance of oxidation tank No. 1 to pH 2.39 in the oxidation tank No. 2. According to the formula (1), pH change in the process can be calculated from SO2 content. Measured data of 4 SO2 is that it was changed from 2324 mg/L in raw 4 AMD to 2058 mg/L in the treated water. Assuming that 100% of sulfuric acid is dissociated, SO2 difference of 4 266 mg/L is equivalent to the consumption of H+ 5.5 103 mol. H+ amount in AMD of pH 1.83 is consumed then the pH changes to 2.03. According to the formula (2), oxidation of Fe(II) consumes H+ and changes pH. Measured data is that Fe(II) in AMD is 287 mg/L and after the oxidization Fe(II)
Table 6 Water quality of oxidation process
Volume (L/min) pH Fe2+ (mg/L) T–Fe (mg/L) SO2þ 4 (mg/L) a b
Measured data. Calculated data.
Raw AMDa
Bacteria sludge recyclea
Entrance of oxidation tank No. 1b
Oxidation tank No. 1a
Oxidation tank No. 2a
3.0 1.79 316 374 2342
0.3 3.71 0.2 0.7 2148
3.3 1.83 287 340 2324
2.30 1.9 368 2058
2.39 0.9 308 2004
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decrease to 0.9 mg/L. 1.0 mol of H+ is required to oxidize 1.0 mol of Fe2+. Consumed H+ is 5.1 103 mol. Therefore, pH 2.03 increases to 2.38. On the other hand, soluble Fe decreased 340 mg/L to 308 mg and H+ increased, which is indicated on the formula (3). Assumed all this H+ increase contributes to pH changes, pH change is calculated from 2.38 to 2.32. Measured data on pH change are from 1.83 to 2.39. It was estimated that pH changes from 1.83 to 2.32 based on the theory. It is well indicated that the reaction of the new process is reasonable enough from a theoretical point. 4.2. Sludge volume Sludge volume from the proposed process was calculated and compared to the existing plant’s sludge data. Volume of the generated sludge (dry weight) is shown in Table 7. Sludge generation from the proposed process is about 30% less than from the existing plant. If the aluminum dominated sludge from the thickener is utilized, the volume of the waste will be reduced 25% more. 4.3. Neutralizer Table 8 shows consumption of the neutralizer in the test, such as calcium carbonate and slaked lime, and compared to the existing plant’s consumption data. This data indicate that the new process reduced 15% of consumption, because the bacterial oxidation and pH control were successful. 5. Conclusions In this study, the ‘bacterial oxidation and two step neutralization process’, which enables the bacteria to maintain vitality and to neutralize sufficiently by recycling the sludge generated in the first step process, was proposed and tested.
In the oxidation tank No. 2, oxidation ratio was kept in more than 99% at all times. Retention time at the oxidation tank No. 2 was 60 min, which is practical. It was successful to keep the pH in the vicinity of 2.5, which is well-suited to keep bacteria vitality and oxidation ability. More than 90% of the recycled bacteria sludge, which is the first step sludge, is iron. A little less than 3% is aluminum, and a little less than 2% is arsenic. Calcium is less than 0.2%. Sludge from the thickener, which is the second step, mainly consists of aluminum and gypsum. About 10% of iron, 2% of calcium carbonate, and 0.2% of arsenic are contained, but arsenic content is much smaller than in the first step sludge. The pH behavior in the oxidation tanks was reexamined. Measured data on pH change ranges from 1.83 to 2.39. It was estimated that pH changes from 1.83 to 2.32 based on the theory. It is well indicated that the reaction of the new process is reasonable enough from the theoretical point. Sludge generation from the proposed process is about 30% less than from the existing plant. If the aluminum dominated sludge from the thickener is utilized, the volume of the waste is reduced 25% less on top of 30% reduction. Test data indicates that the new process reduced 15% of consumption of neutralizer, because the bacterial oxidation and pH control were successful. References Imai, Kazumi, 1984. Independent Nourishment Bacteria. Bio-Science Series, p. 187. Japan Oil, Gas and Metals National Corporation, 2007. Report on Technology Development for Sludge Volume Reduction. Metal Mining Agency of Japan, 2003. Report on Technology Development for the Energy Rationalization of Mine Drainage Treatment. Shioda, Hideo, 1989. Microbiology with distinguished functions and it’s application. Biology and Polymer Research 4, 189–211.