Desalination 185 (2005) 275–280
Zero discharge utilization of saline waters from ‘‘Wesola’’ coal-mine Marian Tureka*, Piotr Dydob, Andrzej Surmab a
Silesian University of Technology, Faculty of Chemistry, ul. B. Krzywoustego 6, 44–100 Gliwice, Poland email:
[email protected] b Katowice Cole Holding Company, ul. Damrota 16, 40–022 Katowice Received 28 February 2005; accepted 10 March 2005
Abstract A conception of comprehensive utilization of saline waters from ‘‘Wesola’’ coal-mine was proposed. There the water from level 665 m, that contains about 35.45 g/L Cl, is pretreated by nanofiltration (NF) and then evaporated in twig towers while the water from level 465 m of about 2.25 g/L Cl content is desalinated and simultaneously concentrated by electrodialysis (ED) with univalent permselective membranes. Partially evaporated twig tower brine and ED concentrate undergo further thermal evaporation to NaCl concentration up to 300 g/L and then evaporation with salt (NaCl) crystallization. The NF retentate and post-crystallization lyes are mixed with power station ashes and applied as a hydraulic filling in the mine. The profitability of comprehensive utilization was also estimated. The value of evaporated salt produced ($1.68 million per year) was found to be close to the total cost of utilization ($1.71 million per year). The decrease of charge paid for chloride and sulfate discharge (estimated as $3.45 million per year) was then considered as the utilization benefit, resulting in high economical efficiency of the proposed conception. Keywords: Desalination; Salt production; Electrodialysis; Nanofiltration; Evaporation; Twig tower; Integrated systems; Zero discharge technologies
1. Introduction In Poland, most of the saline mine waters are directly discharged into rivers (mainly into the Vistula river) causing many ecological problems *Corresponding author.
and greatly decreasing the coal mine profits because a high penalty for the discharge of chloride and sulfate ions has to be paid. Thus, a decrease in salt discharge is the main goal of mine water policy. Much research and analysis of the mine water question has led to the conclusion that saline mine water utilization
Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2005.03.082
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is the only rational way for solving the above described problems [1,2]. Currently, the ‘‘Debiensko’’ plant, the only coal-mine waters utilization plant operated up to now, processes the untreated waters, which results in gypsum crystallization in the evaporation step and in many problems related to high magnesium content in the salt crystallization stage. To overcome these problems, a high-energy consumption evaporation method that tolerates gypsum crystallization, as well as crystallizer operating under low salt recovery conditions, are applied, causing low economical efficiency in the aforementioned plant [1,3,4]. At the ‘‘Wesola’’ coal-mine part of the saline mine waters is deposited underground together with the power station ashes as hydraulic filling to decrease the charge paid for chloride and sulfate ions discarding. The amount of such utilized saline water is however limited by the mining void capacity [5]. 2. Wesola coal-mine water question The prognosis of the ‘‘Wesola’’ coal-mine wastewaters quantity and its salinity for the 2005–2015 period are presented in Table 1. There it can be clearly seen that a significant reduction in the amount of salt discharged may be achieved only by the utilization of 665 m level waters, since most of the chlorides and sulfates (about 84%) originate from this mine level.
The estimated volume of waters, which might be deposited in the mines with power station ashes, is 500 m3/d, which is about 6times lower than the amount of water which should be utilized. Moreover, utilization of 465 m level waters, with their 15.1% of the total salts charge, should be also considered. Therefore the reduction of these waters volume by its concentration is necessary. The feasibility of twig tower evaporative concentration of 665 m level waters was examined at ‘‘Wesola’’ demonstration plant [5]. The yearly average power consumption was equal to 4.22 kWh per 1 m3 of distillate (the energy required for brine pumping and recirculation) while the unit cost of evaporation was estimated to be ca $0.75/m3. It was however found that the twig power productivity decreases with salt concentration and is limited to ca 9–10 % of total salinity. This limitation is the result of low vapor pressure over CaCl2 and MgCl2 containing saline waters: the relative humidity of NaCl saturated solution is equal to 75 % (at 25 C) while the relative humidity of CaCl2 and MgCl2 solutions are equal to, respectively, 29 and 33 % only [6]. The ability of up to 2-fold concentration (from ca 5 % to 10 %) only in the ‘‘Wesola’’ twig tower demonstration plant was then proved thus further evaporation is necessary. The performance of the twig tower might be also improved by prior saline water pre-treatment (softening).
Table 1 The quantities of mine wastewaters and their salinity predicted for 2005–2015 period Mine level (m)
230 465 665 Total
Amount (m3/d)
3600 7100 3050 13750
Concentration (g/L)
Charge of Cl&SO4 2
Cl
SO4 2
t/year
fraction (%)
0.100 2.25 35.5 —
0.300 0.524 0.408 —
525.6 7,188.8 39,974.6 47,689.0
1.1 15.1 83.8 100.0
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A membrane (nanofiltration) pretreatment had been suggested prior to thermal evaporation, and was examined in both coal-mine and seawater processing [7–9]. It was found that so far Osmonics Desal 5-L nanofiltration membrane provides the best performance for such a purpose since its NaCl rejection is relatively small (it excess 10 % slightly only) while the Ca2þ, Mg2þ and SO4 2 rejection is high [9]. On the other hand, the electrodialysis with univalent permselective membranes is recommended as a pre-treatment and pre-concentration of low salinity water. Its performance was proved in several industrial salt production plants [10–12] and its usefulness in dual-purpose seawater desalination-salt production [13] as well as in coal-mine saline water processing was demonstrated [2,14,15]. When concentrated, high purity brine (of low Ca2þ, Mg2þ and SO4 2 content) is obtained, which may be then efficiently utilized for evaporated salt production to decrease the total process costs. 3. Conception of comprehensive utilization of saline waters The aforementioned deliberations led us to a conception of comprehensive utilization of saline waters from ‘‘Wesola’’ coal-mine (Fig. 1). According to this conception water from level 665 m, of Cl content about 35.45 g/L is pretreated by nanofiltration (NF) and then evaporated in twig towers. NF permeate, with the significantly reduced content of calcium and sulfates and slightly reduced salinity is then evaporated with no risk of gypsum crystallization using a relatively low cost evaporation method. Also the magnesium content reduction is attained in NF step that provides the better operating conditions of the NaCl crystallization stage and improves evaporated salt purity.
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The softening of ‘‘Wesola’’ coal-mine water from level 665 m using Desal-5L (Osmonics) nanofiltration membrane was investigated in laboratory. The rejection coefficients were found as follows (%): Ca2þ—68.8, Mg2þ— 71.6, SO4 2 —98.4, Cl—11.9, that lets us to estimate the composition of process streams of NF at 85% recovery as shown in Table 2, in the manner presented in [7]. Water from level 465 m that contains about 2.25 g/L of Cl is pre-treated and pre-concentration by electrodialysis with univalent permselective membranes to 54.6 g/L of Cl content. The ED diluate Cl and SO4 2 contents are close to the water from 230 m level (Table 1). Partially evaporated twig tower brine and ED concentrate undergo further evaporation up to NaCl concentration of 300 g/L. Low energy thermal evaporation method is applied as the calcium sulfate crystallization risk is eliminated. The brine with NaCl content close to saturation is then evaporated with salt crystallization. The NF retentate and post-crystallization lyes are mixed with power station ashes and used as ‘‘Wesola’’ coal-mine void hydraulic fillings. The costs of comprehensive utilization using proposed technology was then estimated based on authors’ investigations and literatures’ data. The performance of electrodialysis was assumed to be equal to our laboratory results [13], the twig tower performance was estimated based on the ‘‘Wesola’’ demonstration plant operating data, while the costs of further evaporation and salt crystallization were collected from the literature [16–18]. The costs evaluation of the proposed system is shown in Table 3 while its advantages estimation is shown in Table 4. 4. Conclusions A conception of comprehensive utilization of saline waters from ‘‘Wesola’’ coal-mine was proposed. There the water from level
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water from 665 m level
water from 465 m level
3
7100 m3/d
3050 m /d
retentate 450 m3/d
NANOFILTRATION
TWIG TOWERS brine
ELEKTRODIALYSIS
diluate 6818 m3/d
condensate 250 m3/d concentrate
3
2350 m /d
EVAPORATOR
282 m3/d condensate 2064 m3/d
post-crystallization lyes 50 m3/d hydraulic filling
CRYSTALLIZER
evaporated salt 153 t/d
Fig. 1. The scheme of comprehensive utilization of ‘‘Wesola’’ coal-mine waters.
665 m, that contains about 35.45 g/L Cl, is pretreated by nanofiltration (NF) and the water from level 465 m of about 2.25 g/L Cl content is desalinated and simultaneously concentrated by electrodialysis (ED) with univalent permselective membranes. Both, NF permeate and ED concentrate with reduced content of calcium, magnesium and sulfates are evaporated in proposed process. Calcium sulfate crystallization risk are then eliminated avoiding the use of high energy consuming evaporation method and resulting in low unit cost of the
concentration process. The magnesium content reduction provides the better efficiency of the NaCl crystallization stage and improves evaporated salt purity. Also, the possibility of waste solutions discharge (NF retentate and post-crystallization lyes) as the hydraulic fillings is the advantage of presented concept since their treatment is usually very expensive. The results of economical efficiency evaluation are very promising. The value of evaporated salt produced ($1.68 million per year), when $30/t of salt was assumed [19] is
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M. Turek et al. / Desalination 185 (2005) 275–280 Table 2 Composition of NF process streams at 85% NF recovery Component
Feed Permeate Retentate
Concentration (g/L) Ca2þ
Mg2þ
SO4 2
Cl
1.52 0.80 5.61
0.899 0.44 3.50
0.408 0.01 2.64
35.45 33.87 44.43
Table 3 Cost of the comprehensive utilization of ‘‘Wesola’’ coal-mine waters Item
nanofiltration twig towers thermal evaporation electrodialysis salt crystallizer Total
Costs $
$/d
mln $/year
0.25a 0.75b 1.0b 2.0c 8.0d
650 188 2064 564 1224
0.237 0.068 0.753 0.206 0.447 1.711
a
As related to 1 m3 of permeate. As related to 1 m3 of condensate. c As related to 1 m3 of concentrate. d Per 1 metric ton of salt. b
Table 4 The advantages of coal-mine waters utilization Item
Unit value
mln $/year
evaporated salt decrease of ecological charge
30 $/t 0.2269 PLN/kga
1.68 3.45
a
As the sum of Cl þ SO4 2 , according to Polish regulations.
close to the total costs of utilization ($1.71 million per year). The decrease in penalties paid for chloride and sulfate discharge (estimated as $3.45 million per year) was then found to be the utilization benefit. Acknowledgements This work was financed by Polish Minister of Science and Information Technology,
grant No. 3 T09D 085 28, during 2005–2008 period.
References [1] R.L. Magdziorz, Przeglad Gorniczy, 57(2) (2001) 16–25 (in Polish). [2] M. Turek, Desalination, 162 (2004) 355–359. [3] B. Ericsson and B. Hallmans, Desalination, 105 (1996) 115–123.
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[4] J. Piecha and R. Klimek, Wiadomosci Gornicze, 9 (1998) 379–384 (in Polish). [5] A. Surma, in: R.K.F. Reich, ed., Jakosc Srodowiska Techniki I Technologie, Biblioteka KOMEKO, Gliwice, 2001, 211–220 (in Polish). [6] D.R. Lide, ed., Handbook of Chemistry and Physics, 84th ed., CRC Press, 2003–2004. [7] M. Turek and M. Gonet, 108 (1996) 171–177. [8] A.M. Hassan, A.M. Farooque, A.T.M. Jamaluddin, A.S. Al-Amoudi, M.A.K. Al-Sofi and A.F. AlRubaian, et al Desalination, 131 (2000) 157–171. [9] M. Turek and P. Dydo, Desalination, 157 (2003) 51–56. [10] K. Takashima, S. Futaki, F. Hanada, M. Yamamoto, in: R.M. Geertman, ed., 8th World Salt Symposium, Elsevier, 1 (2000) 641–646.
[11] Ion exchange membrane and its applications Asahi Chemical Industry Co., LTD.. [12] Selemion ion exchange membranes, Asahi Glass Company. [13] M. Turek, Desalination, 153 (2002) 377–381. [14] M. Turek, Chem Pap., 1 (2003) 50–52. [15] M. Turek, P. Dydo and R. Klimek, Desalination, 184 (2005) 439–446. [16] Y.M. El-Sayed, Desalination Water Reuse Quaterly, 9/2 (1999) 71. [17] K. Swist, Conference Material, Vistula Desalination Projects, Krakow, 1993. [18] J. Chowaniec Conference Material, Vistula Desalination Projects, Krakow, 1993. [19] A. Barr, Filtr. Sep., 38 (2001) 18–20.