7/29/2019 v112n02p057 Economics of Mine Water Treatment

1/3

Journal

Paper

Introduction

Lignite coal ranks first among the mineralresources extracted in the Czech Republic.Opencast mines are the deepest sites in thelandscape relief, and as such they areextremely sensitive to the inflow of bothsurface water and groundwater as well asrainfall water. Therefore, each opencast minehas to be equipped with a drainage system forcontrolled drainage of the mine water from theworking area of the opencast mine. Thedefinition of mine water is included in theMining Act of the Czech Republic1. The Actalso enables the mining company, among

other things, to use the mine water free ofcharge and to discharge the excess amount.The objective of the drainage is to ensure

the smooth and safe running of an opencastmine. For this purpose, the Mining Act hasbeen supplemented with other regulations ofthe State Mining Authority, the Water Act2, theAct on Environmental Impact Assessment3,etc. The above legal regulations deal not onlywith safety and technical issues of waterdrainage and operation of opencast mines, butalso environmental protection, notably theprotection of surface water and groundwater.

The drainage system of an opencast mineincludes the safeguarding of the foreground

against surface water and the drainage of themine and the inside dump. The formerinvolves the creation of drainage channelsand, if required, watercourse rerouting in theprospective exploitation area. As a rule, the

drainage of overburden benches and coal cutsis carried out by means of a system of opendrainage channels. The exposed bed, i.e. thebase of the prospective inside dump, is drainedthrough a system of covered drainagechannels. The water from the opencast mineand the inside dump drains into a reservoirthat is sunk at the lowest part of the mine. Thesize of the reservoir has to be determinedtaking into consideration the local rainfall andwater inflow. As a rule, the water is pumpedfrom the reservoir by a pumping plant into themine-water treatment plants. The final effluentis discharged into surface watercourses andthe amount and quality are monitored as themethod and conditions of mine water

discharge into surface watercourses are set bythe self-administration authorities. Watermanagement practices that contravenes theWater Act are subject to penalization.

Technological process of mine water

treatment

The area of our concern is two coal opencastlignite mines. As we will be discussingsensitive data on the costs of mine watertreatment at the treatment plants of therespective mines, we consider it appropriatenot to give a detailed description of the minesin question. They are situated in the northernpart of the Czech Republic and will be referredto as Locality 1 and Locality 2.

Mine water accumulates in the lowest levelof the mine and can be acid (pH 2.54) toneutral (pH 67). The acidity of water is theresult of the biochemical dissolution process of

Economics of mine water treatment

by J. Dvoracek*, J. Vidlar*, J. S terba*, S. Heviankova*,M. Vanek*, and P. Bartak

Synopsis

Mine water poses a significant problem in lignite coal mining. Thedrainage of mine water is the fundamental prerequisite of mining

operations. Under the legislation of the Czech Republic, mine waterthat discharges into surface watercourse is subject to the permissionof the state administration body in the water management sector.The permission also stipulates the limits for mine water pollution.Therefore, mine water has to be purified prior to discharge.

Although all mining companies operate mine water treatmentplants, there is hardly any utilization of the final effluent. The highexpenditure involved in the more sophisticated methods of minewater treatment has fuelled the search for the commercialutilization of the effluent.

Keywords

mine water treatment, economics of treatment process, waterutilization.

* VSB-Technical University of Ostrava, CzechRepublic.

Kamen Ostrome r, Czech Republic. The Southern African Institute of Mining and

Metallurgy, 2012. SA ISSN 0038223X/3.00 +0.00. Paper received Jul. 2010; revised paperreceived Oct. 2011.

157The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 112 FEBRUARY 2012 L

7/29/2019 v112n02p057 Economics of Mine Water Treatment

2/3

Economics of mine water treatment

iron disulphides contained in the coal. In order to decrease

the corrosive action of mine waters, a pre-treatment isperformed by partial neutralization, which is effected bydosing of lime wash to accumulation ponds.

The process continues with the sedimentation ofsuspended solids in large sedimentation tanks by means ofgravity. Preliminary coagulation may also take place. Thefollowing stage of treatment involves oxidation to remove thesoluble forms of iron and manganese from the water bymeans of aeration or addition of an oxidizing reagentpotassium permanganate. Controlled sedimentation is used toseparate sludge from water. Thickening, dewatering via acentrifuge/filter press, and the disposal of sludge follow. Thefinal effluent is then discharged into the surface watercourse.

However, this treatment process leaves high concen-trations of sulphate ions (their concentration being between600 to 1500 mg/l) unaffected, which means that the surface

water lawful discharge limits are exceeded and that the watercannot even be utilized for industrial purposes.

The Faculty of Mining and Geology of the VB-TechnicalUniversity of Ostrava, Czech Republic developed and testedon site a technology for the desulphation of mine water(Vidl, et al.46) that enables the removal of sulphates aswell as residual heavy metals. The chemical precipitationprocess is initiated by the addition of lime slurry and sodiumaluminate. After flocculation, the resulting precipitate (theettringite sludge) is dewatered in a filter press after itsdensity has been increased by means of gravity settling. It isof advantage to utilize desulphation node sludge for thereactive precipitation of sulphates (aluminium ion donor) orto use it as silicate additive to improve the hydraulicproperties of gypsum and lime mortars (VidlandHevinkov7). The output of the desulphation technology isdemineralized (industrial, technological) water.

The precipitation reaction chemistry is based on analogywith the well-known reaction that takes place by hydration ofground clinker of calcium aluminate cements, which containsulphates. An outcome of these heterogeneous chemicalreactions is a new low-solubility mineral constituent, calciumaluminate sulphate (ettringite), which, as cement bacillus,was also identified by Bannister8 in 1936. Our desulphationmethod is based on the direct interaction of calcium andaluminium ions with sulphates present in water of highalkalinity (pH about 12.4).

From a theoretical point of view, the probable precipi-tation mechanisms are:

3SO42-

+ 6Ca2+ + 2AlO2

+ 4OH

+ 29H2O = [1]3CaO.Al2O3. 3CaSO4. 31H2O

or also

3Na2SO4 + 6Ca(OH)2 + AlCl3 + 26H2O = [2]Ca6Al2(SO4)3(OH)2 . 26H2O + 6NaCl

The calcium and hydroxyl ions for these reactions areprovided by calcium hydrate, Ca(OH)2. It is of advantage toadd the aluminium ions in the form of sodium aluminate,NaAlO2, which does not carry any other deleterious ions.

The removal of sulphates can be controlled to achievedesired contents, even below 300 mg/l.

The mine water desulphation technology of this projectalso provides for efficient removal of all Fe, Mn, and othermetal ions, which occurs in the primary stage (highalkalization of input water by lime solution), when all metals

present precipitate in the form of hydroxides. The second

stage consists of sulphate precipitation, and the third stageprovides for final neutralization of the purified water byliquid carbon dioxide, CO2 (Hevinkov and Bestov9).

The desulphation process technology for mine watertreatment is similar to the treatment process to producepotable water from water supply reservoirs, except for thefinal sanitation phase, i.e. disinfection.

Economics of the mine water treatment process

Current mine water treatment involves the following process:in Locality 1, mine water neutralization, removal of insolublesubstances and partial removal of iron. In Locality 2, besidesthe removal of insoluble substances, the reduction of acidityof mine water and removal of manganese and iron areundertaken also. The purified mine water is discharged into

the surface watercourse without further treatment norutilization. One of the reasons is the high content of sulphateions, which at present is not subject to any statutoryrequirements. This is expected to change in the coming years.

The extent of mine water treatment is the determinant ofthe unit costs. At the current Czech crown/US dollar exchangerate, the unit cost at Locality 1 is US$ 0.44/m3 and at Locality2 US$ 1.20/m3 of treated mine water.

However, the final effluent has a high content ofsulphates, and manganese concentration is not satisfactoryeither. It is only in Locality 2 that manganese is removed byoxidation using atmospheric oxygen. As the process becomesless efficient at lower mine water temperatures, potassiumpermanganate is used as oxidizing agent. However, it isadded only when the limit concentrations of manganese inthe effluent have been exceeded. Furthermore, if the optimumdose of oxidizing reagent is exceeded, the concentration ofmanganese in the effluent increases beyond the permissiblelevel. The task force of the Faculty of Mining and Geology(Hevinkov and Bestov9) proposed a modified process forremoving high manganese levels from acid mine water. Theprocess proposed includes alkalification using calciumhydroxide and chemical oxidation including neutralization bycarbon dioxide.

However, the removal of sulphate ions and manganesefrom mine water involves higher operational costs.

L158 FEBRUARY 2012 VOLUME 112 The Journal of The Southern African Institute of Mining and Metallurgy

Figure 1A sample of chemically precipitated calcium aluminate

sulphate ettringite

7/29/2019 v112n02p057 Economics of Mine Water Treatment

3/3

The sulphates removal process entails the depreciation

expense for buildings and technological equipment, higheroperational costs, increased consumption of chemicals forwater treatment, and capital lock-up in the inventory ofchemicals. The estimated maximum capacity of the sulphatesremoval facility is 1 752 thousand cubic meters of treatedmine water per year.

The manganese removal process involves greaterconsumption of chemicals, increased storage costs, increasein the depreciation expense for carbon dioxide storage, andhigher freight cost.

At the current Czech crown/US dollar exchange rate, theestimated cost of sulphate removal from mine water isUS$1.92 per cubic meter of treated mine water. The estimatedcost of manganese removal using the technology proposed isUS$0.10 per cubic meter of treated mine water.

Based on the above, it is possible to compare the unit

costs of treated mine water using different treatmentmethods. The outcome is given in Table I. It is based on theactual data from the mine water treatment plants at both thelocalities, supplemented with the prospective methods of amore sophisticated mine water treatment process, i.e. removalof manganese and sulphates. Table I contains:

Actual cost of mine water treatment at Locality 1 andLocality 2

Actual cost of mine water treatment at Locality 1 plusthe manganese removal proposal

Actual cost of mine water treatment at Locality 1 plusthe sulphate removal proposal

Actual cost of mine water treatment at Locality 2 plusthe sulphates removal proposal.

To arrive at the figures in Table I, the total costs weresplit into fixed and variable costs so that the unit costs could

be determined with regard to the capacity harmonization (i.e.the annual amount of treated mine water) of individualtechnological elements of the mine water treatment plants.The table shows that improved quality of the final effluententails a significant increase in unit costs.

The higher treatment costs can be partially offset byusing the mines own water rather than purchasing water fortechnological purposes from a waterworks. Another option isto sell the treated mine water to other entities for techno-logical or other purposes.

The analysis of the options for the respective Localitiesleads to the following results:

1. Use of mines own water: the estimated amount ofwater purchased by a mining company is 200300

thousand cubic meters per year. The use of the mines

own water instead would lead to savings worth tens ofthousands of dollars.2. Sale of the treated mine water: in principle, such water

can be used for agricultural, industrial, andrecreational purposes. As a rule, agricultural activitydoes not take place in the vicinity of opencast minesdue to the impairment of the environment as a resultof mining operations. Industrial companies alreadyhave their own water sources. Theoretically, artificialwater bodies set up for recreational purposes may bean option, but these would involve substantialinvestments. Moreover, they would have to competewith natural water bodies in the region.

The yearly volume of treated mine water ranges fromclose to 1 million cubic meters at Locality 1 to 2 million cubicmeters at Locality 2. The cost of mine water treatment is so

high that it cannot be offset by the benefits stemming fromthe utilization of the treated mine water.

Conclusion

At present, the treated mine water from lignite coal mining isnot used for commercial purposes in the Czech Republic.However, due to climatic changes or emergency situations, itmay constitute an important prospective resource of surfacewater in the future. It has been proven that the quality of thefinal effluent from opencast lignite coal mining (after theexisting technology is supplemented with the processesproposed) conforms to the requirements of a strategic watersupply resource.

Acknowledgements

We would like to thank the Research Centre for IntegratedSystem Development Concerning the Utilization of By-product of Energy Resource Mining and Processing for theirsupport in the study.

References

1. CZECH REPUBLIC. Act No. 44/1988 Coll. on the Protection and Utilization ofMineral Resources (The Mining Act). www.cbusbs.cz/prehled-platnych.aspxAccessed 9 February 2010.

2. CZECH REPUBLIC. Act No. 254/2001 Coll. on Water and Amendments toSome Acts (The Water Act). Ministry of the Environment of the CzechRepublic. www.mzp.cz/www/platnalegislativa.nsf/0/20f9c15060cad3aec1256ae30038d05c?OpenDocument&ClickAccessed 25 January 2010.

3. CZECH REPUBLIC. Act No. 100/2001 Coll. on the Assessment of Impacts onthe Environment and Amendments to Some Acts (Act on EnvironmentalImpact Assessment). Ministry of the Environment of the Czech Republic

www.mzp.cz/www/platnalegislativa.nsf/d79c09c54250df0dc1256e8900296e32/8a12b8f25817a234c125729d0039d956?OpenDocumentAccessed

25 January 2010.4. VIDL, J., URBAN, V., and KINDIGER, G.Zpsob itn odpadnch vod s

nadlimitnmi obsahy sran. Czech Pat. 290953: Appl. 6 March 1998, Acc.13 November 2002.

5. VIDL, J., NMEC, J., OSNER, Z., RACLAVSK, J., SCHEJBAL, C., and STRNADOV,N. Zpsob pravy dlnch vod hlinitanem sodnm. Czech Pat. 295200:Appl. 30 December 1998, Acc. 15 June 2005.

6. VIDL, J. et al. Nvrh technologickho odstraovn sran arozputnch ltek z dlnchvod lokality Kateina-Radvanice (VUD). VB-TUO, erven 1998.

7. VIDL, J. and HEVINKOV, S. Utility Model, CZ .21442/2010: Donor ofaluminium ions for sulphate precipitation.

8. BANNISTER, F.A., HEY, M.H., and BERNAL, J.D. Ettringite from Scawt Hill, Co.Antrim,Mineralogical Magazine, vol. 24, 1936. pp. 324329.

9. HEVINKOV, S. and BESTOV, I. Odstraovn manganu z kyselch dlnchvod. Uhl, Rudy, Geologickprzkum, vol. 15, no. 3, 2008.pp. 1214. N

Economics of mine water treatment Journal

Paper

159The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 112 FEBRUARY 2012 L

Table I

Unit costs of different methods of mine water

treatment (US$/m3)

Locality 1 Locality 2

Neutralization, partial removal of Fe 0.44 -

Neutralization, removal of Fe and Mn - 1.20

Neutralization, partial removal of 0.54 -

Fe, removal of Mn

Neutralization, partial removal of Fe, 2.49 -

removal of sulphates

Neutralization, removal of Fe, Mn, - 3.20

and sulphates