Investigation of water decant from underground collieries in Mpumalanga

Abstract

English: This investigation was intended: 1. To investigate and describe the status quo in terms of mining methods, scheduling, geology, geohydrology, hydrochemistry, water and salt balances at six underground collieries that are in the process of decanting or where decanting is imminent. 2. To investigate management options whereby the quality of the mine water can be influenced in operating underground collieries, thus minimising the long-term salt load that will be released into the environment. 3. To document the six case histories to be used for future reference, demonstrating the time-dependency of these systems. 4. To extract and discuss management options that should be applied in operating collieries to achieve the long-term goal of minimising the salt load to the environment. Six collieries where mining has ceased have been selected for detailed geohydrological and hydrochemical investigation. The scope is to provide sufficient time-related information with which the industry and authorities can associate to promote standardisation of water management methodologies. The mines selected for this investigation are:

This investigation was intended: 1. To investigate and describe the status quo in terms of mining methods, scheduling, geology, geohydrology, hydrochemistry, water and salt balances at six underground collieries that are in the process of decanting or where decanting is imminent. 2. To investigate management options whereby the quality of the mine water can be influenced in operating underground collieries, thus minimising the long-term salt load that will be released into the environment. 3. To document the six case histories to be used for future reference, demonstrating the time-dependency of these systems. 4. To extract and discuss management options that should be applied in operating collieries to achieve the long-term goal of minimising the salt load to the environment. Six collieries where mining has ceased have been selected for detailed geohydrological and hydrochemical investigation. The scope is to provide sufficient time-related information with which the industry and authorities can associate to promote standardisation of water management methodologies. The mines selected for this investigation are: • Minnaar -Small underground mine; compartments artificial recharge; mine water irrigation;- Bord-and-pillar • Ermelo - Large underground mine, partially stooped; in Ermelo the process of filling up with water; flushing option considered.- Bord-and-pillar stooping • TNC – Complex arrangement of underground and opencast mining; partially filled with water; water quality management is possible through mixing – Bord-and pillar stooping opencast • New Largo – underground mine with very little subsidence; water balance calculations and seepage losses – Bord-and-pillar limited stooping • Schoongezicht – Underground mine; currently decanting; water and salt balance studies – Bord-and-pillar opencast • Kromdraai – Underground mine currently being reworked by opencast methods; impact of change in mining method – Bord-and pillar secondary opencast The main conclusions are: • The mining method and associated mining geometry dictates the end result in terms of water influx and water quality. • On a regional scale, there is no significant variation in the geology, mineralogy and geohydrology of the collieries in Mpumalanga (Pinetown, 2003). • Water balances are of overriding importance in determining areas of recharge, water loss and reaction rates. These vary from mine to mine. Overriding factors are the method of mining, depth of mining and the surface hydrology. High extraction methods invariably disturb the overlying strata more than bord-and-pillar methods. A summary of the percentage influx to be expected for the various mining methods is as follows: • Shallow bord-and-pillar 5-10% of the rainfall. • Deep bord-and-pillar with no subsidence - 1% of the rainfall. • Stooping 4 - 12% of the rainfall. • Longwall 6 - 15% of the rainfall. • Opencast 14- 20% of the rainfall. The longwall mining value (Hodgson and Krantz, 1998) was not covered in this investigation, but is included for reference purposes. e Daily sulphate production rates range from 0.4 - 2.7 kg/ha in the mines. Sulphate constitutes about 50% of the salt load and concentrations typically ranging from 400 - 2 000 mg/L. • Flooding of the mines should minimise oxygen ingress and sulphate production will decline under alkaline conditions. Four of the six collieries currently have alkaline water. Long-term projections indicate that three of these collieries should remain alkaline. • It is suggested in this document that more should be done in terms of planning mine-water handling, during and after mining. Projections indicate that flushing is a viable option to reduce salinity in the mine water. Mine water from the northern collieries is low in sodium content, which makes it usable for the irrigation of crop. In the southern part, the Mpumalanga Collieries generally have high sodium concentrations, which limits the possible applications of this water. • Management options for mine water during and after mining can be reduced to a few simple concepts. Endless branching into more sophisticated, sometimes more complex, procedures is possible. The following is a list of simple management options that include all the important issues: • Select the mining method based on environmental considerations. • Mine from low-lying areas to high ground. • Flood the mine workings as soon as possible. • Flush the mines after being flooded.Afrikaans: Die doel van die ondersoek was die volgende: 1. Om die mynbou metodes, skedulering, geologie, geohidrologie, hidrochemie, water- en sout balanse van ondergrondse steenkoolmyne wat of besig is om oor te loop, of waar oorloop onafwendbaar is, te ondersoek en te beskryf. 2. Om bestuursopsies, waardeur die kwaliteit van water in bestaande steenkoolmyne beinvloed kan word, te ondersoek en sodoende die langtermyn soutladings wat in die omgewing vrygestel gaan word, te verminder. 3. Om ses gevalle studies te dokumenteer vir toekomstige verwysings, en om sodoende die tydsafhanklikheid van die sisteem te bewys. 4. Om bestuursopsies vir bestaande steenkoolmyne uit te lig en te bespreek, rakende die langtemyndoelwit om die versouting van die omgewing te minimaliseer. Ses steenkoolmyne waar produksie reeds gestaak is, is gekies vir 'n deeglike geohidrologiese en hidrochemiese ondersoek. Die doel daarmee is om voldoende inligting aan die owerheid en die industrie beskikbaar te stel, sodat eenvormigheid oor die bestuur van water bereik kan word. Die myne wat gekies is vir die ondersoek, is as volg: • Myn – Eienskappe • Minnaar - Groot ondergrondse myn met gedeeltelike pilaar herwinning; besig om vol te loop; deurspoeling 'n opsie. • TNC - Komplekse samestelling van ondergrondse en oopgroef mynboumetodes; gedeeltelik gevul met water; bestuur van waterkwaliteit is moontlik deur vermen in. • New Largo - Ondergrondse myn met baie min insakkings; waterbalansberekeninge; waterverlies deur uitsypeling • Schoongezicht - Ondergrondse myn; huidiglik vloei dit oor; water- en Soutbalanse • Kromdraai - Ondergrondse myn wat huidiglik herwerk word deur oopgroefmetodes; impak van veranderde metodes ondersoek. Die belangrikste gevolgtrekkings is as volg: • Die mynboumetode en gepaardgaande mynligging eindresultaat rakende waterinvloei en -kwaliteit. • Op' n regionale skaal is daar nie betekenisvolle verskille tussen die steenkoolmyne in Mpumalanga wat betref geologie, mineralogie en geohidrologie nie. • Waterbalanse is die belangrikste aspek in die bepaling van aanvulling, waterverliese en reaksietempos in myne. Dit verskil egter van myn tot myn. Faktore wat in ag geneem moet word, is die mynbou metodes, diepte van mynbou en oppervlakhidrologie. Hoë ekstraksiemetodes versteur die oorliggende gesteentes meer, as wanneer kamer-en-pilaar metodes toegepas word. 'n Opsomming van die persentasie aanvulling wat verwag kan word vir die verskillende metodes van mynbou, is as volg: • Diep kamer-en-pilaar met geen insakking 1%van die reënval • Pilaar herwinning - 4-12% van die reënval • Strookafbouing - 6-15% van die reënval • Oopgroef - 14-20% van die reënval

• Daaglikse sulfaat generasietempos wissel van 0.4 - 2.7 kg/ha in die myne. Sulfaat maak omtrent 50% van die totale soutlading uit en konsentrasies wissel meestal tussen 400 - 2000 mg/L. • Oorstroming van die myne verminder suurstofinvloei, en gevolglik behoort sulfaatgenerasie te verminder onder alkaliese toestande. Vier van die ses myne het tans alkaliese water. Langtermynprojeksies dui daarop dat drie van hierdie myne alkalies behoort te bly. • In hierdie dokument word voorgestel dat meer gedoen word met die beplanning van die hantering van myn water, gedurende produksie en na die sluiting van 'n myn. Projeksies dui daarop dat spoeling 'n werkbare opsie is om die soutgehalte van mynwater te verminder. Water van die noordelike steenkoolmyne het 'n lae natrium inhoud, wat dit geskik maak vir besproeiing. Die suidelike steenkoolmyne in Mpumalanga het gewoonlik 'n hoë natrium inhoud, wat veroorsaak dat die nuttige aanwending van hierdie water beperk is. • Bestuursopsies vir mynwater, gedurende produksie en daarna, kan tot 'n paar eenvoudige opsies beperk word. Die uitbreiding hiervan tot meer gesofistikeerde, soms meer komplekse metodes, is wel moontlik. 'n Lys van eenvoudige bestuursopsies, wat al die belangrike aspekte insluit, is as volg: • Kies die mynbou metode volgens omgewingsvereistes. • Myn vanaf laagliggende gebiede na hoër dele. • Oorstroom die myn so spoedig moontlik. • Deurspoel die myn nadat dit gevul is.