Chemical aspects between water and minerals in view of building

geothermal power plants

A short introduction into mineralogy from Martin Weber

Msc. chem., Switzerland

Analysis of thermal water from the richest mineral source in Switzerland • Place of the fountain: Baden (Switzerland)• Temperature: 46.5°C• pH-data: 6.43• Total mineralisation: 435 mg/L• Boric and silicic acid: 71 mg/L• Solved gas: 292 mg/L

Analysis of thermal water from the richest mineral source in Switzerland

Kations mg/L• Ammonia 0.78• Lithium 4.8• Sodium 720• Potassium 63• Magnesia 99• Calcium 503• Strontium 6.2• Iron 0.013• Manganese 0.016• Copper <0.005• Zinc <0.01• Lead 0.002• Aluminium 0.018• Total 1397

Anions mg/L• Fluorine 3.1• Chlorine 1185• Bromine 2.5• Iodine 0.009• Nitrate <0.5• Hydrogen-

carbonate 487• Sulfate 1375• Hydrogen-

phosphate 0.05• Hydrogen-

arsenate 0.1• Molybdate <0.005• Total 3053

Solid precipitations from mineral water

some frequent examplesSolvation product

• Limestone CaCO3 4.7 x 10-9

• Dolomithe MgCa(SO4)2 2.6 x 10-6

• Plaster CaSO4 2.4 x 10-5

• Strontianithe SrCO3 1.6 x 10-9

• Barythe BaSO4 1.5 x 10-9

• Silicates CauMgvAlw(SixOy)z

• Borates NauCavMgw(BxOy)z

Problems with precipitating minerals from water

• Formation of mineralic layers and coatings on pipes and heat exchangers.

• Reduced heat transitions trough mineralic coatings and sedimentations.

• Reduced water flow through the primary water cycle and risk of a transition from a laminar to a turbulent flow.

Formation of limestone CaCO3

• Mineral water contains Calcium (Ca2+) and Hydro-gencarbonate ions (HCO3

-). These ions are easily soluble and stay in a chemical equilibrium with CaCO3, which is heavily soluble:

Ca2+ (aq) + 2 HCO3- (aq) CaCO3 (s) + H2O + CO2

easily soluble heavily soluble

In cold water the equilibrium is on the left side, where the soluble particles exist. Heat will change the equilibrium from the left to the right side, so the formation of limestone is preferred.

Formation of Silicates

• Mineral water contains considerable amounts of silicic acid H4SiO4. The range of concentration spreads from 5 mg/Litre up to 75 mg/Litre.

• Silicic acid is moderately soluble, but by splitting off water it tends to form higher poly-silicates, which are less soluble and precipitate as solids:2 H4SiO4 (aq) H6Si2O7 (s) + H2O3 H4SiO4 (aq) H8Si3O10 (s) + 2 H2O4 H4SiO4 (aq) H10Si4O13 (s) + 3 H2O etc.

Formation of Silicates

• The anion of (monomeric) silicic acid SiO44-

represents a tetrahedral geometry.

• Polysilicic acids may be formed by connecting corners, edges or planes of monomeric, tetrahedral silicates as the following picture is showing us:

schematicstructure

Formation of Silicates

ortho-silicatemonomer

group-silicate

chain-silicate

ring-silicate

band-silicate

ring-silicate

Formation of Silicates• Connecting of corners leads to the formation of

either groups, chains, bands, rings or layers, connecting edges or planes leads to the formation of cages from polymeric silicic acid:

layer structure of polysilicate mineralslike clay and mica[(Si2O5)2-]x

Formation of SilicatesCombined with different kations we come across the different naturally existing silicates which are known by alternative names.

Type of silicate formula of Mineral- name and formulathe anion class of a representative mineral

monosilicates SiO44- Olivines Fayalit Fe2SiO4

group silicates Si2O76- Barysilit Pb3Si2O7

ring or cyclosilicates Si3O9

2- Beryll Al2Be3Si6O18

chain silicates [SiO32-]x Pyroxenes Diopsid CaMg[SiO3]2

band silicates [Si4O116-]x Amphiboles Tremolit Ca2Mg5(OH)2[Si4O11]2

layer silicates [Si2O52-]x clays Kaolinit Al2(OH)4[Si2O5]

Talcum Mg3(OH)2[Si2O5]2 Mica Biotit K{(Mn)3(OH)2[AlSi3O10]}

Asbestos Serpentin Mg3(OH)4[Si2O5]cage silicates [AlySi4-yO8

y-]x Feldspars Orthoclas K[AlSi3O8] Ceoliths Mordenit

Na2[Al2Si10O24] . 6 H2OQuartz SiO2

Weathering decay of silicates• Products from weathering decay of silicates are:• SiO2 or silicic acid H4SiO4

• Hydroxide of aluminium Al(OH)3

• Clay minerals

• Weathering decay of feldspar:KAlSi3O8 + 4 H+ + 4 H2O K+ + Al3+ + 3 H4SiO4

• Reformation of Kaolinit (a clay mineral)2 KAlSi3O8 + 4 H+ + 9 H2O 2 K+ + AlSi2O5(OH)4 + 4 H4SiO4

Resistivity of weathering decay from different silicates

Resistivity of weathering decay increases

Iland-silicates

Chain-silicates

Layer-silicates

Cage-silicates

OlivinsSiO4

4-

PyroxenesSiO3

2-

AmphibolesSi4O11

6-

Clay and MicaSi2O5

2-

Feldspars andQuartz SiO2

Band-silicates

Solutions for encountering mineralic precipitations on pipes and heat

exchangers

• Electrophoresis• Exchangeable heat exchangers• Filters for gelatinous polysilicic acids• Cyclone (centrifugal separation)