Solution mining

Introduction Technology of solution

mining Technology of the salt

production Geological conditions Pros and cons

Solution mining - Introduction

The method of solution mining is a very old technological process. Natural brine sources were already used in antiquity.

Brine was produced by squirting water into mining chambers as well as by injection of water in the deposit through wells.

In the 50th and 60th new fields of the technology were developed. The present status of equipment of the well was developed.

The erected cavern by solution mining often used as underground storages for gas or oil.

Besides the exploitation of rock salt other minerals obtains a great importance (i.e. sylvinite, carnallite, bischofite, trona).

The control and measurement methods for steering the size and the form of the cavities were elaborated.

Technology of Solution Mining (1)

A bore hole was drilled from the surface of the earth to the bottom of the salt layer.

- A casing was worked in the bore well and was cemented from the surface to the top side of the deposit. The cement must shut tight against the pressure of the blanket.

- The surface of the bore hole in the area of the deposit is free. The salt can be dissolved.

Technology of Solution Mining (2)

The dissolution of the salt begins with the solution of a cavern sump. The sump shall be accommodate the insolubles of the deposit near the casings in the well.

- During the solution of the sump only water is used .- The water current is directly, that means that the current of brine in the

cavern has the same direction as in the production casing.- The solution of the sump can be ended if the diameter of the cavern is 5 –

10 m.

Technology of Solution Mining (3)

The next step is the undercut phase. The injected water is going trough the outer casing and the brine leave the cavern trough the inner casing. This current direction is named indirectly.

Important for the forming of the cavern is the precise controlling of the blanket level.

Technology of Solution Mining (4)

For winning of the salt in the deposit the level of the casings and the blanket was arranged higher. Because in the cavern the density of the brine increases from the top to the bottom, the brine current goes from the end of the outer casing under the blanket level to the side and then it flows to the inner casing and to the surface.

Technology of Solution Mining (5)

The last step is reached, if the cavern arrives the top of the deposit.

Technology of Solution Mining (6)

Last of all the tubes were removed and the bore hole will be cemented.

Technology of Solution Mining (7)

The equipment of the brine place is very simply. For the production of brine is needed:(1) a building for a control room and an office, (2) a workshop and a storage, (3) a building for pumps, (4) a blanket station, (5-7) tanks for water and brine

Technology of Solution Mining (8)

Another technology is used for the erection of underground storages. In this case the salt was dissolved after the undercut in only one step. The entry of the solvent into the cavern is trough the inner tube. From there the solvent rises up, dissolves the salt and goes to the outer casing.The sides of this cavern are more straightly as the caverns which is leached with the step-by-step technology. A disadvantage of this procedure is that the brine is in the most cases not saturated.

Technology of Solution Mining (9)

Methods to control the size of the caverns

1. Measurement of radial distance between the well and the cavern surface with ultrasonic sondes (sonar).

2. Measurement of the area by addition of blanket into the cavern and determination of height difference of the blanket level.

3. Mass- and volume balance of solvent injection and brine recovery

This three methods used together allows an precise assessment of the cavern area and size.

Technology of the salt production (1)

The most important salt minerals, which produced by solution mining are:

- Rock salt (NaCl)

- Sylvinite (NaCl + KCl)

- Carnallite (MgCl2 * KCl * 6H2O)

Because these minerals have very different thermodynamic properties, the production technology for each salt had to developed specifically.

How we have to see later for the winning of carnallite the development of a new solution mining technology was necessary.

Technology of the salt production (2) –Rock salt

The most important property of NaCl is the independence of the saturation concentration from the temperature.

Because of this the whole water contains in the brine must be evaporated.

For example contains 1000 l saturated NaCl brine 317 kg KCl and 882 kg H2O.

That means the production of NaCl is a very energetic intensive process.

In locations with arid climate often used the evaporation in solar ponds.

Otherwise the evaporation must take place in technical plants. As energy base is used steam or electrical power.

Technology of the salt production (3) –Rock salt

Flowsheet of NaCl production in a solar pond process

Solar pond

Brine

Crushing, screening

Harvested crystalline crop

Drying

Storage

Oil or gas Water

NaCl

WashingWater Soiled brine

Technology of the salt production (4) –Rock salt

Flowsheet of NaCl production in a technical process

Chemical purification, precipitation of Mg++, Ca++,SO4

--

Brine

Evaporation, crystallisation

Drying

Storage

Oil or gas

Water

NaCl

Steam or electrical power

Water

Technology of the salt production (5) –Sylvinite

Sylvinite is a mixture of NaCl and KCl.

In the case of contact with water by solution mining will be dissolved both components. At first in relation of their concentration in the raw salt and later the dissolution is appraoching to the invariant point M (red line), as shown in the following picture.

10°C 90°C50°C

Brine

NaCl - crystallisation

Evaporation

KCl - crystallisation by cooling

Mixing with ML

Solution mining

0

50

100

150

200

250

300

350

400

0 50 100 150 200 250 300 350 400 450 500 550 600

KCl g/kg H2O

NaC

l g/k

g H

2O

Technology of the salt production (6) Sylvinite

Flowsheet of NaCl + KCl production in a technical process

Chemical purification, precipitation of Mg++, Ca++,SO4

--

Brine

Evaporation,

NaCl crystallisation

Drying

Storage

Oil or gas

Water

KCl

Steam or

electrical power NaCl

Drying

Storage

Oil or gas Water

NaCl

Washing

Vaccum cooling,

KCl crystallisation

Water Soiled brine

Technology of the salt production (7) –Carnallite

Carnallite is a double salt of MgCl2, KCl and six crystall water (MgCl2 * KCl * 6 H2O).

The solubility of the system Mg – K – Cl – H2O is shown in the following diagram.

MgSO4=0 g/kg H2O

20°C

0

100

200

300

400

500

0 50 100 150 200 250

KCl g/kg H2O

MgC

l2 g

/kg

H2O

80°C

Cold

leac

hing

KCl loss by decomposition

Cold leaching

Carnallite crystallisation by vacuum cooling

Eva

pora

tion

Technology of the salt production (8) Carnallite

How we can see the cold leaching has no efficiency, because

- the brine is not high concentrated and many water must evaporated

- the losses of KCl by decomposition of carnallite are very high

Therefore the hot leaching technology for solution mining of carnallite must used. This procedure has not the named disadvantages and has the following advantages:

- The brine is high concentrated. Carnallite can be crystallised by evaporation of a few amount of water and cooling the brine

- The solvent is saturated on NaCl. Therefore halite and also kieserite remain in the cavern as residue.

- In the cavern remains a high concentrated brine, which not worries the environment.

Because the solvent has a high temperature, the cavern has two wells as shown in the following picture. In only one well would exchange the heat between the concentric inner and outher tube or casing.

Technology of the salt production (9) Carnallite

Solution mining of carnallitite with:

- two wells

- selective dissolution

- hot leaching

Technology of the salt production (10)

Carnallite

brine

life steam condensate

hot saturated brine

condensate

slurrymother liquor 1:solvent for solution miningor prodoction of bischofiteor discharge liquor

carnallite, halite

water decomposition liquor

sylvite, halite

hot mother liquor 2 halite, wet

hot brine, KCl saturated

water condensate

slurry

mother liquor

KCl

Flow sheet for the production of KCl from carnallite brine

evaporator

evaporator, vacuum cooling, carnallite crystallisation

vacuum cooling, KCl cristallisation

vacuum cooling, KCl cristallisation

thickener

decomposition

hot leaching

Geological conditions

The very best to use the solution mining technology is:

- a great height of the deposit

- and a low depth

But by using new developed technologies the winning of mineral salts in deposits with low height is possible. This new technology is named solution mining with „tunnel caverns“. In this case one bore hole was drilled verticaly and the other was drilled at first verticaly and then it follows in the deposit the direction of the salt layer with a deviation.

This technologie is not usable if the deposit has tectonical breakdown and other disturbances or great changes in the direction.

The drilling of the bore holes can be complicated and expensivly if the overburden contains gas or water.

Pros and cons

Pros:- In the most cases solution mining has a very high economic efficiency because:

- The investment costs are low. (We don‘t need a mine)

- The drilling of the bore holes are running costs

- The demand of manpower is low

- Solution mining can also used by difficult hydrogeological conditions.

- The first step of the potash mill (hot leaching) is in the underground. There are no costs for this equipment.

- Residue and high concentrated brine stays in the cavern, therefore there environmental burdens are low.

Cons:- If the geological and tectonical conditions are very difficult, the solution mining is not usable.