The KNeW Process

Revolutionary water recovery solutions

The KNeW process was developed to solve the problem of removing dissolved cations and anions from acid mines drainage (AMD), thus producing excellent quality water at an affordable cost for inland agriculture.

 

AMD is caused by rainwater permeating through mine dumps which contain vast amounts of pyrite (iron sulfide). This Pyrite reacts with the oxygen from the air to form free sulfuric acid and a residual ugly rust coloured stain where it has been lying. This acidic solution flows down into mine workings and continues to eat away at the dolorites in the reefs underground, which then release a cocktail of elements, including heavy metals and radioactivity. This unpleasant mixture is pumped to the surface to allow underground mining to continue thus creating the “AMD” scourge that is being dealt with in some areas by neutralizing with lime. Neutralizing with lime is not a permanent solution as the product gypsum – which is usually stored in the mine area - will re-dissolve with time and convert back to being a sulfate source for renewed pollution. It is, also, unable to remove any of the sodium present and only partially reduces the sulfate level to 1200ppm – allowable limits are 200ppm.

South Africa uses 65% of its fresh water for this important activity. However, one of the worst pollutants for agriculture in irrigation water is its sodium content. Sodium attaches to clay particles in the soil and renders it impervious to water penetration and to oxygen ingress to keep the vital organic life in the soil alive. In most AMD the level of sodium is around 900ppm and with an average of the SA AMD flow at 350ML/day (Turton 2014). This equates to 115 000 tons of sodium per year being poured into our rivers. As 65% of this goes to irrigation that means that 75 000 tons per year of sodium is applied to our soils because of the AMD discharge – and this effect is cumulative.

In most countries of the world over 60% of all fresh water is used for irrigation –

South Africa uses 65% of its fresh water for this important activity.

Did you know?

65%

Reverse osmosis (RO) is able to remove all the dissolved solids but this process delivers excellent water at a very high price while creating a brine that has to be stored for ever – at the coast this is returned to the sea but inland this problem becomes severe.

 

Coal mines mostly produce a neutral effluent (NMD), which contains sodium sulfate and sodium chloride in large amounts. This effluent is created because the soil layer above all SA’s coal mines contains a high level of salt – said to have formed when the sea above the coal seam dried up millions of years ago. When the overburden is disturbed to remove the coal underneath, the salt is leached out by rainwater falling on it and this saline solution then runs down into the opencast or underground workings, from where it is pumped out into the nearest river. When this NMD is disposed of it will also contribute to the destruction of irrigated farmlands just as AMD does.

How does

it work?

In the KNeW process, acidity is removed by neutralizing with Soda Ash. Thus, the usual large gypsum precipitate is avoided, and the minor quantities of heavy metal pollutants can be removed as a concentrate and rendered immobile by fixing in cement. This will ensure that they are never released again into the environment.

  • The effluent solution is then filtered and fed to an ion exchange plant that removes the cations (which will be mostly sodium) and the anions (which will be entirely sulfates and chlorides).
  • The loaded resins are then regenerated using nitric acid for the cations (to form sodium nitrate) and ammonia for the anions (to form ammonium sulfate/chloride).
  • The sodium nitrate solution is converted in a double decomposition reaction to sodium chloride and potassium nitrate. As sodium chloride is the least soluble salt in the mixture at boiling point, it will precipitate out when the solution is concentrated by evaporation and this is separated in a centrifuge, washed and dried for supply to the industrial market. The remaining solution is then cooled and potassium nitrate crystallizes out. This is centrifuged off, washed and dried, for supply to the hydroponics market.
  • The overall result is that the dissolved solids in the AMD or NMD are converted to useful products, leaving clean and usable water. The sale of the fertilizers offset the costs of the process so that the water can be supplied at an affordable price.

The advantages

of the KNeW process

  • Removes all the toxic and inhibiting dissolved solids from AMD and NMD without creating unmanageable wastes and doing all this at no overall cost.
  • The process does not rely on having to sell its product water at domestic prices to try to offset the unaffordable cost attributed to all other processes.

Solubility Chart