Magazine Article on Acid Mine Drainage For Profit
THE KNeW PROCESS™ – Trailblazer Technologies (Pty) Ltd
RECOVERY OF ACID MINES WATER PROFITABLY
J. Bewsey CE, CSci
Patent Owner Trailblazer Technologies (Pty) Ltd, Box 680 Featherbrooke, Krugersdorp, South Africa 1746 firstname.lastname@example.org
Acidic mines effluent water has presented an expensive problem to recover up until the advent of the KNeW process (Potassium Nitrate ex Waste) patented in 2011 by Trailblazer Technologies. The AMD is neutralised without creating any gypsum, filtered to remove coarse particles and precipitated heavy metals, then pumped through an ion exchange battery to remove all the dissolved ions leaving a water of any required quality.
The cation resins are regenerated with dilute nitric acid and the anion resins with ammonia solution. The residual sodium nitrate solution is mixed with an equimolal amount of potassium chloride and evaporated with the result that the least soluble salt – sodium chloride – crystallizes out. On cooling potassium nitrate is isolated as a primary fertilizer.
Ammonium sulfate is separated from the anion regeneration.
The sale of the fertilizer products pays for all the operating costs and leaves an interesting profit.
The most difficult pollutant to remove effectively from any effluent is sodium as all its salts are soluble. Sodium pollution causes more devastation on the soil than all other dissolved solids together. The KNeW process achieves this removal using minimal energy and converts it to a usable raw material thus saving the environment as over 60% of all fresh water is used for irrigation and this pollution is cumulative and debilitating to agriculture.
The mining of minerals from the earth’s crust will always leave a void from where the more valuable material has been removed and this void will fill with pristine rain water when the wet season arrives. During this water’s sojourn in the manmade cavity it will dissolve any soluble salts that it contacts and if the water ingress is excessive then this now polluted volume will spill over into the nearest water course together with its load of salts spreading its problem into the rivers that feed the nation.
This pollution that is generally called Acid Mines Drainage (AMD) can be quite variable in its make-up due to the wide variety of mining operations in South Africa. In the gold, uranium and platinum mining areas the dissolved salts are mostly sulfate oriented and acidic due to the pyrite ore associated with the reefs that are being mined, while coal mines create a neutral salinity problem due to the coal fields being fossilized sea beds with dried up salt lakes above them and that salinity re-dissolves into the rainwater influx. (Zhao, 2010)
Most of the research effort up until the present has been devoted towards developing processes to remove the major dissolved ion in the gold mining areas – sulfate – along with iron and heavy metals such as manganese and chromium. (de Beer M and Greben H, 2010) Little work has been directed towards the removal of dissolved sodium but as will be shown in this paper this is by far the most dangerous pollutant in this mines effluent.
Sodium is a quiet killer of the soil as it attaches to the clay particles in the soil and is not readily removed. When the sodium ion is attached to the clay particles it hydrates and causes the clay to swell making the soil impervious to water and air. Without water and oxygen penetration into the soil the agricultural yield drops quite drastically and this is a far more serious long term problem than any other associated with AMD. Between 60% (USA) and 95% (Australia) of any country’s fresh water is used by agriculture so South Africa’s water scarcity affects food production far more than it does the much heralded possibility of potable water shortages in urban areas. It is, therefore, of the utmost importance that any process used to clarify AMD be able to remove the sodium content of the water from the environment – as well as the toxic heavy metals and, where applicable, residual radio-activity. It is a misdirection of effort to be concentrating only on sulfate reduction or removal as this ion is essential for agriculture and is purchased in large quantities by agriculture to add to soil to enhance the protein content of the crops.
To put this argument into perspective it is sobering to think that the daily off-take of the Rand Water Board is over 4100ML/day (RWB 2012) while all of the AMD arising across the mining area is estimated to be about 350ML/day. (Turton A 2012) So to be putting a large and expensive effort into partially or completely cleaning AMD for drinking water purposes seems to be misguided and is usually promoted as a revenue source to try to justify the viability of a particular process. It is far more important that AMD be brought to a quality suitable for agriculture and to be returned to our water courses for irrigation purposes than to be trying to supplement the lesser requirements of the urban user.
Thus it became clear that effort had to be put into not only removing and delisting all the toxic heavy metals which are relatively small in volume but to beneficiate the permanently soluble salts like sodium and chloride which cannot be easily separated and to convert them to good use effectively removing them from the environment and at as little cost as possible.
As none of the precipitation processes could achieve this requirement as most sodium and chloride salts are quite soluble this lead to reviewing the work done in reverse osmosis (RO) and in ion exchange (IX) to find a suitable process.
RO produces the required product water but concentrates the brine problem, uses large amounts of power and is expensive to run. Traditional IX suffers similarly except that it does not use much power. It was, thus, essential that a process be created to convert the arising concentrates to useful products that would remove the permanently soluble salts from the environment and would, if possible, cover its cost of operation.
A solution, the KNeW process, an acronym for Potassium Nitrate ex Waste, was developed by Trailblazer Technologies in 2010 which successfully removes all the dissolved salts and converts them into beneficial raw materials for agriculture and industry. The KNeW process covers all the costs of the operation from the sale of these end products, uses very little power, creates much needed jobs in the chemicals processing and does not rely on the revenue from the sale of water for its economic justification.
2.0 THE KNeW PROCESS
It must be stressed that the KNeW process is a means of turning the regeneration mixes from an ion exchange operation into useful and relatively valuable exit products as opposed to all other processes that extract the pollution salts from AMD but produce valueless or troublesome brines and solids. The KNeW process can be fitted after IX or RO and convert them into profitable operations but it makes little sense to do this after RO as it is comparatively expensive to operate while IX can achieve approximately the same result at low operating pressure and cost.
|Figure 1 – General flowsheet for AMD remediation by ion exchange|
Figure 1 is a schematic flowsheet of a standard IX operation where radio-activity – if present – is first removed at low pH for recovery followed by the removal of any acidity using sodium or potassium carbonate. This will cause the magnesium and calcium present to precipitate as a dolomite together with the heavy metals such as iron, manganese and chromium as insoluble hydroxides. These can be removed by centrifuging or filtration and be simply disposed of by immobilising as their volume is relativity small. (In the case of coal mining the level of these cations is low). The remaining singly charged cations in the water are then removed on the cation resin followed by the removal of the anions on the anion resin leaving clean residual water. The catex and annex resins are regenerated when exhausted using nitric acid and ammonia to give a solution of nitrate salts of the cations that were present and ammonium salts of the anions that were present. (Robinson & Gussman 2001)
|Figure 2 – The KNeW cation process|
Potassium chloride is then added to the cation nitrate regenerant solution in an amount equimolal to the sodium present. This effectively leaves a solution of sodium chloride and potassium nitrate but the important property is that the solubility curve of sodium chloride is essentially flat – see figure 3. (Lange’s Handbook)
|Figure 3 – solubility curves|
On evaporation of the catex regeneration solution the least soluble salt will crystallize out – sodium chloride – and this is centrifuged out and washed to give a pure NaCl which can be sold to the chlor-alkali industry. The residual hot mother liquor is cooled giving crystalline potassium nitrate which is centrifuged off, washed, dried and bagged for sale to the horticultural industry. The remaining mother liquor is recycled with fresh feed to give complete conversion and recovery of the dissolved salts.
The anion regenerant is essentially ammonium sulfate and ammonium chloride and this solution is evaporated to saturation level at 50°C and then 30% methanol by volume is added to the crystalliser causing the ammonium sulfate to crystallize out as a pure fine solid. (See figure 5) This is centrifuged off, washed, dried and bagged off for sale to agriculture.
|Figure 4 – KneW anion process|
The remaining mother liquor is pumped to a distillation column for the recovery of the methanol for reuse in the ammonium sulfate precipitation. The bottoms from the distillation has the residual sulfate separated out by the addition of a calcium nitrate solution to give a small amount of gypsum for agriculture while caustic soda is added to the residual liquor to give ammonia for recycle to the anion regeneration solution and more sodium chloride for the chlor-alkali industry.
|Figure 5 – Ammonium Sulfate precipitation using methanol ]|
The effect of the KNeW process is to convert all the sodium and chloride in the effluent to the only sodium salt with an on-going market, sodium chloride, while the sulfate is almost entirely converted to ammonium sulfate for agriculture. All the nitrate from the nitric acid catex regenerant becomes potassium nitrate for horticulture and the ammonia from the annex regenerant becomes ammonium sulfate pure for agriculture. The KNeW process develops only a small amount of gypsum for agriculture (instead of other processes creating large amounts needing beneficiation), is able to remove radioactivity beneficially and is only left with the smaller, more concentrated waste stream of heavy metals to dispose of which is a safe and simple operation as the heavy metals all present as insoluble, safe hydroxides.
2.1 CONTINUOUS ION EXCHANGE PROCESS DEVELOPED
As most of the neutral waters contain carbonates the operation of the traditional column technology became unmanageable due to the release of CO₂ in the cation column disrupting the resin bed completely. We thus adopted a CSTR approach where the resin is moved using air pumps (to protect the resin) and rotating trommel screens countercurrent to the treated water with each reactor being operated and stirred by only one pump. There is thus no need to rinse the resin as it is separated and drip dried at each stage.
Strong solutions can be made in the regeneration streams reducing the cost of concentration quite dramatically. The KNeW process can operate successfully at very high concentrations (tests have been successful at 175000 ppm TDS in the feed stream) and as it does not require any rinsing it can recover in excess of 90% of the feed water – this can usually only be done up to 10000 ppm TDS when using normal column technology; above this level it becomes a nett water user.
3.0 MARKET VIABILITY
The world market for potassium nitrate is about 1.5 million tons per annum (Trailblazer 2013) growing at 10% per annum. The maximum potential production of potassium nitrate from the KNeW process in the South African context will be about 300 000 tons per annum equal to two year’s growth in the world market.
The world market for ammonium sulfate is about 28 million tons (Trailblazer 2013) which means that there is sufficient market for the proposed KNeW product which on the same basis as above would be 150 000 tons per annum. The SA salt market is around 1 million tons per year (Trailblazer 2013) so there will never be a difficulty in finding a usable sink for the KNeW chemically pure salt.
The KNeW ion-exchange process is the only process presently available that can convert AMD and brak coal mining effluent water into acceptable reusable water and valuable raw materials needed in large quantities by agriculture and industry. It also does this conversion at a financial ratio that makes the cost of operating anywhere between a breakeven to a comfortable profit and creates an interesting number of badly needed jobs in the chemical processing plants.
de Beer M and Greben H, (Feb 2010) Positive management of mine effluents ReSource pp 53 – 57
Lange’s Handbook of Chemistry (11th edition) Ch 10 (Ed Lange N.A) MacGraw-Hill Book Company
Robinson R.E & Gussman H.W (2001) The evolution of economically viable waste water treatment processes based on ion exchange resins – figure 5 – Fer-IX process
(RWB) Rand Water Board Annual Report (2012) pp16
Trailblazer Technologies internal market research 2013
Turton A (2012) private mail to author
Zhao B (2010) – The genetic relationship of coal mine water quality and its source rocks in South Africa: a summary – University of Fort Hare