Forthe technical nerds, excerpts from An Environmental Critique...

Forthe technical nerds, excerpts from An Environmental Critique of In Situ Leach Mining: The Case Against Uranium Solution Miningby Gavin Mudd,Victoria University of Technology.

 

I don’t much care for the underlying theme and/orconclusions of this paper, but it has some interesting technical commentarythat may be of interest to some of you.

 

Clay Interference

 

The unique plate-like mineral structure ofclays leads to them to have a strong influence on the chemistry of groundwater(Langmuir, 1997). Clays have a high surface-charge density to mass ratio(usually a negatively charged mineral surface), meaning that positively chargedelements in solution tend to adsorb onto the surface of clays in an aquifer(Freeze & Cherry, 1979). This leads clays to have a large capacity toundergo the processes of ion exchange and adsorption, which can significantlyaffect the overall chemistry observed in groundwater.

 

Often in groundwater systems, the claysfound within the aquifer materials contain a particular positively chargedelement on their surface, such as calcium. If the chemistry of the groundwateris altered, the clays can undergo ion exchange, whereby the calcium is replacedby sodium or ammonium, for example. Since there is only a positive charge ofone on a sodium or ammonium ion in solution and a positive charge of two for calcium,it takes two sodium ions to replace one calcium ion on the clay surface. In orderto accommodate the extra ions, the clay will expand or swell (Langmuir, 1997). Theability of a clay to undergo ion exchange with positively charged elements in solutionis known as the CEC or Cation Exchange Capacity (Fetter, 1993).

 

One particular clay mineral that has thisproperty is montmorillonite, a member of the smectite group of highly reactiveclay minerals (Langmuir, 1997). The swelling caused by clay minerals undergoingion exchange can lead to structural instability and a significant reduction inpermeability of the aquifer material involved (Langmuir, 1997). This loss ofpermeability means lower flow rates through the aquifer and higher pumpingcosts for an ISL mine.

 

At many early ISL sites in the USA, sodiumor ammonium ions (since many early ISL trial mines used sodium or ammoniumbicarbonate leaching chemistry) that were undergoing ion exchange with clayscaused severe problems with loss of permeability (Kasper et al., 1979; Charbeneau, 1984).

 

Another important property of clays is thatof adsorption. In much the same way that clays can undergo ion exchange, ifthere is available space on the surface of a clay mineral, they can adsorbelements onto their surface with no release of another element. This can leadto say, ammonium for example, being adsorbed onto the clay surface and notremaining in solution until the chemistry of the groundwater is altered onceagain. Adsorption is an equilibrium process, meaning that there is always abalance between the amount on the surface of a clay and that in solution. It isalso a critical process for many trace elements, as they tend to adsorb verystrongly to clays (Fetter, 1993).

 

The processes of ion exchange and adsorptionon clays lead to three important consequences for In Situ Leach mining:

 

(i) they can affect the overall leachingchemistry due to removal of, say, sodium, from solution and reduce theeffectiveness of the dissolving the element and mineral of interest (such asuranium or copper minerals);

 

(ii) any element that is adsorbed onto aclay, can later be released upon a change in overall groundwater chemistry,hampering the restoration of groundwater quality;

 

(iii) the exchange of ions between solutionand clays can lead to swelling of the clay, and a subsequent permeabilityreduction in the aquifer.

 

The Scaling Problem

 

The process of ISL involves a number ofoften competing phenomena, and it is the engineer’s job to exercise appropriatejudgement to use laboratory scale data to design and construct a full scalefacility. This is by no means an easy task.

 

In order to develop an ISL deposit tocommercial production, samples from the orebody are first subjected to columnleaching tests in a laboratory. The lixiviant chemistry, before, during andafter leaching, the ore chemistry and physical properties are all monitoredvery closely during a column test. Such testing may also include different configurations,such as different oxidants or leaching solutions on alternate ore samples. Thedata collected from these tests is then used to plan design the overall ISLmine, beginning initially with small scale trials. However, the problem ofscaling laboratory data to a full size facility is a fundamental engineeringproblem, whether used for the design of multi-storey buildings, or even watersupply dams. In ISL, the laboratory data may tend to underestimate the chemicalor physical aspects involved, such as the quantity of acid required to leach acertain amount of uranium, the permeability of the material and problems ofclay swelling, or the degree to which other contaminants may leach.

 

A good case in point is the ISL trial ofsulphuric acid at the Nine Mile Lake site near Casper, Wyoming in the UnitedStates, reported in Nigbor et al., 1981 & 1982. The uranium deposit was thought to be amenable to the use of sulphuric acid leaching chemistry since the deposit contained less than 0.1% calcite (CaCO3, a strong acid consumer), thereby avoiding the restoration problems of sodium or ammonia-based lixiviants. Laboratory tests conducted on samples with sulphuric acid and carbonate lixiviants indicated that savings in chemical costs could be made by the use of sulphuric acid. These results and calculations were shown byfield trials to be misleading (my emphasis). In the laboratory, only 800 mg/l of acid was required to obtain a pH of 2.0; but in the field trials, 3,000 to 5,000 mg/l of acid was required to attain the same pH. This significant difference was explained by the higher concentration of gangue minerals in the field which consume the lixiviant, and the lower water content (or liquid-to-solid ratio) in the field compared to the laboratory. A similar case was also presented for the

oxidant used (hydrogen peroxide, H2O2). Thecase demonstrates that despite the best indications laboratory data can give, theyare not by their nature the most effective method to scale and engineer thedesign for an ISL mine or trial (my emphasis).

 

Sorry for the longpost.