The technology sounds promising, but your description leaves a few key questions unanswered. High-density sludge treatment using lime is a mature, well-established technology. Few practitioners will look at an alternative unless a compelling case is made.

• Were your demonstrations at a lab-scale or pilot-scale? Do they provide information on metal removal rates, final metal concentrations (particularly for Cd, Cu, Pb or Zn), and reagent consumption or sludge production rates?
• Saying:"the cost of treatment may be similar to lime" is not good enough. How does a ton of your reagent compare to a ton of lime? How do costs compare for 1,000 gallon (or 4 cu.m.) ARD treated? A proper side-by-side comparison is required to make your case more convincing.

I don't mean to be discouraging, but want you to be clear about the challenges that lie ahead.

Thanks for your comments but you do sound very negative! Perhaps reading the papers and digesting their contents will answer your questions (I did offer to send them to you). And yes, I am well aware of lime and its use in the mining industry. This is why I pursued an alternative approach, particularly in light of some well documented problems with lime and often decreasing purity/increasing cost. You will appreciate that one cannot give an exact cost comparison as it will vary on a case by case basis in terms of water quality, lime or other reagent cost, other costs associated with infrastructure, materials handling/dewatering, possible metal recovery/cost offsets, discharge or reuse criteria etc. Is this good enough?

Our work suggests something in the order of 20% of HT solids is generated compared to lime treatment. It all depends on the initial water quality. Two papers outline the technique and its efficiency.

As the HT forms there are a series of intermediate stages where cations are initially incorporated. As the pH increases, anions and oxyanions are also incorporated as interlayer species. If we take the case of Al and its amphoteric nature, it is quantitatively removed in the HT such that none is left in solution at say pH 8-10 due to its incorporation in the HT. I can send you the manuscripts that outline all of this and give some examples where we look at a range of metals.

I will be in touch. To be clear, approximately 20% reduction or 20% on w/w basis with lime treatment on the "same" effluent? I am looking forward to receiving your publications, and by all means, carry on.

I am neutral towards the technology you are proposing. I am interested in and welcome any new development that overcomes the limitations of lime treatment. However, I am familiar with the strengths and simplicity of lime treatment, particularly in high-density sludge systems, and expect that any new technology developer will acknowledge that the bar is set high.

Second, it is important to know that many proponents with alternatives to lime have come and gone. I recall the Kleeco process that was pushed about ten years ago and proved to be uncompetitive. Currently, the ABC process (Alkali-Barium-Calcium) and the MBO process (Magnesium-Barium-Oxide) are being tested in South Africa, but the pilot studies that were (supposedly) completed a year ago have not yet been presented, and my correspondence with the developers goes unanswered.

Third, I know well the formidable challenges of bringing a technology out of the lab and into the plant. That seems to be the current hang up with ABC and MBO, though both processes hold so much promise.

Lastly, I would indeed appreciate receive your papers. I commented here because I want to hear about your work, though you should expect with the critical mind of a long-time practitioner in these arts.

Around 20% of the solids compared to lime treatment. We do not generate gypsum in any significant quantity other than for say some minor Ca impurities in added Mg.

I am familiar with lime treatment. As stated before, this was the basis for looking at developing a new technology. As with all technologies it will be a case of "horses for courses". Robust criticism is fine, but a vision beyond the status quo is better so that we have more options available for industry.

Over 15 years ago, I was involved in an AMD project where we used MgO as an alternative to lime treatment because we had limited space for storing sludge. MgO was slaked on site (a rather touchy process) and added to AMD. Be aware that MgO will only raise the pH to about 8.5, which is fine for most metals, but may not be sufficient for cadmium precipitation. I am concerned, Grant, about patents on this process. Too often, a process used by the industry is claimed by a company when it really should be considered accepted practice. I hope that the patent is narrow enough that typical applications are not included.

After reading your papers, I feel that your proposed technology is worth knowing and that it will soon (hopefully) has a place as an alternative treatment option for ARD.

Obviously, some real world applications are now needed to determine its strengths and weaknesses. For instance, high-density sludge (HDS) lime treatment is most powerful in removing toxic metals (Cd/Cu/Co/Pb/Ni/Zn) to very low concentrations. Its drawback is the high volumes of sludge that is generated, lack of sulphate removal (past gypsum solubility limits) and problems in dealing with high aluminum/low iron waters.

If we can add the ABC, MBO and/or MgO-hydrotalcite processes to our repertoire, we may provide more viable alternatives to lime and find economic alternatives to membrane filtration processes (reverse osmosis) that are being pushed by stringent regulations.

Thank you for your input - much appreciated. I am aware of the use of MgOas an alternative to CaO. What differentiates the hydrotalcite (HT)-basedtechnique is the requirement to optimise the M2+ to M3+ mol ratios, (alsotaking into account the relative amount of Fe2+/3+ to other M2+/M3+)

generally by addition of Mg and/or Al with by simple salts such aschlorides or if/as required by the addition of MgO or aluminate to addneutralisation capacity. In addition, we also exploit the Mg and or Alalready in the solute, both of which are generally present, to reducefurther reagent addition, and hence cost. Most MgO also contains some CaOas an impurity which in practice generally raises the pH higher than 8.5,but yes, MgO if pure will generally buffer around pH 8.5-9.0. Where
required, NaOH is also added to complete the HT precipitation process byfurther raising the pH, often to 10. We have found though that generallywithin a few days the supernatant buffers back to pH 8.5 which is veryconvenient as many discharge criteria have this as an upper pH.

A further point of difference is that one may vary the pH endpoint betweensay 8.0 and 10 or even higher depending on the metals present and theirspeciation to facilitate incorporation into the HT. In practice we havefound most "difficult" metals are quantitatively removed. Please refer tomy papers.

Finally, we also remove many anions, often those present asmetallo-oxyanions as interlayer species as the HT forms. This complementsthe incorporation of metals in the primary M2+/M3+ mixed metal hydroxidelayers. Other theoretical and operational advantages have been outlined in
earlier discussions in this forum.

The technique that you describe of using MgO as a method to simply raise pHis no doubt effective in some cases, but we have taken it much further withdetailed experimental work and many trials using a variety of real AMD andother waters, not all of which we have published (yet!). To my knowledge,from 25+ years research experience, the scientific literature, and frompatent searches augmented by many other searches when being assessed inother patent jurisdictions, the processes described above, particularlywith reference to HT formation, the optimisation of such using existing Mgand/or Al in solution, where required, the concurrent optimisation and useof alkalinity-producing Mg and/or Al species is not covered by any existingpatents or techniques.

I am happy to continue the discussion. In particular, I want to get thisout there and used. As a government scientist I have no financialinterest. My "reward" is successful commercialisation leading to improvedoutcomes. I have been down this (often bruising)research-patent-commercialisation path before with Phoslock, a P-absorbentclay now used in around 25 countries.