crunch time

another fine example of continuous disclosure by pst ...


are't those guys getting serious, or what ?
maybe you can answer ? he,he
I like to watch





Tuesday, April 26, 2005 11:44 AM "Bauxsol Questions for prof David McConchie"



Bauxsol is a manufactured dry red solid comprising a complex mixture of minerals that include abundant hematite, boehmite, gibbsite, sodalite, quartz, cancrinite, brucite, calcite, diaspore, ferrihydrite, gypsum, hydrocalumite, hydrotalcite, p-aluminohydrocalcite, portlandite, minor aragonite and a few low solubility trace minerals (Clark, 2000). We have been asked to assess its potential as an agent to treat an acid rock drainage problem at Skytop. A following list of questions are identified by author (B for Barnes, D for Doden, G for Gold, and R for Rose) and listed under topic categories. I recognize some overlap in the questions, and I realize that one answer may satisfy more than one question.

Physics of Bauxsol in loose rock

G1. One involves the textural nature of the “fill” and waste material, the abundance of voids (porosity) and the communication between voids (permeability). Is it necessary for bauxsol to fill the voids? How does one get around the problem of piping and channeling that is prevalent in a heterogeneous “aggregate-fill”, such as one would encounter at Skytop?

G2. What are the scales of the pilot tests (e.g., Mt. Carrington and Gilt Edge) done to date? How much bauxsol should be applied for the Skytop material? Is the proportion of bauxsol to “fill” determined by (a) a “mass balance” to ensure all the pyrite can be consumed over an extended period of time, or (b) a “rate balance” in which pyrite is coated by iron oxyhydroxide minerals and effectively rendered inert (i.e., taken out of the oxidation reaction cycle)?

D2. Given the geometry and nature of the Buttress site (2:1 slope, < 10 feet (?) thick pile of compacted material on a limestone drainage layer (for part of the Buttress), very heterogeneous particle size distribution, etc.), how do you ensure that the injected Bauxsol solution can be reasonably well distributed in three dimensions throughout the target region?

D3. Following on #2, it would be helpful to see a plan and cross-section to illustrate the proposed sites for injection wells and monitoring/water-return wells on the Buttress. Although more information is needed to better understand the movement of groundwater through the Buttress and underlying bedrock, how can you ensure that water tested for pH, etc. from the monitoring wells is representative of more than only narrow channels that might (and probably do) exist between points of entry (injection wells) and exit from the Buttress?

D4. How long must a Bauxsol solution be in contact with the pyrite-bearing aggregate to effectively render it neutral? How much time is needed for water tested in the monitoring wells for pH, sulfate concentration, etc. to reflect this neutralization? In other words, reaction rates seem important here; how much time is required for pyrite to be encapsulated effectively, metals to be adsorbed, etc. and would the short-term pH tests from the monitoring wells actually record these processes?

D5. What is the device or method used to inject the Bauxsol solution into the drill holes, and does it inject only at the bottom of the hole or does it apply pressure throughout the column?

B4. Is there a danger of loss of the fine Bauxsol as a suspension that may be flushed down higher permeability channels of the waste piles prior to reacting?

R4. How do you determine how much Bauxsol is needed in a mass of broken rock? For long-term (10's of years) protection, I should think you would need enough at least enough neutralization potential to balance the pyrite oxidation, plus some more for loss by leaching, plus enough to fill essentially all paths of water leaving the mass of rocks. This seems like a very large amount.

R6. How do you determine the drill spacing for injection?

Chemistry of reaction of Bauxsol

G3. What are the roles of hematite and quartz in the bauxsol mix? Both should be “insoluble” in the “heap-leach” conditions envisages in the “aggregate fill” and waste rock piles. If a critical reaction in bauxsol involves the Fe2+ - Fe3+ then surely wustite or even magnetite would be a better candidate. If not, is hematite acting as a sorbant for metallic cations?

B1. Hematite has been a very slow reactant in our various lab experiments where it affects, if at all, only redox processes and has no pH influence except of acidic solutions below about pH 3. What does it beneficially contribute to the Bauxsol mix? Isn’t it primarily an inert dead load?

B2. Isn’t the advantage of Bauxsol simply its alkaline buffering capacity due to its mix of 5 minerals: boehmite, gibbsite, brucite, hydrocalumite, and hydrotalcite? Then the other minerals of the mix, sodalite, quartz, cancrinite, diaspore, and hematite are so slow reacting that they are of no positive value? If so, wouldn’t a brucite slurry with its larger buffering capacity be a superior means of maintaining the pH between 8 and 10 where the product of pyrite oxidation is armoring iron oxyhydroxides or sulfates and nearly all other metals are also insoluble?

B5. Because the kinetics of reaction of the Bauxsol-saturated solution with pyrite is controlled by the surface area of that mineral, can the grain sizes and porosity of the waste be mostly ignored in estimating the quantities of Bauxsol that may be necessary to control acid-generation in a waste bank?

R7. What is the rate of neutralization by alkali silicates in Bauxsol (sodalite), as contrasted to carbonates and hydroxycarbonates? Does this have any pH effect, or is the neutralization effect all from carbonates and the like?

R8. What information do you have on the host for trace metals in the Bauxsol? Is it adsorbed on Fe oxides, incorporated in newly grown minerals (and what minerals), or what?

S1. We were told at our meeting on the 24th that the components of Bauxsol were positively charged and attracted negatively charged components of the pyrite alteration. I have taken that to mean that the mechanisms of sequestration are ion exchange. Is this interpretation of the explanation that was given to us correct? If it is, can you clarify the nature of the sorbing substances, what anions are sorbed and any preferences [i.e. Kd’s].
If the explanation is incorrect, can you provide the correct mechanism with the rationale?.

S2. From your presentation at the December workshop, you had indicated the mineralogical composition of Bauxsol as containing six primary phases? The majority of these phases are inert and the remaining phases do not have ion exchange capabilities. Can you therefore clarify the mechanism for the power remediative action of Bauxsol? Are you relying on the modified red mud mineralogy to perform the sequestration or are you relying upon in situ alteration of the red mud host rock interactions for the sequestration?

S3. Is the sequestration mechanism the same for grouted Bauxsol as it is for the addition to sed ponds and/or soil amendment applications?

S4. At this same meeting we were told that during the performance cycle of Bauxsol that crystals nucleated and grew which incorporated heavy metals into their structures. This observation could very well account for the decreasing concentrations of heavy metals reported to us. Can you provide the evidence for these statements? What phases form? and what is the projected effective life time for the chemical reactions?

Properties of Bauxsol

D1. It has been suggested that the composition of Bauxsol vary depending on its intended use. What is the proposed composition for use at Skytop?

R5. What is the physical state of Bauxsol? Particle size, density, porosity, permeability?

S6. We were also told that the blend of phases in Bauxsol was adjusted for site specific applications. Can you clarify this statement? What is blended with Bauxsol? How are decisions made with respect to what is blended? How much material by mass is blended?

S7. It was stated that Bauxsol consisted of 10 micron sized particles yet the sample that PENNDOT purchased is very coarse. This observation would lead me to believe that different particle sizes may be used for different applications. Is that true? What are the effects of surface area on Bauxsol applications? Is there any differentiation in mineral content as a function of size?


Information on previous tests

R1. At Mt. Carrington, please furnish more details of the tests on the haul road material, as described in a Virotec brochure sent to Gary Byron. This appears to be the only case study of material similar to the I-99 situation. What is the % sulfide in the rock waste? How much Terra B was brought in? How deeply was it mixed with the rock waste? In other words, what is the mixing ratio of Terra B and rock? I infer it was mixed to less than 50 cm, since the lysimeters at 50 cm are said to be below the zone of Terra B. What is the variance of the 3 lysimeters per test? What is Terra B? How does it differ from simple Bauxsol? What is its neutralization potential? What is the climate at Mt. Carrington?

R3. At Gilt Edge, please furnish specific reports and descriptions of work on piles of rock waste. I am able to find only references to a permeable reactive barrier, and experiments with barrels of rock.


Other Questions

B3. Intermittently immersed by seasonally dependent precipitation? If so, can a lower amount of Bauxsol be used in such circumstances where the oxidant may be mostly air, a slower reaction than with aqueous oxygen?

B6. Have you used an in situ pH measurement to track when the buffering capacity of Bauxsol becomes exhausted within a waste pile and the pH falls below 7? Such monitoring could signal the minimum amount of Bauxsol that could be applied risk-free to control the pH at moderately alkaline conditions at minimum cost. To continually follow the pH, is there a rugged and better pH sensor than the electrode offered by Thermo Orion, which is specified to measure to within 0.05 pH units without recalibration for at least a year. As a bonus, it also has an integral temperature sensor to permit detecting any increase due to the enthalpy of pyrite oxidation.

S5. From my experience, ‘red mud’ has an environmental problem associated with the presence of hexa-valent chromium. Why isn’t hex-chrome a problem for Bauxsol?