ASTM D5744-12

The laboratory weathering procedure will generate data that can be used to: (1) determine whether a solid material will produce an acidic, alkaline, or neutral effluent, (2) identify solutes in the effluent that represent dissolved weathering products formed during a specified period of time, (3) determine the mass of solute release, and (4) determine the rate at which solutes are released (from the solids into the effluent) under the closely controlled conditions of the test.

Data generated by the laboratory weathering procedure can be used to address the following objectives: (1) determine the variation of drainage quality as a function of compositional variations (for example, iron sulfide and calcium+magnesium carbonate contents) within individual mine-rock lithologies, (2) determine the amount of NP accessible in a mine-rock sample to neutralize acid and maintain drainage pH≥6.0 under the conditions of the test, (3) estimate mine-rock weathering rates to aid in predicting the environmental behavior of mine rock, and (4) determine mine-rock weathering rates to aid in experimental design of site-specific kinetic tests.

The laboratory-weathering procedure provides conditions conducive to oxidation of solid material constituents and enhances the transport of weathering reaction products contained in the resulting weekly effluent. This is accomplished by controlling the exposure of the solid material sample to such environmental parameters as reaction environment temperature and application rate of water and oxygen.

Because efficient removal of reaction products is vital to track mineral dissolution rates during the procedure, laboratory leach volumes are large per unit mass of rock to promote the rinsing of weathering-reaction products from the mine-rock sample. A comparison of laboratory kinetic tests with field tests has shown that more reaction products from mineral dissolution are consistently released per unit weight and unit time in laboratory weathering tests (9). For example, sulfate release rates observed in laboratory tests of metal-mine rock have been reported to be 3 to 8 times those for small-scale field test piles of Duluth Complex rock (10), and from 2 to 20 times those for small-scale field test piles of Archean greenstone rock (11). A greater increase is anticipated when laboratory rates are compared with field rates measured from operational waste-rock piles.

Fundamental assumptions governing Options A and B of the procedure:

Option AAn excess amount of air pumped up through the sample during the dry- and wet-air portions of the weekly cycle reduces the potential for oxidation reaction rates being limited by low-oxygen concentrations. Weekly leaches with low ionic strength water promote the removal of leachable mineral dissolution products produced from the previous week's weathering cycle. The purpose of the three-day dry-air portion of the weekly cycle is to evaporate some of the water that remains in the pores of the sample after the weekly leach without totally drying out the sample. Consequently, sample saturation is reduced and air flow is enhanced. During the dry-air portion of the cycle, the oxygen diffusion rate through the sample may increase several orders of magnitude as compared to its diffusion rate under more saturated conditions of the leach. This increase in the diffusion rate under near-dryness conditions helps promote the oxidation of such constituents as iron sulfide. Additionally, evaporation from the three days of dry air increases pore water cation/anion concentrations and may also cause increased acidity (for example, by increasing the concentration of hydrogen ion generated from previously oxidized iron sulfide). Increased acid generation will enhance the dissolution of additional sample constituents. As evaporation continues, the remaining water may become over-saturated with respect to some mineral phases, consequently causing them to precipitate. Some precipitated minerals are potential sources of acidity when re-dissolved (for example, melanterite, FeSO₄·7H₂O; and jarosite, K₂Fe₆(OH)₁₂(SO₄)₄). Compared to the three days of dry air where the pore-water mass decreases over time, the wet (saturated)-air portion of the weekly cycle helps maintain a relatively constant mass of pore water in the sample (12)). This may help promote some diffusion of weathering products (for example, re-dissolved precipitation products) in the remaining pore water without totally saturating the sample and adversely affecting oxygen diffusion.

Note 1—Under idealized conditions (that is, infinite dilution in air and water), published oxygen diffusion rates in air are five orders of magnitude greater than in water (0.178 cm² s^-1 versus 2.5 × 10^-5 cm² · s^-1 at 0 and 25°C, respectively) (13).

Option BIn contrast to Option A, Option B protocol does not include dry air or wet air introduction to the humidity cells during the weekly cycle. Instead, Option B requires that temperature and relative humidity be maintained within a constant range by storing the cells in an environmentally controlled enclosure during the 6 days following the weekly 500- or 1000-mL leach. Consequently, oxygen is delivered to the cells by diffusion (and possibly advection) of ambient air, rather than by pumping. Because it lacks a dry-air cycle, more interstitial water is retained in the Option B sample than in the Option A sample during the weekly cycle. Furthermore, the interstitial water content for Option B is more constant than that in Option A during the weekly dry-air cycle. In addition, the interstitial water content for Option B is less variable over the course of testing than that in Option A (14).

This test method has been conducted on metal-mine wastes to classify their tendencies to produce acidic, alkaline, or neutral effluent, and to subsequently measure the concentrations of selected inorganic components leached from the waste (2-3, 14-16).

Note 2—Interlaboratory testing of this method to date has been confined to mine waste rock. The method has not been tested for applicability to metallurgical-processing waste. Although the method has been applied by some practitioners to finely ground metallurgical-processing wastes such as mill tailings, those materials were not included in the interlaboratory testing of the method. Consequently, modifications of this method might be necessary to deal with problems associated with finely ground materials, which would make this method as written, inappropriate for kinetic testing of finely ground materials. For kinetic testing of finely ground materials, please refer to the biological acid production potential method in the appendix of Test Methods E1915 or other kinetic methods accepted by the regulatory jurisdiction.

The following are examples of parameters for which the scheduled weekly, semi-monthly, or monthly collected effluent may be analyzed (see 11.5.2 for suggested effluent collection frequency):

pH, Eh (oxidation/reduction potential), and conductivity (see Test Methods D1293, Practice D1498, and Test Methods D1125, respectively, for guidance);

Alkalinity/acidity values (see Test Methods D1067 for guidance);

Cation and anion concentrations;

Metals and trace metals concentrations.

An assumption used in this test method is that the pH of each of the leachates reflects the progressive interaction of the interstitial water with the acid-generating or acid-neutralizing capacity, or both, of the solid material under specified laboratory conditions.

This test method produces leachates that are amenable to the determination of both major and minor constituents. It is important that precautions be taken in sample collection, filtration, preservation, storage, and handling to prevent possible contamination of the samples or alteration of the concentrations of constituents through sorption or precipitation.

The leaching technique, rate of leach water addition, liquid-to-solid ratio, and apparatus size may not be suitable for all types of solid material.

Notable differences have been observed between Option A and Option B protocols:

Water retention in the solid-material sample between weekly leaches is more variable for Option A than in Option B; for Option A, standard deviations from the mean water retention can range from 20 to 60 % of the mean value; comparable values for Option B have been reported at less than 9 % (14).

Greater water retention in Option B cells may favor dissolution of, and consequent acid neutralization by, magnesium-bearing minerals; increased retention may facilitate transport of acidic reaction products from iron-sulfide minerals to magnesium-bearing minerals (14).

Comparisons of sulfate mass release from the same sample subjected to Option A and Option B protocols indicate no significant difference in sulfate concentration as a result of water-retention variation between protocols (14). This suggests the increased water retention of Option B does not limit oxygen diffusion to the extent that sulfide mineral oxidation rates are reduced (14). However, samples containing greater than 7 % sulfur have not as yet been subjected to comparable Option A and Option B protocol studies.

Note 3—Examples of products from the test include the following: (1) effluent pH, acidity/alkalinity, and specific conductance, (2) cumulative mass release of individual solutes, and (3) release rates for individual solutes (for example, the average release of μg sulfate/g of solid material sample/week). The dissolution time required for NP depletion and the subsequent duration of acid generation can be estimated using the values generated in items (2) and (3) above (15).