I would like to thank Tim O'Hearn of BC Research Inc., Vancouver, for the use of the photographs of sub-aerial and sub-aqueous testwork columns.

Introduction

Acid Base Accounting procedures are used as a screening process to categorise materials into potentially acid generating, potentially non-acid generating and uncertain groups. For material where the potential for acid generation is uncertain, kinetic testwork is performed to attempt to define acid generation characteristics. The term kinetic is used to describe a group of testwork procedures wherein the acid generation (and metal solubilization and transport) characteristics of a sample are measured with respect to time. Acid Base Accounting procedures are referred to as static because measurements are made over a short and fixed period of time. Procedures are described below for humidity cells, columns and lysimeters, which are the three most commonly used methods of determining kinetic ARD characteristics of drill core, waste and other rock samples, and tailings. A procedure for determining the kinetic ARD characteristics of in-situ rock such as pit walls and the rock surfaces of adits, stopes and other underground workings has been developed through the MEND Program and the British Columbia ARD Task Force. This procedure is called "Minewall" (MEND, 1995). Humidity cells are typically laboratory units using a sample size of about 1 to 15 kg. Columns may be of laboratory, pilot plant or site scale with sample size ranging from a few kg to hundreds of kg. Lysimeters are site scale units with sample size typically in the tonne range. The Minewall Procedure has been used on exposed rock surfaces no greater than 1 m by 1 m (Price, 1997). For an extensive review of kinetic geochemical processes and their relevance to the assessment of waste rock ARD potential, see Perkins et al. (1995)

All kinetic testwork procedures contain the following elements:

As will be discussed below, calculated rates of acid generation, neutralization capability depletion, and metal release are for the test sample used, and the equipment configuration and conditions used, and cannot be used directly for the prediction of field water quality (unless the sample is truly representative of its source material and field conditions are used, as would be the case with a large lysimeter test on site).

Humidity Cell and Column Testwork Equipment Arrangement

The following Figure 1 shows generic schematic diagrams for the arrangement of a humidity cell, a sub-aerial testwork column, and one possible configuration for a sub-aqueous testwork column.There are other possible configurations for sub-aqueous testwork (Lawrence, 1995).

Humidity cells have, in the past, varied considerably in dimensions.However the laboratories servicing the mining industry have now adopted a degree of standardisation. In Designation: D5744-96 Standard Test Method for Accelerated Weathering of Solid Materials Using a Modified Humidity Cell (ASTM, 1996), a cell 203 mm (8.0 inch) in height by 102 mm (4.0 inch) diameter is specified for material crushed to 100% passing 6.3 mm (crushed core or waste rock, and coarse tailings), and a cell 102 mm (4.0 inch) in height by 203 mm (8.0 inch) diameter is specified for material passing 150 m (fine tailings). Price (1997), while recommending similar dimensions, states that the dry and humid air flow should be directed across the surface of tailings samples, rather than through the samples from below. In both of the above arrangements the sample mass is 1 kg which typically gives a bed depth of about 80-120 mm in a 102 mm diameter cell and 20-40 mm in a 203 mm diameter cell, depending upon sample bulk density. Soregaroli and Lawrence (1998) have reported the use of a 3 kg sample with a 102 mm diameter cell and a 15 kg sample with a 254 mm diameter cell, and compared results with those obtained with a 1 kg sample and 102 mm diameter cell.

Figure 1: Schematic Arrangements of Humidity Cell and Sub-Aerial and Sub-Aqueous Columns

For details and specifications of dry and humid air supply systems, materials of construction and other mechanical details see Designation: D5744-96 Standard Test Method for Accelerated Weathering of Solid Materials Using a Modified Humidity Cell (ASTM, 1996). The dry and humid air systems must be capable of delivering a controlled rate of 1 to 10 l/min to each humidity cell, and flow meters are required for flow rate measurement (ASTM, 1996).

Columns for sub-aerial and sub-aqueous testwork are typically 76, 102 or 152 mm (3, 4 or 6 inch) in diameter, and from about 1 m to more than 3 m in height. The following two photographs show both sub-aerial and sub-aqueous column testwork in progress.

Photograph 1: Sub-Aerial Testwork Columns (Left) and Photograph 2: Sub-Aqueous Testwork Columns (Right)

Photographs Courtesy of BC Research Inc., Vancouver, BC

 

Humidity Cell Procedure

The humidity cell operational procedure is a cyclic one, during which the sample is subjected to three days of dry air permeation, three days of humid (water saturated) air permeation, and one day of water washing (with a fixed volume of water). Caruccio (1995) has suggested that coal spoils from the Eastern United States should be subjected to six days of humid air, rather than three days of dry and three days of humid air.

The procedure is usually referred to as an accelerated weathering procedure (ASTM, 1996) because it is designed to accelerate the natural weathering rate of potentially acid-generating samples and reduce the length of time for which testwork must be run. The artificial nature of the procedure is a consequence of regulatory constraints, rather than natural ones.

Designation D 5744-96 (ASTM, 1996) states:

"1.1 This test method covers a procedure that accelerates the natural weathering rate of a solid material sample so that diagnostic weathering products can be produced, collected, and quantified. Soluble weathering products are mobilized by a fixed-volume aqueous leach that is performed, collected and analyzed weekly. When conducted in accordance with the following protocol, this laboratory test method has accelerated metal-mine waste-rock weathering rates by at least an order of magnitude greater than observed field rates.

1.6 This test method is not intended to provide leachates that are identical to the actual leachate produced from a solid material in the field or to produce leachates to be used as the sole basis of engineering design.

1.7 This method is not intended to simulate site-specific leaching conditions. It has not been demonstrated to simulate actual disposal site leaching conditions."

The ASTM Procedure (ASTM, 1996) requires a minimum test duration of 20 weeks, while Price (1997) recommends a minimum of 40 weeks. It is common in Western Canada for humidity cells to be run for periods in excess of two years (104 weeks). It is possible that some tropical areas of the world experience climates similar to that of a humidity cell, but for North American mines the atmospheric conditions of the humidity cell are considerably harsher than the natural ones to which waste rock and tailings may be exposed. This is because:

  1. Testwork is usually conducted in a laboratory at room temperature, which is normally greater than the atmospheric temperature to which mining waste material is, or will be, exposed. Lower temperatures slow both chemical and biological reactions involved in acid generation.

  2. Testwork ensures a rigourous dry air/moist air/water cycle to accelerate sulphide oxidation and to maximise oxidation product flushing. Most sites experience neither the regularity of the dry air/moist air cycle nor the regularity and intensity of wet precipitation corresponding the water cycle of the humidity cell.

  3. The water flush cycle of the humidity cell is conducted to ensure as completely as possible the wetting and flushing of the entire sample. Precipitation influx through waste rock dumps, and to a lesser extent tailings, is non-uniform due to channelling, and complete wetting and flushing is not achieved. Thus the conditions of full oxygen and water supply to liberated sulphide minerals will not be achieved in practice.

  4. Size reduction of waste rock or drill core, or the use of only fine fractions of waste rock, to meet the requirements of the humidity cell procedure produces sample material with a substantially greater specific surface area (m2/kg) than the real waste rock, and a substantially higher degree of both sulphide (acid generating) and neutralizing minerals liberation. Therefore, on a sample mass basis, a humidity cell test will potentially produce more acid (mass) and more soluble metals (mass) in a given time than the corresponding waste rock.

A humidity cell test will usually determine if a given sample will "go acid", but not when the material from which the sample was taken will "go acid" since the operation of the humidity cell has intentionally accelerated sulphide mineral oxidation. Similarly, the accelerated rate of oxidation and acid production will result in an accelerated rate of oxidation products generation as dissolved metals and/or precipitated metal compounds. That is, the metal concentrations in the weekly leachate (wash cycle) are likely to be higher than those generated in the field.

Prior to start-up of a humidity cell test, the sample material must be weighed, and characterized by ABA analysis, ICP multi-element analysis, whole rock major element analysis, screen sizing and mineralogical examination. The same weighing and characterization procedure must also be conducted on the humidity cell sample residue at the completion of the humidity cell test. It is particularly important that Acid Generation Potential (AP) and Neutralization Potential (NP) of the initial and final samples are determined using identical methods. It is equally important that the initial and final samples are adequately characterized mineralogically, because some samples undergo complex mineralogical changes during the extended period of a kinetic test. Without "before" and "after" mineralogical information it is sometimes difficult or impossible to evaluate kinetic test data.

Samples from material containing soluble sulphate minerals or previously oxidized material may require pre-treatment prior to humidity cell testwork in order to remove minerals or chemical compounds that may either mask acid generation or delay it.

The preparation of humidity cell test samples should avoid size reduction processes (such as crushing) if possible. Tailings samples can usually be tested "as is", while Price (1997) recommends the use of only the minus 2 mm fraction of existing waste rock dumps. Drill core must be crushed to provide samples suitable for humidity cell tests. Price (1997) recommends 80% passing 6 mm for future waste rock and ore, and 80% passing 150 m for future tailings. The reduction of drill core to 80% passing 150 m requires laboratory crushing and grinding that may produce size and mineral liberation characteristics very different from those of a real mill using several stages of crushing, screening, grinding, classification and other unit operations. It is therefore preferable that samples used for kinetic testwork on future tailings are those produced from metallurgical testwork.

A further particle size-related concern in humidity cell (and column) testwork is the presence of clay minerals (such as illites and smectites), with particle size less than 2 m, in the test sample. In the above arrangement diagrams, the sample is shown supported by a grid which, in practice, is covered with a filter medium to prevent fines loss during flushing. While this filter medium must be chemically inert and non-retentive to water, it must also be porous enough for easy flow of leachate while retaining the test sample. In practice a compromise is made with the use of polypropylene felt, or equivalent, with a filament diameter of 22 m (ASTM, 1996). Such material is unlikely to retain particles of less than 2 m, and samples containing significant quantities of such particles will produce a leachate from flushing that will require further filtration (typically at 0.45 m) before dissolved metals can be determined.

A subject of considerable uncertainty at this time is whether or not humidity cells should be inoculated with bacterial cultures such as Thiobacillus ferrooxidans. ASTM Designation D5744-96 (ASTM, 1996) includes instructions for the preparation of a T. ferrooxidans culture for humidity cell use, and for the method of introduction of the inoculum to the rock sample. It seems likely that samples of existing waste rock, ore or tailings would not require inoculation, whereas drill core samples might require inoculation. However, since it is generally accepted that a consortium of bacteria, rather than T. ferrooxidans alone, is required for the bacterial enhancement of sulphide oxidation rates, and since even T. ferrooxidans is site specific (Leduc and Ferroni, 1994), the potential benefits of an inoculum of laboratory-prepared T. ferrooxidans are questionable.

The operation of humidity cells is straightforward and can be significantly automated with the use of peristaltic pumps and timers. An initial flushing of the cell is conducted by adding 750 ml (for a 1 kg sample cell) de-mineralised water to the top of the cell so that the sample is thoroughly wetted. After a period of 2 hour (4 hour for tailings samples) the leachate is collected, weighed (for volume determination) and analysed for pH, conductivity, acidity, alkalinity, sulphate and dissolved metals. The measurement of leachate Redox potential, Eh, may also prove useful in test interpretation. For the following three days dry air is delivered to the cell at a predetermined rate, then for three more days humid air is delivered to the cell (again, at a predetermined rate). The air flow rates used may be a generic standard, or they may be determined as site-specific parameters. On the seventh day the sample is flushed with 500 ml (for a 1 kg sample cell) de-mineralised water, allowed to stand for 2 hour (4 hour for tailings samples), and the leachate is collected, weighed and analysed for the parameters noted above. The seven day cycle is continued until the test is terminated, whereupon the sample residue is removed from the cell, dried, weighed and subjected to ABA analysis, ICP metals analysis, major component whole rock analysis and mineralogical examination. Price (1997) and ASTM (1996) describe the procedure in detail.

Weekly (cycle) leachate analyses (g/l or mg/l) are used with leachate volume (l) and sample initial mass (kg) to calculated anion and cation leaching rates (g/kg/week or mg/kg/week). These leaching rates, along with conductivity, pH (and Eh) are plotted against cycle number (week number). The resultant curves indicate the progression of acid generation (sulphide oxidation) and neutralization. There are a variety of ways to plot the humidity cell data including cumulative anion or cation leached and residual anion or cation not leached (initial minus cumulative leached). Normally leachate data is plotted on a week by week continuous basis to monitor humidity cell progress, particularly with respect to pH. It is usual to terminate humidity cell tests after pH has stabilised at a constant value (and after the minimum number of cycles recommended by the appropriate regulatory agency). However, the presence of soluble sulphate minerals or the presence of previously oxidised material may mask or delay the onset of acid generation as noted earlier, and some samples may take 60 weeks or more to stabilise (Price, 1997). The point in time at which humidity cells are terminated is usually, therefore, site-specific, and may require considerable judgement by the project supervisor.

The reproducibility of humidity cell data is not well documented. ASTM (1996) give comparisons for duplicates of two samples subjected to humidity cell testing for 120 weeks and shown in Table 1. There is no information on either the origin of the samples or their characteristics. Data is given for sulphate generation (release) rate.

Table 1: Comparison of Sulphate Release Rates from Duplicate Humidity Cell Tests on Two Samples (from ASTM, 1996)

TEST DURATION

(20-WEEK INTERVALS)

AVERAGE SULPHATE RELEASE RATES (mg/kg/week)

SAMPLE 1

SAMPLE 2

CELL 1

CELL 2

CELL 1

CELL 2

0-20

20-40

40-60

60-80

80-100

100-120

182.5

321.8

354.8

306.5

294.0

227.6

184.1

254.6

197.5

192.1

235.0

141.3

523.3

887.1

1985.6

1834.5

1763.4

3165.0

558.7

868.1

1481.1

1292.1

1247.6

1660.7

The differences between duplicates in this example are significant, but may represent variability between sample splits, rather than variability in cell performance. In this case it would have particularly interesting to compare ABA analytical data and mineralogical information for the samples to determine if sulphide liberation characteristics were responsible for the differences.

The interpretation of humidity cell and other kinetic testwork data is discussed on the Kinetic Testwork Interpretation page of this site.

 

Column Testwork Procedure

Column testwork may be undertaken to determine the kinetic behaviour of waste rock, ore or tailings stored on the surface and exposed to atmospheric weathering (sub-aerial storage), or stored under water cover (sub-aqueous storage). In either case the aim is to monitor water (leachate) quality with time by cyclic (weekly or monthly) sampling. Unlike humidity cell procedure, there is little, if any, standardization of column testwork procedure, allowing considerable flexibility. This flexibility permits column operation to be highly site or material specific with regard to material particle size and size range (which for waste rock, ore or drill core is usually greater greater than that used for humidity cell tests, but still less than that of site conditions), sample mass, water infiltration or flow rate and degree of oxygenation. Because of the lack of standardization of procedure some regulatory agencies view column testwork as supplementary to, or confirmation of, humidity cell testwork, rather than an alternative to humidity cell testwork. Price (1997) states that trickle leach column tests have the following disadvantages (over humidity cells):

  1. "The primary weathering products may be retained and therefore leachate chemistry cannot be used as a measure of the relative rates of acid generation and neutralization, and of times to mineral depletion.

  2. They are run at the laboratory temperature often with a reduced particle size, and without seasonal variations and the extremes of both temperature and precipitation. Consequently, they provide poor analogues for heterogeneous drainage and the secondary mineral precipitation and dissolution, the controlling factors for metal leaching under all but the most acidic pH values.

  3. Without the entire load of primary weathering products, leachate results cannot be used either with MINTEQ to predict the extent of secondary precipitation or release, or with field data to predict metal leaching based on the predicted evolution in drainage chemistry (i.e. pH conditions)."

The first listed disadvantage appears to ignore the fact that humidity cell tests are intentionally accelerated and cannot be used to predict field rates of acid generation and neutralization; the second disadvantage is questionable, since column tests are invariably run on coarser material (except for tailings) than humidity cells and can be (and are) run with simulated site seasonal precipitation and temperature variations; the third disadvantage appears to discount the fact that column leachate is produced from a non-accelerated system in which the leachate is in equilibrium with solid oxidation products.

Sub-aerial column (sometimes referred to as "trickle leaching") testwork is conducted to simulate the leaching effects of precipitation infiltration to, and drainage from, material stored at surface and exposed to the atmosphere. Water addition rate to the column may be either fixed (i.e. a certain amount per cycle), or it may be varied to simulate the seasonal variations at site. The column is open to the atmosphere so that there is no oxygen barrier, but there is usually no forced oxygenation as with a humidity cell. Similarly, the column is operated without aggressive flushing so that oxidation products may accumulate at particle surfaces in addition to being removed in leachate. This behaviour parallels field conditions and, as a result, leachate analyses from column testwork are a better indicator of expected water quality than leachate analyses from humidity cells - particularly if column infiltration rate is varied to simulate site conditions. Column operational temperature can also be varied to simulate site conditions, although this is rarely done.

Sub-aqueous column testwork is conducted to simulate the leaching effects of water infiltration to and ex-filtration from material stored under water cover with no physical exposure to the atmosphere. In the case of tailings or waste rock stored under water in a natural or man-made impoundment, infiltration of oxygenated water from the supernatant replaces any water lost by seepage to groundwater. This may be simulated in a column by the slow downward displacement of pore water by freshwater from above. In the case of tailings or waste rock stored under water where water flow or displacement is not influenced by seepage, but by thermal or density gradients (e.g. submarine storage), simulation of flow (and possible leaching) may be achieved by slow upward movement of de-oxygenated water through the column (as shown in the above figure), so that anoxic conditions are maintained within the rock sample and its environment. In either case the column testwork conditions are adjusted to approximate as closely as possible those of the site.

Sample characterization, both before and after the test, is as important for column tests as it is for humidity cell tests. Cyclic samples are measured and analysed (as for humidity cells) weekly or monthly, and results reported as g/l or mg/l anion or cation in leachate, and g/kg/week (or month) or mg/kg/week (or month) anion or cation produced. Samples containing previously oxidized material or soluble sulphate minerals that may mask or delay the onset of acid generation will usually require pre-treatment.

The interpretation of column and other kinetic testwork data is discussed on the Kinetic Testwork Interpretation page of this site.

 

Mine Wall Washing Procedure ("Minewall")

The Minewall Procedure, which has seen limited use to date because it is still in the developmental phase as a procedure, requires the cyclic irrigation of a small (c. 1 m by 1 m), physically isolated, area of exposed pit wall or underground rock exposure, and the collection of leachate. Physical isolation is achieved by sealing (with silicone bathroom sealant) a plastic frame (with a horizontal top, two vertical sides and an inclined bottom for leachate capture and collection) to a rock exposure. A polyethylene sheet is used to cover the Minewall assembly during leaching.

The area of exposed rock within the plastic frame is irrigated with 200 ml or more of distilled water. The run-off (leachate) is collected and analyzed. The procedure is repeated weekly and results are expressed in g/square metre/week or mg/square metre/week of anion or cation (Price, 1997).

The procedure is limited by the requirement that the isolated rock area be without fractures, which increase the surface area of the test by an unknown factor.

 

Other Procedures

Other kinetic procedures such as on-site lysimeters, pilot scale kinetic test pads and monitoring of the actual mine components represent a major increase in scale from the procedures described above. As scale increases test conditions approach those of the operating (or closed) mine, so that these procedures may be better described as "monitoring" rather than "testwork". Most of these larger scale options are unavailable to an ARD testwork program for a new mine, where quantities of material for testwork are usually severely limited. Monitoring procedures will be considered on other pages of this web site.

 

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REFERENCES

American Society for Testing and Materials (1996), ASTM Designation: D 5744 - 96 - Standard Test Method for Accelerated Weathering of Solid Materials Using a Modified Humidity Cell, ASTM, West Conshohocken, PA, 13p.

Caruccio, F.T. (1995), personal communication cited in ASTM, 1996.

Lawrence, R.W. (1995), Prediction of Acid Rock Drainage - Fundamentals and Tools, notes from MEND Workshop, Montreal, PQ, December 7-8, 27p.

Leduc, L.G. and Ferroni, G.D. (1994), The Need for Thiobacillus Ferrooxidans Strain Selection in Applications of Bioleaching, Proc. Biominet, 10th Ann. Gen. Meet., Minister of Supply and Services Canada, Ottawa, 25-42.

MEND, 1995, MINEWALL 2.0, Series of four reports: Literature Review, User's Guide, Application of MINEWALL to Three Minesites and Programmer's Notes and Source Code, plus one diskette, MEND, Ottawa, ON.

Perkins, E.H., Nesbitt, H.W., Gunter, W.D., St-Arnaud, L.C. and Mycroft, J.R. (1995), Critical Review of Geochemical Processes and Geochemical Models Adaptable for Prediction of Acidic Drainage from Waste Rock, MEND Report, No. 1.42.1, MEND, Ottawa, ON, 120p.

Price, W.A. (1997), DRAFT Guidelines and Recommended Methods for the Prediction of Metal Leaching and Acid Rock Drainage at Minesites in British Columbia, British Columbia Ministry of Employment and Investment, Energy and Minerals Division, Smithers, BC, (April), 143p.

Soregaroli, B.A. and Lawrence, R.W. (1998), Update on Waste Characterization Studies, paper presented at Mine Design, Operations & Closure Conference, Polson, MT, 10p.


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