Disposal scenarios of acid producing waste rock

Tommi Kaartinen, Markku Juvankoski, Jutta Laine-Ylijoki, Elina Merta, Ulla-Maija Mroueh, Jarno Mäkinen, Emma Niemeläinen, Henna Punkkinen, & Margareta Wahlström, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland.

Introduction

These studies were part of CLOSEDURE project’s Work Package 4: “Wastes and Waste facilities”. As part of this work package the possibilities and challenges to compare mining waste disposal scenarios in laboratory scale have been investigated. The research has concentrated on assessing the mining waste leaching behaviour in different scenarios.

In this report a study to assess the differences between two disposal scenarios – under water and on ground – of acid producing waste rock has been presented.

Background

The possible advantages and disadvantages of disposing acid producing mine waste under water can be for instance the following (Steffen Robertson and Kirsten Inc. 1989, Tremblay & Hogan 2001, INAP 2009):

• Under water disposal decreases the contact between the waste material and oxygen (oxygen diffusion in water some 10,000 times slower than in the air) thus slowing down sulphide oxidation
• Under water disposal increases both the contact time between the waste material and water and the volume of the water in contact with the waste thus possibly increasing the amounts of substances leached

Execution of the study

The studies presented here were conducted for a single waste rock sample received from the Kylylahti mine, Finland. Table 1 shows the study program.

Table 1. Contents of the study.

Elemental composition of the sample was determined by using Axios mAX 3 kV X-ray spectrometer and semi-quantitative fundamental parameters program (RRFPO). The method is applicable for fluorine and elements heavier than fluorine, and a typical detection limit is approximately 0.01 w-%.

For the analysis of sulphide sulphur the possible sulphates were removed from the sample by treating with sodium carbonate solution. The remaining sulphur is considered as sulphides, and it is determined by combusting the sample in oxygen stream in a LECO induction furnace in high temperature. The sulphur dioxide formed in combustion is measured with infrared detector.

The carbonate carbon content of the sample was determined by subtracting the amount of non-carbonate carbon from total carbon. Total carbon was determined with Eltra CS-2000 device by combusting the sample in oxygen stream and measuring the amount of CO₂ formed in combustion with infrared absorption. Non carbonate carbon was determined by dissolving the carbonates in the sample with hydrochloric acid and determining the remaining carbon as described above.

Static test for determination of acid potential and neutralization potential of sulphidic waste (EN 15875) is used to determine the potential of sulphide bearing materials for the formation of acidic drainage. Acid potential (AP) of a material is generally based on oxidation of sulphides and according to EN 15875 it is calculated on the basis of total- or sulphide sulphur content. Neutralization potential as described in EN 15875 is determined by digesting a finely ground (<0.125 mm) sample for 24 hours in 90 ml of HCl-solution. The digestion aims at final pH of 2.0-2.5 and the amount of 1 M HCl added in the suspension at the start of the test is determined on the basis of the content of carbonate-carbon of the sample. After digestion the suspension is back titrated to pH 8.3 to measure the amount of acid left in the solution. Acid consumption in the test is converted to neutralization potential (NP) of the sample. The potential for the formation of acidic drainage is estimated by the ratio of NP and AP.

For the comparison of two different waste rock disposal scenarios a test set-up illustrated in Figure 1 was used.

Figure 1. Test set-up for comparison of waste rock disposal scenarios.

The waste rock under water disposal test set-up is shown on the left side of Figure 1. 500 g of waste rock sample ground to particle size <4 mm was placed on the bottom of a cylindrical column with diameter of 10 cm and height of 20 cm. The rest of the column was filled with water, and a constant slow water flow, approximately 3 ml/h (500 ml/week) was established with a peristaltic pump from the bottom to simulate the water flow in e.g. a flooded open pit. The water flown through the column was collected from the top outlet into a HDPE bottle, and after a week the collection vessel was changed. The water was analysed every week for sulphate and every four weeks for major and minor elements. Every week the pH and the electric conductivity were also measured from the collected water. The test was continued for 30 weeks.

The waste rock on ground disposal scenario is shown on the right side of Figure 1. The same amount, 500 g, of <4 mm waste rock was placed in the column. The waste rock was flushed from above once a week with 500 ml water, which was then in contact with the waste rock for one hour before opening a tap at the bottom of the column to let leachate drain into a collection vessel. The amount of leachate collected was recorded and analysed similarly to the underwater scenario. From week 22 until the end of the experiment the waste rock sample in this scenario was exposed to more air contact in order to accelerate the oxidation reactions in the sample. Air was introduced to the column through the bottom tap with a constant flow rate of 1 litre/minute. Air flow was ceased during the weekly flushing and leachate collection.

In this test set-up the volume of water in contact with the waste rock on a weekly basis was the same in both scenarios. This was done in order to enable comparison between scenarios and to reduce the amount of factors affecting the leaching behaviour. Weekly loads of leached substances (mg/kg/week) were calculated by multiplying the volume of water collected with the concentration of the substance in the leachate and dividing this by the mass of the sample in the column.

Results

Table 2 shows the semi-quantitative elemental composition of the waste rock sample as determined by X-ray fluorescence analysis XRF.

Table 2. Semi-quantitative elemental composition of the waste rock sample. Concentrations of elements are expressed as percentages. The detection limit of the method is around 0.01 %.

Results related to the acid base accounting have been compiled into Table 3.

Table 3. Carbonate carbon and sulphide sulphur mass fractions of the waste rock sample expressed as percentages and results from acid base accounting.

Figure 2 summarises the most essential results from the comparison of the two disposal scenarios for Kylylahti waste rock sample. The leached substances chosen to these graphs were such that the concentrations remained above the analytical detection limits throughout the experiments.

Figure 2. Evolution of essential parameters during the 30 weeks test period in two different waste rock disposal scenarios.

Discussion

Composition and acid base accounting

In the XRF analysis elements like As, Ni, Cu and Zn were found to appear in elevated concentrations in the waste rock sample in comparison to e.g. natural soils. With possibly changing pH environment as a result of sulphide oxidation there is most likely potential to increased contaminant leaching from this type of waste material.

Based on the acid-base accounting the studied sample could be classified as an acid producing waste rock, since the acid potential AP of the sample was higher than the neutralisation potential NP, and thus the Neutralisation Potential Ratio (NPR) was below 1.

Comparison of disposal scenarios

Altogether the amounts of contaminants leached from the waste rock sample throughout the whole test period of 30 weeks were really small in both scenarios. This applies to all other measured substances (data not shown) than those shown in Figure 2.

It can be seen from Figure 2, that the pH of the leachates from both scenarios remained quite similar and close to neutral throughout the whole 30 weeks test period. This indicates that enough reactive neutralisation potential has been present in the material to neutralise the acid produced in possible sulphide oxidation. On the other hand the graph of weekly sulphate loads is also quite equal in both scenarios during the first 22 weeks of the experiments indicating very slow sulphide oxidation, if at all. After starting the air introduction into the “On ground” column at week 22 the weekly sulphate loads decreased quite significantly in this scenario. This was due to both decreased sulphate concentrations in the leachate and decreased volume of water collected during these weeks, both of which decrease the load of sulphate coming out from the column. The decreased amount of water collected during these weeks was a result of the sample drying during the air introduction and thus the flushing water retaining in the sample. The reason for decreased sulphate concentrations in the leachate during the accelerated air introduction period remains unknown.

Conclusions

No significant oxidation of sulphides was observed during the long 30 week experiment in the on ground disposal scenario. This phenomenon would have been needed to better show differences between the two disposal scenarios in terms of leaching behaviour. As a main conclusion the presented results highlight the challenges in simulating all of the weathering reactions taking place in mining waste at least in small scale. These reactions may take years or tens of years in the field and many factors affect them. In the field e.g. the bacterial activity has been considered as one essential factor in accelerating the sulphide oxidation. Also, as mentioned earlier, the volumes of water flushing the mining waste in different disposal scenarios most likely differ a lot, something that cannot be easily approached in laboratory scale when still aiming to produce comparable results.

References

EN 15875. Characterization of waste – Static test for determination of acid potential and neutralisation potential of sulfidic waste.

INAP 2009. The GARD Guide. The Global Acid Rock Drainage Guide. The International Network for Acid Prevention (INAP). http://www.gardguide.com

Steffen Robertson and Kirsten Inc. 1989. Draft Acid Rock Drainage Technical Guide, Volume 1. British Columbia Acid Mine Drainage Task Force Report.

Tremblay, G.A. & Hogan, C.M. 2001. MEND Manual, Volume 4. Prevention and Control. MEND Report 5.4.2d.