Laboratory scale experiments for assessing the impacts of organic cover layer on heavy metal leaching from tailings

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 impacts of organic cover layer on mining waste leaching behaviour has been presented.

Background

The possible advantages and disadvantages of using organic cover layer on mining waste have been presented in Table 1.

Table 1. Possible advantages and disadvantages of using organic cover layers on mining waste (MEND 1994, Peppas et al. 2000, Steffen Robertson and Kirsten Inc. 2001, INAP 2009, Lottermoser 2010).

Method	Description	Advantages	Disadvantages
Covering of mining waste with oxygen consuming layer, e.g. organic cover	Covering of metal (and sulphide) bearing mining waste with organic cover e.g. wood based waste, sludge or peat	Minimises the intrusion of oxygen into contact with the waste Low costs if materials are available locally Utilization routes for organic materials (waste)	Only applicable for areas with uniform distribution of humidity throughout the year Long-term functioning not well known Availability of materials and transport costs Material pre-treatment need Leachate quality?

Execution of the study

Organic materials such as peat have been used as mining waste cover layers primarily to consume the oxygen and thus minimize sulphide oxidation in the mining waste. On the other hand, the complexation of DOC with heavy metals such as copper is a widely known phenomenon in the context of environmental behaviour of waste materials. The detailed research question in this study was whether the DOC leached from the cover layer would increase the leaching of heavy metals from the tailings

The studies presented here were conducted for a single tailings sample received from Luikonlahti mill, Finland. Table 2 shows the study program.

Table 2. Contents of the study.

Study	Objectives + additional information
XRF	Semi-quantitative elemental composition, screening of contaminants
Up-flow percolation tests	Leaching test to simulate leaching behaviour and the impacts of DOC on leaching. Pure water and humic acid solution compared as leachants
pH-dependence leaching tests	Leaching tests to assess the impacts of pH and DOC on leaching, input to geochemical modelling. Pure water and humic acid solution compared as leachants.

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-%.

The up-flow percolation test (CEN/TS 14405) is used to simulate the leaching behaviour of a granular or powdery material in placement conditions at different time frames. In the test granular material is packed into a vertical column with diameter of 5 cm to a bed height of 30 cm. A peristaltic pump is used to pass demineralised water through the column. Liquid (the eluate) appearing at the outlet is collected in 7 fractions ranging from a liquid to solid (L/S) ratio of 0.1 l/kg to 10 l/kg. The eluate fractions are filtered and prepared for subsequent chemical analysis. The results are expressed in terms of accumulated leached amounts (mg/kg) as a function of L/S. Time to reach a certain liquid to solid ratio at the site of placement can be roughly estimated when annual infiltration through the material bed is known.

In the pH-dependence test (CEN/TS 14997) the sample (15, 30 or 60 g) is mixed with distilled water for 48-hours at L/S ratio about 10. The pH-level of the mixture is kept at the predetermined pH-value (pH range 4-12) by using an automated pH-titrator. The eluate is separated by filtration. Constituents of interest are determined from the eluates.

Results

Table 3 shows the semi-quantitative elemental composition of the tailings sample as determined by X-ray fluorescence analysis XRF.

Table 3. Semi-quantitative elemental composition of the tailings sample. Concentrations of elements heavier than fluorine are expressed as percentages. The detection limit of the method is around 0.01 %.

Element, %	Luikonlahti tailings	Element, %	Luikonlahti tailings
Na	0.23	Cr	0.06
Mg	4.5	Mn	0.04
Al	1.5	Fe	3.9
Si	25	Co	0.04
P	0.06	Ni	0.06
S	2.1	Cu	0.16
K	0.42	Zn	0.10
Ca	9.5	As	0.01
Ti	0.21	Zr	0.01
V	0.02

To assess the impacts of dissolved organic carbon on leaching of contaminants from a tailings sample, a standard humic acid solution (Sigma Aldrich) was prepared. Table 3 shows the composition of the prepared solution. The load caused by the humic acid solution to the leaching results were always subtracted from the results to be able to better compare the scenarios. In reality it is always possible that some leaching of contaminants takes place also from the cover materials.

Table 4. Composition of the prepared Sigma Aldrich humic acid solution.

Component	Humic acid solution (mg/l)	Component	Humic acid solution (mg/l)
Al	0.5	Sb	0.0001
As	0.001	Se	0.003
Ba	0.003	Sr	0.003
Be	0.000164	V	0.003
Co	0.001	Zn	0.005
Cr	0.00123	Ca	1.54
Cu	0.004	Fe	0.41
K	0.348	Mg	0.18
Mn	0.0004	Na	18
Mo	0.0003	Si	0.32
Ni	0.003	Cl^–	2.6
P	0.017	SO₄^2-	0.8
Pb	0.00015	DOC	31
Rb	0.0001

The results from up-flow percolation tests have been compiled into Table 5.

Table 5. Results from the up-flow percolation tests. The amounts of leached substances have been expressed as mg/kg dry matter in cumulative liquid to solid (L/S) ration of 10.

	Luikonlahti tailings (mg/kg) Up-flow percolation test, leachant: de-ionized water	Luikonlahti tailings (mg/kg) Up-flow percolation test, leachant: Huimc acid solution		Luikonlahti tailings (mg/kg) Up-flow percolation test, leachant: de-ionized water	Luikonlahti tailings (mg/kg) Up-flow percolation test, leachant: Huimc acid solution
Eluate pH	7.9 – 8.2 *	7.9 – 8.3 *		7.9 – 8.2 *	7.9 – 8.3 *
*Leached substances, mg/kg*
Ag	<0.01	<0.01	Sb	0.21	0.27
Al	0.39	2.3	Se	0.02	0.02
As	0.02	0.08	Sr	0.54	0.71
B	0.04	0.44	Th	<0.01	<0.01
Ba	0.03	0.08	Tl	<0.01	<0.01
Be	<0.01	<0.01	U	0.06	0.72
Bi	<0.01	<0.01	V	<0.01	0.08
Cd	<0.01	<0.01	Zn	0.11	0.71
Co	0.13	1.2	Ca	350	470
Cr	<0.01	0.03	Fe	0.52	3.4
Cu	0.01	0.32	Mg	39	42
K	37	54	Na	14	180
Li	0.02	0.03	Si	24	24
Mn	0.82	0.69	S	270	390
Mo	0.05	0.27	Cl^–	3.4	32
Ni	0.12	0.87	F^–	2.0	5.2
P	0.18	0.84	SO₄^2-	770	1100
Pb	<0.01	0.03	DOC	11	110
Rb	0.04	0.08

* range of pH’s in seven collected eluate fractions.

The results from pH-dependence tests have been compiled into Table 6 (de-ionized water as leachant) and Table 7 (humic acid solution as leachant).

Table 6. Results from pH-dependence tests with de-ionized (pure) water as leachant. The amounts of leached substances have been expressed as mg/kg dry matter (L/S 10).

Target-pH	2.0	4.0	6.0	7.0	Natural pH (8.1)	10.0	12.0
Acid/base consumption, mol H⁺/OH^–/kg	3.77 (H⁺)	3.34 (H⁺)	1.63 (H⁺)	0.56 (H⁺)	–	0.05 (OH^–)	0.43 (OH^–)
*Leached substances, mg/kg*
Ag	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Al	550	120	0.57	0.56	0.14	3.8	27
As	2.2	0.04	0.01	0.01	0.01	0.33	1.3
B	0.09	0.12	0.04	0.06	<0.01	0.02	0.06
Ba	3.7	3.0	1.8	0.67	0.14	0.01	0.01
Be	0.01	0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Bi	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Cd	0.17	0.08	0.04	0.01	<0.01	<0.01	<0.01
Co	260	220	21	47	0.45	0.02	<0.01
Cr	5.9	0.69	0.01	<0.01	0.01	0.07	0.44
Cu	61	38	0.06	0.03	0.03	0.05	0.09
K	1600	750	520	1000	350	170	400
Li	0.31	0.13	0.07	0.03	0.03	0.02	0.03
Mn	210	200	110	38	0.29	0.02	<0.01
Mo	0.06	0.01	<0.01	0.03	0.05	0.08	0.15
Ni	170	150	30	32	0.69	0.03	0.01
P	330	2.1	1.4	0.46	0.69	0.10	0.59
Pb	6.6	3.4	<0.01	<0.01	<0.01	<0.01	<0.01
Rb	1.5	1.1	0.63	0.19	0.08	0.02	0.06
Sb	0.25	0.07	0.01	0.09	0.40	0.39	0.40
Se	0.16	0.13	0.07	0.05	0.03	0.02	0.15
Sr	28	27	16	6.4	0.78	0.04	0.01
Th	0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Tl	0.08	0.06	0.02	<0.01	<0.01	<0.01	<0.01
U	5.5	4.0	0.02	0.28	0.14	0.02	0.03
V	2.7	0.01	<0.01	<0.01	0.01	0.07	0.23
Zn	95	45	15	3.9	0.13	0.07	0.16
Ca	49000	47000	29000	11000	660	12	5.4
Fe	4900	2200	4.6	0.50	0.50	1.1	1.2
Mg	10000	9800	1800	260	83	2.1	0.48
Na	30	18	15	14	10	1200	9400
Si	530	100	32	25	33	26	54
S	490	350	490	520	500	1100	11000
DOC	34	27	28	21	13	9.6	14

Table 7. Results from pH-dependence tests with humic acid solution as leachant. The amounts of leached substances have been expressed as mg/kg dry matter (L/S 10).

Target-pH	2.0	4.0	6.0	7.0	Natural pH (8.2)	10.0	12.0
Acid/base consumption. mol H⁺/OH^–/kg	3.7 (H⁺)	3.2 (H⁺)	1.3 (H⁺)	0.74 (H⁺)	–	0.08 (OH^–)	1.1 (OH^–)
*Leached substances, mg/kg*
Ag	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Al	550	68	0.31	0.35	0.64	11	39
As	1.4	0.03	0.01	0.01	0.02	0.68	2.6
B	0.13	0.07	0.05	0.07	0.03	0.06	0.13
Ba	3.8	2.8	1.71	1.0	0.11	0.06	0.01
Be	0.01	0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Bi	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Cd	0.11	0.08	0.04	0.01	<0.01	<0.01	<0.01
Co	350	120	61.81	40	0.19	2.2	0.01
Cr	8.1	0.11	0.01	0.05	0.07	0.18	0.61
Cu	0.57	19	0.03	0.03	0.08	1.6	0.74
K	1300	570	410	580	420	560	460
Li	0.26	0.11	0.05	0.04	0.02	0.02	0.01
Mn	200	190	88	48	0.14	0.24	<0.01
Mo	0.05	0.01	0.01	0.04	0.06	0.22	0.22
Ni	220	80	46	26	0.40	2.7	0.03
P	340	1.2	0.61	0.43	0.10	1.8	0.95
Pb	4.6	1.1	<0.01	<0.01	<0.01	0.16	<0.01
Rb	1.6	1.1	0.62	0.35	0.08	0.06	0.20
Sb	0.26	0.06	0.03	0.09	0.30	0.46	0.45
Se	0.18	0.11	0.04	0.04	0.03	0.08	0.78
Sr	29	30	14	8.9	0.72	0.10	0.01
Th	0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Tl	0.05	0.05	0.01	<0.01	<0.01	<0.01	<0.01
U	3.8	0.65	0.05	0.13	0.30	0.31	0.26
V	2.7	<0.01	<0.01	<0.01	<0.01	0.09	0.33
Zn	60	45	11	1.5	0.04	1.7	2.6
Ca	50000	46000	24000	15000	560	38	8.6
Fe	4300	1100	0.49	0.51	0.50	46	1.8
Mg	10000	9200	710	330	56	9.3	0.49
Na	220	190	180	190	200	2000	26000
Si	530	77	28	30	30	30	94
S	490	459	500	530	480	910	4600
DOC	64	25	20	21	29	120	570

Discussion

Percolation tests

When simulating the impacts of an organic cover layer with up-flow percolation leaching tests it was observed that the leaching of several inorganic contaminants increased substantially. Some of the most important findings are shown in Figure 1.

Figure 1. Impacts of humic acid solution on leaching of Co, Cu and Ni from a tailings sample. The load caused by the humic acid solution has been subtracted from the results.

With Co, Cu, and Ni the leached amounts when using the humic acid solution as leachant were some tenfold compared to the scenario where pure water was used.

pH-dependence tests

With the pH-dependence tests the impacts of dissolved organic carbon (DOC) on leaching of metals were not as straightforward to observe. This is due to at least the following:

Leaching of many metals are highly dependent on pH
Leaching of DOC is also dependent on pH (with low pH values the DOC added is contained in the solid matrix)

Therefore, the reasons to observed behaviour were not easy to explain. Figure 2 shows an example of the leaching behaviour as a function of pH. In the case of pH-dependence tests the load coming from humic acid solution can only be seen as theoretic, because some of the dissolved substance in the humic acid solution may take a solid form at different pH values than the solutions original pH.

Figure 2. Leaching of Cu as a function of pH. The (theoretic) load coming from humic acid solution is subtracted from the results with humic acid solution.

With lower pH’s (<8) the leaching of Cu is lower in the tests with pure water as leachant. With pH’s 10 and above the leaching of Cu is again some ten times higher with humic acid as leachant. The reasons behind the observed behaviour are likely to be a) retaining of DOC (possibly together with Cu) in the solid matrix at lower pH’s and b) increased leaching of DOC at higher pH’s and complexation with Cu to increase Cu leaching.

Conclusions

The results gained in this study suggest that the dissolved organic carbon (DOC) can be a significant factor in controlling the leaching of several heavy metals from waste materials. Based on these lab-scale results, the increased leaching of heavy metals should be taken into account when comparing different covering options for mining wastes. Also, the benefits from using organic material as cover layer for mining waste should be weighed against increased risk of elevated heavy metal leaching as a result of complexation with DOC.

References

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

MEND 1994. Evaluation of Alternate Dry Covers for the Inhibition of Acid Mine Drainage from Tailings. MEND Project 2.20.1. March 1994. Prepared by: SENES Consultants Limited. Mine Environment Neutral Drainage Program (MEND).

Peppas, A., Komnitsas, K. & Halikia, I. 2000. Use of Organic Covers for Acid Mine Drainage Control. Minerals Engineering 13, No. 5, 563-574

Steffen Robertson and Kirsten Inc. 2001. Methods for Delaying the Onset of Acidic Drainage – A Case Study Review, Final Report. Mend Project 2.37.2.

CEN/TS 14405:2004. Characterization of waste – Leaching behaviour tests – Up-flow percolation test (under specified conditions).

CEN/TS 14997:2006. Characterization of waste – Leaching behaviour tests – Influence of pH on leaching with continuous pH-control.