R.H.P. Ringeling, H.J. Glass, M. Olijve*
Subfaculty of Applied Earth Sciences, Delft University of Technology
Mijnbouwstraat 120, 2628 RX Delft, The Netherlands
* BodemSanering Nederland in Weert, The Netherlands
Abstract
The cleaning of contaminated soil with coal spirals has been studied by monitoring the cleaning of six batches at an industrial facility. The coal spiral allows separation of consistently high percentages of organic substances.
Further evidence of the high separation efficiency was obtained by performing sink-float tests. Sink-float tests were used to establish which fraction of a contaminant is present as free particles and which fraction is sorbed to the soil particles. A surprising side-effect was the separation of certain heavy metals from the soil. This is attributed to the nature of the soil rather than the spiral design.
1. Introduction
Easy to operate, spirals are often applied in soil remediation processes. Spirals essentially separate particles on the basis of differences in density and size.
However, the separation principle is not easily understood. Starting point for analysing the separation of particles is characterization of the spiral. The following characteristics are relevant:
• the inner and outer diameter of the spiral trough,
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• the axial and radial inclination,
• the length of the spiral, and
• the presence of weirs.
When the axial and radial inclination is small, it is attempted to concentrate soil in the inner flow.
Contaminating particles should remain in or move to the outer flow, which implies that these particles have to be relatively light.
This is often the case with organic contaminants, especially when these are present as discrete particles. Furthermore, the optimum flow rate and density of the slurry have to be selected. As the axial and radial inclination increases, the soil increasingly moves into the outer flow. In that case, relatively heavy particles can be separated when these move into the inner flow. This could lead to separation of particles containing heavy metals.
The coal spiral was designed for coal washing, i.e. to separate coal particles from gangue like quartz. The small axial and radial inclination makes the coal spiral suitable for separating relatively light contaminants, notably Polycyclic
Aromatic Hydrocarbons (PAH) and other organic substances. The validity of this expectation will be studied in this contribution. For this purpose, the performance of coal spirals is monitored at an industrial facility in the Netherlands. Figure 1 shows an illustration of a coal spiral.
The cleaning of soil has been monitored at the facility of BodemSanering
Nederland (BSN), located in Weert, The Netherlands. The process employed by
BSN is schematically represented in Figure 2.
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The process is based on treating fractions of particles according to size. First, particles larger than 30 mm are screened out and stored for disposal. The remaining bulk of the soil is fed to a high-pressure unit where the contaminants attached to the soil particles are liberated. Once the contaminants are present as free particles, further classification according to size and density is performed to clean the soil. Wet screening serves to separate the particles larger than 1 mm which consist mainly of organic material and gravel.
Hydrocycloning is used to remove the fraction of particles smaller than 63  m, which are generally highly contaminated with heavy metals as well as organic components. The bulk of the soil, consisting of particles with a size between 63
 m and 1 mm, are first subjected to wet magnetic separation before reaching the spirals. Two types of spirals are employed: coal spirals for removing relatively light contaminants and Humphrey spirals for removing relatively heavy contaminants. Finally, the cleaned soil exiting from the spirals is dewatered.
2. Contamination of the soils
The cleaning of six batches of polluted soil has been monitored. Each of the batches is sampled in order to establish the particle size distribution, the
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concentrations of PAH, other organic substances and heavy metals.
Furthermore, it is important to establish the degree in which the contaminants are present as free particles. This is done by sink-float analysis. The history of the soil may be usefull additional information, in order to verify the concentration and the presence of the contaminants. In the following, the history of each of the soils, labelled A through F, will be briefly discussed. Soil A is an artificial type of soil called sieve sand. Sieve sand is the fine residual fraction of building waste. It originates from dry sieving of the building waste, usually on 8 mm. Soil
B is obtained from the Ertskade (‘Ore Docks’) in the harbour of Amsterdam.
Because the location has been scheduled for housing development, the site, which was used for storage of chemicals, is investigated. The topsoil (0-0.5 m depth), consisting of light gravel, carbon- and sintel-bearing coarse sand, contains high concentrations of PAH and heavy metals. At greater depths, localized contamination with heavy metals is observed. In addition, the ground water is contaminated with zinc, nickel and benzene. Also present in the ground water are relatively high concentrations of mineral oil. Soil C is derived from the municipality Zwolle. Formerly farmland, it is adjacent to a railway line and is the scene of former railway buildings. Major pollutants are the heavy metal chromium and mineral oil. Soil D originates from Helmond at a site where a machine factory, a foundry, and a fertilizerplant were located. This could produce contamination with PAH, mineral oil, acids and various heavy metals.
Soil E is from the site of butchery in Nijmegen. The area is bordered by three rivers and has a history of agricultural use. However, a factory of the company
Honig is of a more recent date, possibly causing contamination. Soil F is obtained from the centre of the town of Weert. About 80 years ago, it was the site of a butchery, followed by a diary plant. The soil is contaminated with PAH and zinc.
Comparison of the origin of the batches reveals that all locations were sites of
(former) industrial activities. This is not quite unexpected as site investigation is often triggered by records of previous industrial activities.
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3. Performance of coal spirals
For each soil, the performance of the coal spiral was monitored by comparing the inlet and outlet concentrations of pollutants. The indication of the separation efficiency can then be used to verify whether the expectation, namely a high efficiency for organic substances, is correct.
Characterization of the soil occurs by analysing samples drawn from the inlet and the outlet of the coal spiral. The particle size distribution is determined by wet screening. Figure 3 compares the particle size distributions of the different soils.
Whereas the inlet consists of a single flow, splitters in the outlet create three flows. The distribution of soil at the outlet is influenced by the operating variables slurry density and slurry flow rate. During cleaning of the soil, these were roughly constant and equal to about 30 wt% solids and 4 m 3 /hr respectively. For these settings, the bulk of the soil is present in the innermost flow. Therefore, only the innermost flow was sampled. The concentrations of organic contaminants and humus in the inlet and outlet are summarized in Table 1.
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F
E
D
C
B
Soil
A
Table 1: Concentrations of organic contaminants (in ppm) and humus (in wt%) in the soil before and after the coal spiral.
PAH EOX in 58.0 out 13.0 in 14.0 out 1.8 in 5.9 out 0.8 in 1.8 out 0.3 in 17.0 out 5.3
20.0
0.5
0.6
0.2
1.0
0.4
0.2
< 0.1
0.2
< 0.1 in 33.0 0.4 out 8.2 0.3
Min. oil
680
180
95
16
100
< 50
< 50
< 50
150
< 50
180
50
0.8
0.8
0.5
< 0.1
1.6
0.6
0.8
Humus
2.1
0.8
2.8
0.1
2.6
Without exception, the concentration of organic substances in the soil is reduced. In other words, organics are separated and captured in the outer flows of the spiral trough. It should be remembered that the soils have already passed a number of process stages prior to entering the coal spiral. This has probably enhanced the separation efficiency of the coal spiral considerably.
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With respect to the heavy metals, no reduction or even a slight increase in the concentration is expected. However, in some cases a clear reduction of the concentration of the heavy metals (especially copper, zinc and lead) is observed. A plausible explanation is that these heavy metals co-exist with organic substances or are present as relatively small particles. In this way, heavy metals are found in the relatively light particles.
As can be seen in figure 3, the "clean" product of the spiral has a coarser grain size distribution. A reasonable explanation for this is finer particles moving to the outer flow of the spiral.
4. Sink-float analysis
The separation performance of the coal spiral was evaluated by comparing the concentration of contaminants before and after the spiral. Although this provides a measure for the overall separation efficiency of the coal spiral, other measures are also possible. For example, the efficiency can also be calculated after correcting for the fraction of the contaminants which is sorbed in the soil.
Normally, the fraction sorbed to the particles cannot be separated with a coal spiral and should not be included in the definition of the efficiency.
Determination of the sorbed fraction is possible by performing sink-float tests.
During a sink-float test, the contaminants present as free particles are separated from the soil before measuring the residual concentration of contaminants in the soil. Particles consisting (largely) of contaminants have a markedly higher or lower density than the soil particles. In order to separate these relatively high- and low-density particles, a dense suspension is prepared using sample material, sodium polytungstate and moisture. By preparing a suspension with a density of 1.9 g/cm 3 , the light fraction (PAH, organics) will be floated. After removing the floating material, the density is increased to 2.9 g/cm 3 . After centrifuging, the soil (mainly sand) particles will float. Analysis of the soil fraction indicates the concentration of contaminants which is sorbed to soil particles. Comparison of the residual and the initial concentration of
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contaminants reveals the maximum cleaning potential of soil washing, i.e. using physical separation techniques like the coal spiral. Measurement of the sorbed concentration of contaminants allows definition of a efficiency which depends only on the concentration of available or free contaminants. This will be denoted the effective efficiency. In equation, the two efficiencies are defined as follows:
B
C
Soil
A where C denotes concentration and the subscripts i, o and s denote inlet, outlet and sorbed respectively. The definition of the effective efficiency allows for differences between the sorbed concentration of the inlet and the outlet.
Differences could arise from additional contaminants being liberated during the passage through the spiral or from sampling error.
Sink-float tests were carried out with samples from all soils collected before and after the coal spiral. The calculated efficiencies are summarized in Table 2.
Table 2: Concentration reduction (in %).
PAH EOX Min. oil effective 84 overall 78
98
98
81
74 effective 99 overall 87 effective 86
50
67
60
83
83
> 50
Humus
95
62
> 96
> 96
63
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D
F
E overall 86 effective 100 overall 83 effective 68 overall 69 effective 80 overall 75
60
100
> 50
-
> 50 -
50 > 67
> 50 > 67
> 25 > 62
25 72
80
69
50
40
63
100
88
The effective efficiency of the spiral with respect to the removal of various organic materials is consistently high, proving that the coal spiral is highly efficient for the separation of liberated low-density contaminants.
5. Final remarks
This study has shown that application of coal spirals is attractive when the lowdensity, organic contaminants are present as free particles. This emphasizes the importance of preliminary characterization of the (contamination in the) soil.
Removal of smaller particles is required to enhance the mobility of the individual soil particles and prevent deposition of a smooth layer in the trough. Liberation of the contaminants can be performed by scrubbing, either by mechanical agitation or by impact at high velocities.
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