Wire meshes for separating particulate secondary building materials

1 Introduction

Wire meshes are generally used for droplet separation and for separating liquid/liquid systems [1-4]. However, as this article shows, these wire meshes can also be used very effectively for separating coarse-grained and irregularly shaped particle systems. Wire meshes can be used with excellent separation performance in the recycling of building materials, the processing of plastics or renewable raw materials, and in the agricultural sector, for example. Mineral bulk materials and secondary raw materials from the construction and ceramics industries very often have a broad particle size distribution, so that the coarse material can be separated very successfully in a first process step. In these areas, it is particularly important to achieve high separation performance with low pressure drop.

Due to their design, wire mesh filters are a combination of surface and depth filters. With suitable fluids, the systems can be cleaned after a certain number of operating hours and then reused. Wire mesh filters are therefore very well suited for separating particle systems with a wide size distribution and sufficiently coarse particles.

2 Materials and methods

2.1 Mixed construction waste

As part of this work, mixed construction waste from the building construction recycling sector was examined. Figure 1 shows the mixed construction waste used, which was kindly provided by Franken Baustoff Recycling GmbH & Co. KG.

This construction waste was crushed using hammer and ball mills and then fractionated by sieving. Figure 2 shows the particle size distribution Q₃(x) of the four fractions examined (M1 to M4).

The average particle sizes are as follows: x_50,3(M1) = 10.9 µm, x_50,3(M2) = 55.1 µm, x_50,3(M3) = 103.3 µm and x_50,3(M4) = 153.8 µm. The average density of this mixed construction waste is ρ = 2.6 g/cm³. Figure 3 shows an example of a fraction of the mixed construction waste after crushing in a scanning electron microscope (SEM) image.

It can be seen that after crushing and screening, particle fractions of the mixed construction waste are produced that have different sizes and shapes.

2.2 Wire meshs

The wire meshes examined in this study were provided by Rhodius GmbH, Weißenburg. The wire meshes used have a diameter of D = 140 ± 4 mm and a thickness of s = 5 mm. They are made of wires with a diameter of d = 0.14 mm and a density of 1000 kg/m³. Figure 4 shows a wire mesh used in the test.

2.3 Test facility

The facility used to investigate the possibilities for separating construction waste from gas-particle flows is shown in Figure 5. The plant essentially consists of two components, the raw gas chamber and the clean gas chamber, which are separated by the wire mesh. The plant also has a pressure connection, a unit for dosing the building material particles, pressure measuring units in each gas chamber, and a hot-wire anemometer for measuring the flow velocity at the end of the plant.

3 Test results

The tests were carried out using the test setup shown in Figure 5 with a particle mass flow of  = 1,1 g/min and a fluid mass flow velocity of u = 2.1 m/s.

With a test duration of t = 90 min, all tests resulted in a pressure loss across the wire mesh of Δp ~ 210 Pa. Figure 6 shows the wire mesh after a test duration of 1.5 hours.

Figure 7 shows the volume sum function Q₃(x) for one test as an example. The construction waste particles separated by the wire mesh are in the size range 25 µm ≤ x ≤ 400 µm.

The average particle size in the raw gas area is x₅₀ = 223.6 µm. As already mentioned in the previous chapters, the wire mesh is a separation device that cannot be considered an “absolute filter”. This means that there is also an unseparated particle fraction with a mean diameter of x₅₀ = 73.3 µm in the clean gas area. This is clearly shown in Figure 8 and Figure 9.

Figure 8 shows the particle fraction retained by the wire mesh in 1.5 hours at the volume flow rate specified above. This clearly shows that wire meshes are very well suited for separating coarse fractions, for example in the recycling of building materials.The separation efficiency ξ is determined according to equation (1):

⇥(1)

Here, m_RG is the separated fraction in the raw gas duct, Δm_DG is the fraction of particles separated inside the wire mesh, and m_AG is the mass of the feed material. The tests carried out in this project yielded separation efficiencies in the range of 0.895 ≤ ξ ≤ 0.932. An average separation efficiency across all tests performed is ξ_m = 0.921.

Figure 9 shows the clean gas chamber of the test rig after 1.5 hours of operation. It can be seen that the fine fraction of the recycled construction waste cannot be separated by the wire mesh used, but also that no coarse particles pass through the wire mesh.

4 Conclusion

Wire mesh has typically been used for droplet separation and for separating liquid/liquid systems. This article has shown that wire mesh, which is a combination of depth and surface filters, can also be used to separate coarse-grained particle systems. This work has demonstrated that effective separation is possible for the mixed construction waste material system. Using the wire mesh used and the fractionated particle systems with a maximum particle size of x_max = 400 µm, particles with a size of x_min = 25 µm could be separated. With separation efficiencies in the range of 90 %, wire meshes are very well suited for the separation of particle systems with a high proportion of coarse material.