Investigation of lab and pilot scale electric-pulse fragmentation systems for the recycling of ultra-high performance fibre-reinforced concrete

Abstract

Ultra-High Performance Fibre-Reinforced Concrete (UHPFRC) such as LafargeHolcim Ductal® is a new concrete product that incorporates large amounts of fine metal fibres, and is designed to have multiple advantages over traditional concrete products. These fibres, while providing additional strength, represent a new recycling challenge as they may block or increase wear of conventional mechanical apparatus, or be broken during processing rendering them unusable. High voltage electric-pulse fragmentation (EPF) systems such as those produced by SELFRAG AG use repeated electric discharges to selectively fragment composite materials along phase boundaries, overcoming compressive strength and preventing damage to metallic fibres. Initial tests in a laboratory scale system at a range of specific energy levels up to 60 kWh/t showed that Ductal® sample with a compressive strength of 170 MPa was amenable to EPF with good recovery rate of the steel fibres, which were fully liberated in the 0/2 mm product size fraction. Upscaled tests were performed on two Ductal® samples with compressive strengths of 170 and 210 MPa respectively using the 'Pre-Weakening Test Station' (PWTS), a continuous EPF system. Tests with specific energy levels up to 27 kWh/t showed similar results for both Ductal® samples: fibre liberation correlates with increasing specific energy input up to a plateau at about 13 kWh/t where increased energy produces little to no additional breakage. About 60% of fibres were recovered after just one treatment step performed at 13.4 kWh/t. These promising results obtained at pilot-scale indicate that this technology is suitable for UHPFRC recycling and fibre recovery, and that scaling-up the process to a commercial level is technically feasible.

Comparative laboratory study of conventional and Electric Pulse Fragmentation (EPF) technologies on the performances of the comminution and concentration steps for the beneficiation of a scheelite skarn ore

“…Above 9 kWh/t, the rate of size reduction decreases before levelling off in the high energy range (>15 kWh/t). Similar trends were reported in van der Wielen et al 2013, Zuo et al (2015) and Bru et al (2018), with the additional observation of an initial phase at low energy levels (<3 kWh/t) where little size reduction occurred. This initial phase was explained by the requirement of a material-specific threshold discharged energy to fully overcome particle strength and lead to the breakage of the majority of the particles (van der Wielen, 2013).…”

“…The operating parameters that can be changed are the voltage, electrode gap, pulse repetition rate and number of electric pulses applied to the sample. More details about the SELFRAG Lab can be found in Bru et al (2018) and van der Wielen (2013). In particular regarding the energy delivery of the lab system, two types of energy can be considered:…”

Pilot-scale investigation of two Electric Pulse Fragmentation (EPF) approaches for the mineral processing of a low-grade cassiterite schist ore

“…Increasing the discharged energy input from 6.5 to 12.5 kWh/t led to a reduction in product size (Test 2), however overall product size reduction was not linearly correlated with energy input: multiplying the energy input by a factor 1.9 (from 6.5 to 12.5 kWh/t) only led to a reduction in P80 by a factor 1.4 (from 15.0 to 11.0 mm). This is due to the non-linear relationship between energy input and size reduction as already observed by several authors (van der Wielen et al, 2013;Zuo et al, 2015;Bru et al, 2018). Fig.…”

“…taking into account the energy used for the first fragmentation step (either with conventional or with the EPF method), is given in Table 4 with the energy consumption of the jaw crushing steps implemented in the conventional pathway instead of the EPF treatment estimated using the Bond formula law and the crushability index of the ore. Table 4 shows that using an EPF treatment in the comminution circuit leads to an increase of the overall comminution energy compared to the conventional route. This comparison should be considered carefully, however, since the discharged energy value of 6.5 kWh/t was chosen based on previous tests performed at labscale, and it was shown that the energy consumption of a batch lab-scale EPF equipment exceeds the one of a continuous pilot-scale [Instruction: EDF should be replaced here by "EPF" (typo mistake)]EDF system for similar performances ( Bru et al, 2018 ). It could therefore be possible that an EPF treatment performed at a discharged energy lower than 6.5 kWh/t still induces a marked weakening of the sample leading to a lower energy consumption of the whole comminution circuit.…”

“…The combination of the shock wave with acoustic discontinuities concentrates tensile stress at phase interfaces (Andres et al, 1999) causing breakage at these boundaries, allowing full liberation of components from the feed material. The potential of this technique to improve liberation and grindability of ores was already demonstrated by several authors (Andres et al, 1999;Wang et al, 2012;Shi et al, 2013;Bru et al, 2018) but its application to a schist ore with a low cassiterite grade has not yet been reported.…”