**3. Slurry ERT literature review**

Improving the accuracy and utility of ERT for slurry applications has been the subject of many articles over the years, and a selection of these is reviewed here. A recent comprehensive overall review of slurry applications of ERT is given in [4].

Ref. [5] has presented a general iterative algorithm to improve the quantitative accuracy of ERT. This used a 2D model for the forward problem and was shown to be superior to the linear backprojection (LBP) algorithm used for rapid image reconstruction.

A further paper from the same group demonstrated ERT in use to understand the differing flow regimes in pipe flows [6]. They used slurry concentrations up to 20% by volume in a pipe loop with diameter 3 inches. The ERT was shown to readily visualize the flow regimes from homogenous flow to stratified flow. The transitions between these flow regimes could also be readily detected.

Ref. [7] examined the solids distribution in suspension flows with ERT. They used both neutrally buoyant, low- and high-density particles. Their algorithm used a 2D inverse problem and a 3D forward problem called a "pseudo three-dimensional algorithm." They attempted to model the observed particle distributions with limited success.

Ref. [8] examined bidispersed suspensions of negatively buoyant particles in a low Reynolds number pressure-driven pipe flow. A nonlinear ERT reconstruction algorithm was used. They used PMMA particles and silver particles. Due to the conductivity of the silver particles, the two types of particles were able to be imaged separately. These data were used for comparison with a suspension model, and another interesting result was the observation of the hindering of particle settling of one phase by the other phase.

Ref. [9] used a high-speed ERT system to measure concentration and velocity profiles in pipe flow. An 80-mm diameter pipe rig was used for all the tests. Two electrode planes of 100 mm apart were used for a cross-correlation technique to measure velocities. Kaolin carrier fluids and silica sand coarse particles were used to make the slurries.

Ref. [10] described one of the earliest ERT results for pipe flow. They used 60-mm and 100-mm diameter pipe loops with slurries made from water and sand or glass particles. Solids concentrations ranged from 6.8 to 13.7% by weight. Among other results, they were able to demonstrate ERT detection of a dispersed suspension compared with a saltating suspension. They proposed that a deposition velocity could be determined from the ERT image.

A recent example of ERT in conjunction with CFD applied to slurry tank flows is [11] where the mixing performance of a coaxial system comprises a wall scraper and a pitch blade central impeller. It was found that the coaxial mixer system provided better particle suspension than a single-impeller system.

A combination of ERT and magnetic flow meter has recently become commercially available as discussed in [12, 13]. The device was trialed in a dredging application. A significant advantage of the new device is that it eliminates the safety concerns of nuclear sources on board dredging vessels. The ERT is integrated into a single spool together with the magnetic flow meter.

Ref. [13] also describes other industrial applications of ERT in slurry flows, including mineral processing, tunneling, and deep-sea mining. Again, the motivation for much of this work is replacement of nuclear density gauges.
