The true density of total tailings from the CCM was 2.77 g/cm3, and the corresponding bulk density was 1.46 g/cm3. The porosity was 47.29vol%. The true solid densities of the tailings were determined using Eq. (1), based on the specific gravity test results.
where Gs is the specific gravity of dry tailings, ρs is the true solid density of dry tailings, and ρw is the density of water at 20°C, which is 998.23 g/cm3. The specific gravity (Gs) of the tailings was calculated following our previous study .
The cumulative particle distribution of the tailings was as follows: 36.39% were under 25 μm, 46.66% were under 38 μm, 64.50% were under 74 μm, and 17.08% were coarser than 0.18 mm. The particle size distribution of the total tailings samples was determined by artificial sieving methods, as shown in Fig. 1.
The mineralogical analysis of the micronized tailings was carried out using X-ray fluorescence, the result of which is given in Table 1. The main chemical composition of the tailings sample was SiO2, with a mass fraction of 47.82%.
SiO2 Al2O3 CaO MgO K2O Na2O S P Cu Co Pb Zn Fe Loss on ignition Total 47.82 9.71 11.5 8.89 4.51 0.2 0.35 0.06 0.1 0.03 0.02 0.02 1.94 9.38 94.53
Table 1. Chemical composition of the tailings
As determined from the flocculant selection experiment, Magnafloc 5250 from Badische Anilin-und-Soda-Fabrik (BASF), a German chemical company, was a suitable flocculant for the CCM tailings (rapid settling and a relatively low dosage). The optimal flocculant dosage was 20 g/t. The molecular weight of the flocculant was 13 million.
A self-made dynamic dewatering device was reconstructed. It was based on a Brookfield R/S Plus rheometer slow-speed rotating motor (Fig. 2). The device has an adjustable rotational speed (0.01−800 r/min) and can monitor parameters such as shear stress (0−2000 Pa), viscosity (0.001−80000 Pa·s), and torque (0.05−50 mN·m) during the mixing of tailings slurry. The torque resolution and angular resolution are 0.01 mN·m and 0.8 mrad, respectively. The accuracy of the device is ±1% of the maximum torque value.
The primary aim of this study was to establish a protocol to measure the solid mass fraction (i.e., dewatering performance) using a combination of experimental and theoretical tools. To this end, one rotation rate was set to attain such modeling prediction, although other rotation rates would lead to models serving the same purpose. The rotational speed in our experiments was selected considering the very low rake rotation of the DCT in engineering practices. While a high speed is prone to produce a larger torque value during the measurement, the low-speed measurement cycle could cause mud layer consolidation and a larger measurement torque value, all of which may cause a sudden stop of the rake. Therefore, an appropriate speed must be selected. We selected a low rotation speed of 5 r/min on the basis of the previous small-scale experimental study . In this study, we employed a self-made rake and connected it with the rheometer. Since the shear stress and apparent viscosity tested by the self-made rake were not accurate, the torque tested in this study was used to predict the dewatering performance of tailings. Generally, for the case when a vane rotor rheometer is used, the conversion formula between the shear stress and torque is given as follows [30,34]:
where H and R are the height and radius dimensions of the paddle rotor, respectively; T and τ are torque and shear stress, respectively. The self-made rake is not paddle-shaped, so it is difficult to obtain the regular integral area and thus the shear stress from the measured torque value. Therefore, shear stress and shear rate are also used in this study. The torque value is accurate, while the shear stress value is the relative value that can reflect the evolution rule of shear stress.
The device was used for the dynamic thickening experiment. Before the experiment, the mass of the water per millimetre of the measuring cylinder was calculated; in the conversion, the rake needed to be placed in the measuring cylinder experiments given that the rake also occupied a certain volume. When the mass of the water was 1833.9 g, the corresponding height was 212 mm, and when the mass of the water was 1015.0 g, the corresponding height was 58 mm. The mass of water corresponding to 1 mm height was calculated to be 5.3175 g.
In the process of industrial dewatering by the DCT, the flocculant solution is continuously added to the feedwell to mix with the tailings slurry, almost without rest time in the metal mines. However, before the flocculant solution is pumped into the feedwell, it should be stirred for 1 to 2 h, depending on parameters, such as the molecular weight and temperature. In this study, before the flocculant solution was added to the cylinder, we stirred the flocculant solution using a jar tester at 200 r/min for 15 min and then at 125 r/min for 75 min. In addition, the flocculant solution was left for 1 d for maturation before use .
First, 1400 g of water was added to the measuring cylinder, and 16 g of a flocculant solution with a solid mass fraction of 0.1% was further added, and then the flocculant solution was stirred for 3 min. Then, 800 g of tailings was added. By recording the height of the mud layer at different times, one can calculate the corresponding solid mass fraction using the following formula:
where Cw is the solid mass fraction; m1 and m2 are the mass of tailings and water, respectively; H1 represents the scale corresponding to the clear water, and H2 represents the scale corresponding to the mud layer interface.
The experiment was conducted using 600 g of tailings, 1400 g of water, and 12 g of 0.1% flocculant solution. In the previous experiment, the ratio of the final mud layer height to cylinder diameter was obtained as 0.83:1, which is similar to the ratio of mud layer height to diameter of the existing DCTs in other similar mines. The two parameters of shear stress and torque were monitored from the feeding. According to the actual operation of the DCT, the shear stress and torque evolution were investigated under the following three aspects: (1) the effect of the feeding process on the shear stress and torque evolution; (2) the effect of the thickening time on the shear stress and torque evolution; (3) the effect of machine restart on the shear stress and torque evolution.
Reconstructed rheometer for direct monitoring of dewatering performance and torque in tailings thickening process
29 March 2020
Revised: 14 May 2020
Accepted: 8 June 2020
Available online: 10 June 2020
Abstract: To further clarify the dewatering performance and torque evolution during the tailings thickening process, a self-made rake was connected to a rheometer to monitor the shear stress and torque. The dewatering performance of the total tailings was greatly improved to a solid mass fraction of 75.33% in 240 min. The dewatering process could be divided into three stages: the rapid torque growth period, damping torque growth period, and constant torque thickening zone. The machine restart was found to have a significant effect on the rake torque; it could result in rake blockage. Furthermore, the simultaneous evolution of the torque and solid mass fraction of thickened tailings was analyzed. A relationship between the torque and the solid mass fraction was established, which followed a power function. Both the experimental and theoretical results provide a reference for the deep cone thickener design and operation to enhance the dewatering performance.