Online rheometry for multi-phase flows

The need for an accurate and reliable means of measuring suspension rheology in real time has arisen from the greater demands being placed on mineral processing operations. The trends towards finer grinds, higher solids concentration and higher clay contents as a consequence of declining grades and water resources result in complex multi-phase suspensions that need close monitoring to optimise thickener performance, pipeline transport and tailings deposition. In the case of tailings thickening, many operations focus on underflow density, but its actually the underflow rheology that controls or determines thickening performance.

Due to the involved nature of rheological measurement for suspensions and the nuanced interpretation of data necessary to produce useful decisions, rheometry has only seen limited application in process monitoring. A robust unit that can measure, analyse and interpret the rheology of a process stream continuously and unattended is a major part of the answer to this problem.

Operating principle

The iOR™ takes a side stream of the process fluid of interest and measures the pressure drop generated over a pipe of known dimensions for a series of known volumetric flow rates. This is a variation on the standard “capillary viscometer” used for Newtonian fluids. Changing the flow rate within the rheometer allows the non-Newtonian properties to be measured and analysed.

Although a robust measuring method, fluid homogeneity is still required for a correct measurement, which is not always possible for a settling suspension. This is accounted for with the iOR™ by including a Coriolis meter and using two pipes of differing diameter. If the flow is pseudo-homogeneous then the results from both pipes will sit on a single curve in a rheogram. If settling occurs or excess gas is present then they will differ. This is checked by the rheometer’s built-in software.

The inlet to the iOR™ is attached as a bypass of the thickener underflow (or other unit operation) before the underflow pump, or at any point of interest for a slurry pipeline. This ensures the rheology measured is as close to the process condition as possible and not sheared excessively. The unit can be set to run on a fixed time cycle of high-to-low shear rates, generating a fresh rheogram every time. The shear rates and time of shear are adjustable to suit the material being investigated. A regular output of rheograms is generated without operator intervention. An alternative mode is to operate at a single shear rate and continuously monitor change – this is more suitable if rapid changes are expected. If a single representative parameter is required, the yield stress can be calculated automatically and stored in a data file or transmitted as a 4-20 mA signal. Both these options are built-in and can be switched by the user as required. The software has options to run manually or automatically and hosts a web server, so it can be accessed remotely via LAN or wireless mobile communication for analysis and diagnostics.

Why is this instrument different?

Several commercial instruments exist to measure rheology online. These units are not well suited for handling multi-phase flows, often being developed for the food and pharmaceutical industries. The iOR™ has no protrusions into the flow, so wear is reduced. It doesn’t require velocity profile measurement, which is problematic in solids laden flows. The measurement technique allows for the non-Newtonian characteristics often experienced in suspension flow. Some key features:

• Wide shear rate range which is adaptable to different process requirements.
• Unattended operation, following a pre-programmed measurement cycle including the shear rates of interest at the optimum sampling frequency.
• Remote data access via 3G telecommunications.
• Requires only single phase power (110/240V), process water (for flushing) and connection to process stream to be measured.

Two views of the online rheometer field deployable configuration

The iOR™ is capable of measuring a wide range of suspensions on line and without supervision. It provides greater accuracy for in process rheological characterisation of multi-phase suspensions than other techniques. Continuous improvement based on field experience is part of the CSIRO approach to refining this instrument, and as such the iOR™ is capable of being deployed on site with the necessary technical and rheological support to achieve a reliable output.