Mine Tailings Backfill Stations and Grinding Circuit Density Measurement

Backfill Paste at 65%~78% With Coarse Particles: Can an Ultrasonic Density Meter Measure It?

Replacing differential pressure density meters with the ultrasonic acoustic-impedance method for concentration control on backfill transport lines and grinding cyclone feed

Scope: full-tailings cemented backfill plants at metal mines (high-density agitation — piston pump — transport line) and concentrator grinding and classification circuits (ball mill — hydrocyclone — sand return)

▍Project Snapshot / Process Conditions

1 Process Background and Measurement Challenges

▍1.1 Backfill concentration is a narrow band between safety and cost

As green mining and zero-waste mining programmes have advanced, full-tailings cemented backfill has become widespread at large Chinese non-ferrous and ferrous mines: full tailings from the concentrator serve as the aggregate, cement and additives are blended in, a high-density vertical agitator homogenises the mix into a paste (typically 65 % ~ 78 % concentration by weight), and piston pumps deliver it through boreholes and pipework to the mined-out voids underground, at once disposing of the tailings harmlessly and supporting the voids. In the concentrator at the same mine, the feed density in the grinding circuit (ball mill — hydrocyclone — sand return) determines the classification cut size and the circulating load. The control requirements at these two points are not the same:

▪Backfill concentration too low — the paste segregates and bleeds water, the backfill body is not strong enough once set, and the support it gives the void is compromised;

▪Backfill concentration too high — pipeline resistance rises sharply and the risk of blockage or even a burst pipe increases; clearing one blockage and replacing pipe often means several days of lost production;

▪Cyclone feed density too high — the cut size becomes too fine, the circulating load rises, and overgrinding and wasted energy follow;

▪Cyclone feed density too low — the classification runs coarse, particles entering the separation stage are oversize, and metallurgical performance falls with it.

The acceptable concentration band for backfill paste is often only 2 ~ 3 percentage points wide: the lower limit governs the strength of the backfill body, the upper limit governs pipeline safety directly — which means that any drift or lag in the measurement is amplified by the process. A mine's first requirement of a density meter is therefore usually not the accuracy figure itself but that it should not drift, should not block and should not need a shutdown to attend to. At the same time, backfill plants and grinding circuits are moving towards centralised monitoring with few operators, so density has to reach the DCS as a reliable electrical signal rather than depending on manual sampling and laboratory analysis.

▍1.2 Limitations of Existing Measurement Methods

▪Differential pressure / diaphragm density meters: paste backfill has a high solids content with hard particles, so the diaphragm and the impulse lines are under constant abrasion and the zero drifts quickly; the plant reported that the original differential pressure solution needed frequent shutdowns for calibration and replacement, and that the impulse lines blocked readily in high-density paste.

▪Vibrating fork / vibrating element density meters: the tines protrude into the flow path and are continuously scoured in high-density, coarse-particle paste; once worn, their frequency characteristics change and a systematic offset appears in the output, and broken tines have also occurred on site.

▪Gamma (nuclear) density gauges: they suit high-density paste, but backfill plants and underground workings are areas with dense human activity and frequent maintenance, where day-to-day management, annual inspection and decommissioning and disposal of a radioactive source are a heavy burden, and approval for new sources is being tightened in many regions.

▪Issues common to all insertion-type instruments: parts protruding into the flow path readily collect material and form lumps in paste, which both affects their own measurement and can become the starting point of a pipeline blockage — and once a backfill line blocks, dealing with it costs far more than the instrument itself is worth.

Figure 1 Schematic layout of the density measuring points for full-tailings backfill and the grinding circuit at a mine (left: paste density at the backfill plant piston pump discharge / right: grinding cyclone feed density)

2 The PS7000 Technical Approach

▍2.1 Measuring principle: the chirped (linear frequency modulation) acoustic-impedance method

When an ultrasonic wave travelling in a medium meets a solid-liquid interface, the echo amplitude is determined by the difference in acoustic impedance across that interface, and acoustic impedance Z = medium density ρ × sound velocity c is directly related to the density of the medium. The PS7000 uses a single self-transmitting, self-receiving sensor that continuously emits chirped (linear frequency modulation) ultrasonic pulses and acquires the echoes in real time. The wide bandwidth of the chirp signal helps the pulse penetrate bubble layers and suppresses multiple-reflection interference, and the transmitter then resolves the acoustic-impedance signature of the echo to recover the density of the medium.

The fundamental difference from the gamma-ray method is that the PS7000 contains no radioactive source of any kind. It is a non-nuclear instrument: no radiation safety licence is required, and there is no annual source inspection, no radiation-worker qualification and no source decommissioning and disposal.

▍2.2 Non-intrusive measurement: the sensor is wetted but does not protrude into the flow path

The PS7000 measuring spool is flanged into a straight run of the process line and the ultrasonic sensor is mounted on the spool. This is the key structural difference from conventional insertion-type instruments, and the point most often misunderstood during selection, so it is set out here exactly as it is worded in the Operating Manual:

▪Non-intrusive (non-intrusive is not the same as non-contact): the sensor must be wetted by the medium in order to work, but it does not protrude into the flow path — there is nothing projecting into the bore. The manual is explicit on this point: for horizontal installation with the sensor on top of the pipe, a full pipe must be guaranteed, otherwise the sensor loses contact with the liquid and the measurement fails; for remote-mount horizontal installation the sensor should be placed on the underside of the pipe so that it stays wetted.

▪There is therefore none of the leading-edge erosion, build-up at the probe root or downstream flow disturbance associated with insertion probes, and no risk of blocking the line.

▪The acoustic window of the sensor is sapphire: a fine surface finish, wear-resistant and corrosion-resistant. The measuring spool can be supplied with a lining (PTFE / rubber / ceramic) to suit the duty.

▪Heavily scaling media should still be included in routine inspection — the manual lists "scaling on the sensor face" as one of the items to check when calibration fails, the remedy being to clean the sensor and recalibrate.

▪Applicability limits: bubbles in the medium are permitted; the medium should not, however, contain large quantities of suspended or settled coarse particles.

Figure 2 Sectional view of the PS7000 installation — the structural difference from conventional insertion types (schematic drawing of the principle, not a record of measured data)

▍2.3 Key Technical Specifications

※ The typical concentration by weight of backfill paste is 65 % ~ 78 %, close to the upper limit of the 0 ~ 80 % range, so the range should be set to the plant's actual concentration band at the selection stage and the instrument calibrated on the actual medium. Note also the applicability limits in the manual: bubbles in the liquid are permitted, but the medium should not contain large quantities of suspended or settled coarse particles — backfill mixes that include coarse aggregate need their particle size distribution checked first; see section 5.

▍2.4 Selection and Installation Requirements (important)

The following conditions directly determine the stability of the measurement in the field. We recommend confirming them while the measuring point is still being designed; doing so avoids the great majority of post-commissioning disputes over deviation.

▪Vertical installation with upward flow is preferred. For horizontal installation, confirm that the medium is not stratified and that the measuring point is representative.

▪Flow velocity > 1 m/s; the higher the density of the solid phase, the higher the velocity required. Avoid low-velocity or stagnant duties, in which particle settling disturbs accuracy and stability.

▪Straight run requirement: the installation position should be well clear of pumps, valves, bends and other local resistances; for horizontal installation, 10D upstream / 5D downstream of straight run must be provided (5D upstream / 3D downstream as an absolute minimum).

▪Keep the sensor wetted: for integral horizontal installation with the sensor on top of the pipe, the pipe must run full; for remote-mount horizontal installation the sensor should be placed on the underside of the pipe. Where the pipe does not run full, the remote-mount version is preferred.

▪Avoid points where free gas bubbles accumulate. Aeration, twin-pump suction, negative-pressure inlets and similar conditions introduce large quantities of free gas bubbles; the bubbles raise the echo energy and bias the reading high. This is an inherent limitation of the acoustic method and must be handled by the choice of measuring point, not by compensating in calibration.

▪The comparison sampling point should be as close as possible to the measuring point. A sampling point far from the measuring point introduces transport lag, so the laboratory reference value and the displayed reading no longer refer to the same moment, and the difference is easily mistaken for instrument error.

▪Allow enough straight run at the piston pump discharge: backfill piston pumps produce large pressure and flow pulsations, while the manual requires the installation position to be well clear of pumps, valves, bends and other local resistances. Choose a point as far downstream of the pump as possible and preferably on a vertical upward-flow section; if the layout forces the point close to the pump, greater fluctuation in the reading has to be accepted and traded off through the damping setting, rather than corrected by repeated calibration.

▪Explosion protection requirements must be raised at the selection stage: where the underground workings or the backfill plant require explosion protection, the flameproof version is a custom option and the protection rating, the wiring arrangement and the certification requirements must all be stated with the enquiry — they cannot be retrofitted on site after despatch.

▍2.5 Field calibration: the gravimetric sampling method, performed online

Calibration brings the displayed value into agreement with the true density of the medium: several pairs of "instrument reading + corresponding laboratory reference value" are collected and the instrument automatically regresses the correction coefficients. The whole procedure requires no shutdown and can be carried out online provided the process is steady and the sensor is clean.

Process conditions to be confirmed, item by item, before calibration:

The most common situation in the field is that only the process medium is available and clean water cannot be introduced temporarily. In that case two consecutive acquisitions are made on the same medium (about 2 minutes each, about 4 minutes in total): the two measured points plus the virtual zero point added automatically by the system form three regression points, and the instrument uses the goodness of fit r² to cross-check the consistency of the two signals. If bubbles, a step change in flow or an abnormal valve movement during acquisition make the two data sets inconsistent, the system rejects the calibration outright rather than returning an incorrect coefficient. This is the key mechanism that prevents a calibration that looks successful but is in fact wrong.

Recommended calibration interval: calibrate a newly installed instrument once within the first 12 ~ 24 hours of operation; once operation is stable, verify every 6 ~ 12 months; verify every 1 ~ 3 months at measuring points with demanding accuracy requirements; recalibrate immediately after changing the sensor or the medium.

3 Before / After Comparison

4 Field Verification and Operating Record

At a full-tailings cemented backfill plant at a gold mine in Shandong, a PS7000 replaced the original differential pressure density meter on the backfill transport line and has been in service for about a year. Field comparison used the gravimetric sampling method; the displayed values stayed consistent with the manual laboratory reference values within the deviation band acceptable to the process, and all calibration checks were completed online. The plant reports that the concentration variation band of the backfill paste narrowed from ± 2.5 % to ± 0.6 %, and that the pipeline blockages and pipe bursts previously experienced have not recurred since commissioning.

Over the same period a PS7000 was installed on the cyclone feed line of the concentrator's grinding circuit, with the signal taken into the DCS for closed-loop control of the make-up water valve. The plant's figures for the same period show mill energy consumption down by about 4 % and metal recovery up by 0.6 percentage points. It should be said that energy consumption and recovery are influenced by ore properties, feed size, the reagent regime, equipment condition and other factors; the figures above are the plant's own statistics for the period and are not a performance specification of the instrument. The result on any particular project should be judged from that plant's own comparison data.

▍Verifiable Benefits for the Customer

▪The frequent shutdowns for calibration and replacement of differential pressure diaphragms and impulse lines in coarse-particle paste have been eliminated, and calibration checks can be completed online.

▪The concentration variation band of the backfill paste has narrowed (the plant reports ± 2.5 % down to ± 0.6 %), making it easier to hold the concentration within the narrow band the process allows.

▪No pipeline blockage or burst has occurred since commissioning, avoiding the associated production losses and pipework costs.

▪The grinding cyclone feed density reaches the DCS as a stable electrical signal, giving the make-up water valve a usable closed-loop input; the optional 4G module suits centralised mine monitoring with few operators.

5 Frequently Asked Questions

Q1 Backfill paste is 65 % ~ 78 % and contains coarse particles. Can the PS7000 actually measure it?

A It depends on the material, and this point must be settled clearly at the selection stage. Predominantly fine-grained full-tailings paste, made directly from concentrator full tailings, is within the applicable range provided the pipe runs full and the velocity is adequate; the PS7000 range is 0 ~ 80 % concentration by weight, and 78 % is already close to the upper limit, so the range must be set to the plant's actual concentration band and the instrument calibrated on the actual medium. The manual is explicit about the limits of applicability, however: bubbles in the liquid are permitted, but the medium should not contain large quantities of suspended or settled coarse particles — if the backfill mix includes classified tailings, rod mill sand, gobi aggregate or crushed rock as coarse aggregate, the coarse particles will scatter the echo and make the reading jump, and we do not recommend simply ordering the standard model. The correct approach is to supply the particle size distribution curve first (particularly the proportion above + 0.5 mm) together with the actual concentration band, so that we can assess applicability and, if necessary, propose a solution based on a different measuring principle. Setting the limits at the selection stage is far easier than explaining deviations after commissioning.

Q2 Can it be installed at the piston pump discharge?

A Backfill piston pumps produce large pressure and flow pulsations, while the manual requires the installation position to be well clear of pumps, valves, bends and other local resistances. The engineering approach is to choose a point as far downstream of the pump as possible, preferably on a vertical upward-flow section; for horizontal installation, provide 10D upstream / 5D downstream of straight run (5D upstream / 3D downstream as an absolute minimum). If the site layout really allows nothing but a position close to the pump, fluctuation in the reading will increase noticeably and the only remedy is to increase the damping to smooth it — but more damping means more lag, which has a cost at a measuring point used in a closed loop. This is a trade-off; there is no arrangement that gives both, and we recommend fixing the position of the measuring point at the design stage.

Q3 Grinding feed density changes quickly. Can the instrument keep up with closed-loop control?

A The manual gives no response time figure, and we make no claims such as response in seconds. The actual following speed is set mainly by the Basic Settings → Damping parameter: less damping means faster following but a noisier reading; more damping means a smoother reading but noticeable lag. For DCS closed-loop use we recommend tuning the damping on site against the time constant of the control loop and observing how the make-up water valve actually behaves. Note also that during calibration the damping should be reduced temporarily to 2 ~ 4 seconds (a requirement of the manual: damping above 20 seconds delays the acquired signal and degrades calibration accuracy), and restored to the operating value once calibration is complete.

Q4 Does the sensor actually touch the medium? Will it be worn away?

A It does touch it. The manual's formal wording is "non-intrusive", not "non-contact" — the sensor has to be wetted by the medium to work (with horizontal installation and the sensor on top, the pipe must run full or the measurement fails), but it does not protrude into the flow path and there is nothing projecting into the bore, so the main wear mechanism of insertion probes, the leading edge being continuously scoured by fast-moving solids, does not exist. Please note that some material in circulation describes this arrangement as "non-contact", which is not accurate. Within the Pisonics range, the products that genuinely do not touch the medium are the PS7010 clamp-on type (acoustic attenuation method) and the nuclear gamma-ray density gauge.

Q5 How is it calibrated in the field? Is a shutdown needed? How often?

A By the gravimetric sampling method, carried out from the handheld remote menu, with no shutdown — calibration can be done online provided the pipe is full, flow variation is within ± 5 %, no venting is in progress, the temperature is steady and the sensor is clean. Where only one medium is available on site, two consecutive acquisitions are made on that medium (about 4 minutes in total) and the system cross-checks the consistency of the two signals with r², rejecting the calibration outright if they disagree. Interval: first calibration within 12 ~ 24 hours of installation, verification every 6 ~ 12 months once stable, every 1 ~ 3 months for high-accuracy points, and immediate recalibration after changing the sensor or the medium.

Q6 Should the PS7000 or another model be selected for a given measuring point?

A The PS7000 (ultrasonic acoustic impedance) is aimed at the density of bubble-laden, high-solids slurries. If the pipe cannot be tapped, or an ageing line is being retrofitted, choose the PS7010 clamp-on type (acoustic attenuation); for concentration analysis of clean liquids choose the PS7020 (sound velocity); for scaling-prone acids and alkalis in chemical service choose the PS7400 (vibrating fork); for large-bore mains above DN200 the PS7300 (differential pressure) is an option; for custody-transfer accuracy choose the PS7200 (Coriolis); for multi-component concentration analysis choose the PS7100 / 7110 (spectral / refractometric); for aqueous suspensions the PS7600 (microwave) is available. If the selection is not clear-cut, send us the process data and we will work it out for you.

About Pisonics

Xi'an Pisonics Information Technology Co., Ltd. (PISONICS) specialises in ultrasonic inline measurement technology, with products covering slurry density, solution concentration, spectral concentration, level, flow and other industrial process parameters. The PS7000 Series Ultrasonic Slurry Density Meter is the company's own non-nuclear inline slurry density monitor, and is in service in power plant desulphurisation, non-ferrous and ferrous mineral processing, coal preparation, salt lake chemicals, dredging, municipal sludge and other industries.

Ultrasonic · Density · Concentration · Inline Measurement Specialists

※The process data and operating records in this case study are compiled from an actual project; Figures 1 and 2 are schematic diagrams of the principle and the layout, not curves recorded from measurement. The technical specifications of the instrument and its installation and calibration requirements are as stated in the Pisonics Ultrasonic Density Meter Operating Manual supplied with the goods, and are subject to change without notice.