Online Measurement of Slurry Density in Non-ferrous Metallurgical Beneficiation Plants

How Do You Choose a Slurry Density Meter? Non-Nuclear Inline Density Measurement in Mineral Processing Plants

Engineering practice: replacing gamma-ray and differential-pressure density meters with the ultrasonic acoustic-impedance method

Scope: non-ferrous (copper / lead-zinc / molybdenum / nickel / tungsten) and ferrous (iron ore) concentrators — the grinding and classification, flotation and thickening sections

▍Project Snapshot / Process Conditions

1 Process Background and Measurement Challenges

▍1.1 Density is a control variable throughout the concentrator

The core wet-process route of a non-ferrous concentrator (copper, lead, zinc, nickel, molybdenum, tungsten and so on) runs through crushing, grinding, classification, flotation, thickening and filtration; iron ore concentrators follow a closely similar flowsheet. At every one of these stages the slurry density (or solid-to-liquid ratio) is a key process control variable:

▪Mill discharge density — sets the grinding circulating load and directly affects mill power consumption and liner life;

▪Hydrocyclone feed density — sets the classification cut size and directly affects downstream flotation efficiency;

▪Flotation feed concentration — governs reagent performance and directly affects metal recovery;

▪Thickener underflow density — sets the discharge concentration of concentrate and tailings and directly affects the stability of filtration, pressure filtration and tailings transport.

A rule of thumb widely quoted in the industry is that for every percentage point by which the slurry concentration deviates from setpoint, metal recovery falls by roughly 0.3 % ~ 0.5 % and grinding energy consumption rises by roughly 1 % ~ 3 %. These ranges are empirical figures taken from published mineral processing literature; they vary considerably between ore types and flowsheets, and the actual impact at a given mine should be established from that plant's own process test data.

▍1.2 Limitations of Existing Measurement Methods

▪Gamma (nuclear) density gauges: accurate, but they are radiation-based measuring instruments. They require a Radiation Safety Licence, together with annual inspection, qualification training for radiation workers and costly source decommissioning and disposal. Several major mining provinces have placed them on restricted-use lists, and environmental approval for new projects has become harder to obtain.

▪Differential pressure / diaphragm density meters: the diaphragm is directly exposed to highly abrasive slurry, particularly with high-specific-gravity minerals such as iron and copper ores. Noticeable zero drift appears after 3 ~ 6 months of operation and frequent shutdowns for recalibration are needed; scaling on the diaphragm surface amplifies the error further.

▪Vibrating fork / vibrating element density meters: in flotation feed lines, surfactants generate large volumes of fine froth. The vibration frequency of the fork tines is disturbed by the froth, the reading jumps continuously, and the signal is difficult to use in stable closed-loop control.

▪Issues common to all insertion-type instruments: thickener underflow reaches 50 % ~ 70 % solids and often contains coarse particles. Probes that protrude into the flow path are prone to material build-up and blockage, giving high maintenance frequency and low availability.

Figure 1 Schematic process layout of the three slurry density measuring points in a concentrator (PS7000 #1 mill discharge / #2 flotation feed / #3 thickener underflow)

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

▍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.

▪High-specific-gravity minerals need particular attention: iron ore and similar heavy minerals settle and stratify readily in a horizontal pipe at low velocity, and the displayed value is then systematically low — a vertical upward-flow section should be chosen in preference.

▍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 large iron ore concentrator in Xinjiang, a PS7000 replaced the original differential pressure density meter on the mill discharge line. After commissioning, several rounds of field comparison were carried out by the gravimetric sampling method; the displayed values stayed consistent with the manual sampling results within the deviation band acceptable to the process, and long-term accuracy has been stable. The instrument is now the standard fit for equivalent measuring points at that plant. All calibration checks during operation were completed online, and no shutdown has been caused by instrument calibration. The same application has since been rolled out to a non-ferrous concentrator in Qinghai and a copper-molybdenum mine in Inner Mongolia, among other projects.

▍Verifiable Benefits for the Customer

▪No licence application, annual inspection, personnel qualification training or source decommissioning and disposal for radiation-based instruments, which simplifies the environmental approval route for new projects.

▪The 3 ~ 6 month cycle of shutdown calibration and replacement of differential pressure diaphragms has been eliminated, reducing unplanned downtime.

▪Material build-up and blockage no longer occur at the thickener underflow measuring point, raising instrument availability.

▪With the density signal brought into the DCS, the grinding circulating load and the flotation feed concentration now have a stable input for closed-loop control.

5 Frequently Asked Questions

Q1 How much accuracy is given up compared with a gamma-ray density gauge?

A The PS7000 is rated ± 1 % FS (the published datasheet figure is ± 1 % concentration by weight) and can be calibrated on the actual medium. A well-calibrated gamma-ray gauge is more accurate, but it carries the whole-life compliance cost of a radiation licence, annual inspection, personnel qualification and source decommissioning. The choice is really a trade-off between the control accuracy the process needs and the compliance and maintenance cost it carries: for control-type measuring points such as the grinding circuit and the flotation feed, the PS7000 figures are usually sufficient. For custody-transfer grade metering, a Coriolis mass flowmeter or similar should be used instead.

Q2 There is a lot of froth in the flotation feed. Will the measurement still be accurate?

A The wideband chirp signal penetrates fine dispersed bubbles well, and the manual states explicitly that "bubbles in the liquid are permitted", so the behaviour is clearly better than that of a vibrating fork in frothy duties. If, however, large quantities of free gas bubbles are present at the measuring point — an aeration section, or the negative-pressure inlet of twin-pump suction, for example — the bubbles raise the echo energy and bias the reading high. This is an inherent limitation of the acoustic method. The correct response is to adjust the position of the measuring point in line with the principles in section 2.4, rather than to compensate with calibration coefficients. Note also that the medium should not contain large quantities of suspended or settled coarse particles.

Q3 Can it be installed on a horizontal pipe?

A Yes, but the conditions are stricter: the slurry must not be stratified, the measuring point must be representative, the velocity must exceed 1 m/s, and 10D upstream / 5D downstream of straight run must be provided. With the integral version and the sensor on top of the pipe, the pipe must run full; where the pipe does not run full, use the remote-mount version with the sensor on the underside. Heavy minerals such as iron ore settle and stratify readily in a horizontal pipe at low velocity, and the displayed value is then systematically low — this is not an instrument fault, it is the slurry at that point not being representative. The preferred solution is to move to an adjacent vertical upward-flow section.

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.