Density Measurement of Salt Lake Brine Potash Fertilizer Evaporation-Crystallization Solid-Bearing Two-Phase Flow

Measuring Crystal Slurry Density in Salt Lake Potash Production: Dealing With Crystallisation and Scaling on the Sensor

Replacing a gamma-ray density gauge with the ultrasonic acoustic-impedance method to measure the two-phase solids-bearing flow of saturated mother liquor and KCl crystals

Scope: the salt lake chemicals and potash fertiliser industries — raw brine feed, multiple-effect evaporation and crystallisation, cyclone thickening and centrifugal separation; similar evaporation and crystallisation plants (lithium salts, boric acid, magnesium salts and so on) can be specified along the same lines

▍Project Snapshot / Process Conditions

1 Process Background and Measurement Challenges

▍1.1 Density runs right through the feed, crystallisation and separation stages of salt lake potash production

The salt lake regions of Qarhan in Qinghai and Lop Nur in Xinjiang support world-scale potash, lithium salt, boric acid and magnesium salt industries. A typical potash process takes salt lake raw brine as its feedstock and passes it through pre-settling for impurity removal, multiple-effect evaporation and crystallisation, cyclone thickening, centrifugal separation, drying and packing to produce finished KCl. Three slurry density measurements along this route directly determine the operating economics of the plant and the quality of the product:

▪Brine feed concentration — the KCl, NaCl and MgCl₂ composition of the raw brine varies with the climate and the season, and a feed concentration away from the design value drives up evaporation energy consumption and lowers the efficiency of the multiple-effect evaporation;

▪Crystalliser discharge density — the discharge is a two-phase solids-bearing flow of saturated mother liquor and KCl crystals, and its density is the key parameter for judging how far crystallisation has gone, for deciding when to discharge, and for avoiding scale formation in the equipment;

▪Centrifuge feed density — directly determines separation efficiency and mother liquor recovery, and variation in the feed concentration shows up directly in the moisture content of the product.

The real difficulty of salt lake duty lies in the medium itself: a near-saturated high-chloride solution (published industry data put the chloride concentration of raw brine at the order of 200 000 mg/L, about ten times that of seawater) that precipitates crystals on the slightest change of temperature or pressure; the crystallisation stage also contains seed crystals, flash steam and bubbles at the same time. This is both why conventional density instruments fail here and a boundary condition that has to be faced squarely during selection — any measurement solution here has to answer the question of what is to be done about crystal deposits before anything else.

▍1.2 Limitations of Existing Measurement Methods

▪Differential pressure / diaphragm density meters: the diaphragm and the impulse lines are directly exposed to saturated high-chloride brine, with corrosion and crystal abrasion acting together. The plant reported that the original 316L diaphragm solution perforated in less than a year, that the impulse lines were readily blocked by precipitated crystals, and that zero drift was frequent.

▪Vibrating fork / vibrating element density meters: the tines protrude into the flow path and in a near-saturated solution crystals readily form and build up on their surface — the added mass changes the vibration frequency of the tines directly, producing a systematic offset in the output and requiring frequent shutdowns for cleaning.

▪Gamma (nuclear) density gauges: they cope with the corrosion, but salt lake plants are mostly in remote high-altitude areas, where licensing, annual inspection, personnel qualification, transport and decommissioning and disposal of a radioactive source are far harder to arrange than at an inland works.

▪A challenge common to all multiphase flow regimes: the crystallisation stage is a multiphase flow of mother liquor, crystals and flash steam, and solutions that rely on a single differential pressure or on the vibration of a fork give jumping readings in this regime, which makes them difficult to use as a stable input for discharge and feed control.

Figure 1 Schematic layout of the density measuring points in the salt lake brine evaporation and crystallisation process for potash production (PS7000 #1 brine feed concentration / #2 crystal slurry density / #3 centrifuge 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 medium temperature of this duty runs from site ambient to 80 ℃, exactly at the upper limit of the 0 ~ 80 ℃ range of the standard version; if the actual temperature at the crystalliser discharge or another point may exceed 80 ℃, the high-temperature custom version must be selected, and the exact limit should be confirmed with Pisonics at the selection stage. Both the measuring spool lining and the sensor material can be built to suit the corrosivity of the medium; please provide the medium composition and the temperature range with your enquiry.

▍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.

▪Keep clear of the positions where flashing and temperature drop are concentrated: crystals precipitate most heavily just downstream of a pressure-reducing valve or immediately at a heat exchanger outlet, and these positions should be avoided when choosing the point; the measuring point must have a velocity > 1 m/s, and the means of clearing crystal deposits from the spool after the pump stops should be considered in advance.

▪Put an inspection of the sensor face into the maintenance schedule: for media with a strong tendency to crystallise, check the sensor face for crystal deposits and scale at each plant turnaround (or every 1 ~ 3 months), and re-verify the calibration after cleaning — the manual lists "scaling on the sensor face" explicitly as one of the items to check when calibration fails.

▍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 potash plant in Qinghai, a PS7000 replaced the existing gamma-ray density gauge on the centrifuge feed line, with measuring points added at the same time on the raw brine feed and crystalliser discharge lines. Over six months of operation, several rounds of field comparison were carried out by 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 variation band of the centrifuge feed density narrowed from ± 3 % to ± 0.5 %, and that the pass rate for the moisture content of the KCl product rose from 92 % to above 98 %. These are the plant's own figures; moisture content is also affected by crystal size, the condition of the centrifuge and other factors, and the result on any particular project should be judged from that plant's own comparison data. The original nucleonic density gauge was subsequently decommissioned from the main plant.

For this strongly crystallising duty, the project put an inspection of the sensor face into the plant's periodic maintenance schedule at handover: if the sensor face becomes covered by a crystal layer or by scale, the echo signature changes and the reading drifts with it. We explained this to the customer while the solution was still being drawn up — rather than claiming maintenance-free operation, we specify an inspection interval in the delivery documentation, which is easier for both sides.

▍Verifiable Benefits for the Customer

▪No licensing, annual inspection, personnel qualification training, transport or decommissioning and disposal to manage for a radioactive source at a remote high-altitude site.

▪The variation band of the centrifuge feed density has narrowed (the plant reports ± 3 % down to ± 0.5 %), feed stability has improved and the moisture content of the product is easier to hold within specification.

▪With nothing projecting into the bore, the shutdowns previously needed to replace perforated diaphragms and to clear crystal-blocked impulse lines have been eliminated; calibration checks are completed online and do not interrupt continuous operation.

▪The density signal is brought into the DCS over 4-20 mA / MODBUS-RTU, and an optional 4G module allows remote upload to the cloud, suiting the centralised monitoring of unmanned salt lake evaporation sites.

5 Frequently Asked Questions

Q1 Will saturated brine crystallise on the sensor face? Will the measurement still be accurate?

A That risk does exist, and it has to be faced squarely for this duty. A saturated solution precipitates crystals whenever temperature or pressure varies, and once the sensor face is covered by a crystal layer or by scale the echo signature changes and the reading drifts with it; the manual lists "scaling on the sensor face" explicitly as one of the items to check when calibration fails, the remedy being to clean the sensor and recalibrate. We therefore do not use the term maintenance-free for media of this kind, and recommend three things instead: (1) include an inspection of the sensor face in the maintenance schedule at each plant turnaround (or every 1 ~ 3 months); (2) keep the velocity at the measuring point above 1 m/s and avoid the positions where flashing and temperature drop are concentrated; (3) specify a spool lining suited to the duty in order to reduce the tendency for deposits to form. Compared with a diaphragm or a fork protruding into the flow path, the non-intrusive arrangement offers a small area for deposits and is easier to clean, but that makes it more resistant to scaling, not free of scaling.

Q2 What exactly is measured in a two-phase solids-bearing flow? Does it indicate the crystal content?

A The instrument outputs the overall density of the medium across the measuring section, that is the combined density of the saturated mother liquor and the KCl crystals, not the crystal solids content directly. If the process needs solids content, a density-to-solids-content relationship for that particular plant must be established during calibration by sampling (filtering and weighing), bearing in mind one precondition: the density of the mother liquor itself varies with its KCl / NaCl / MgCl₂ composition, so when the mother liquor composition drifts the same solids content corresponds to a different total density. This is an inherent limitation of inferring solids content from a single density parameter, and plants with large swings in composition should correct for it by periodic sampling checks.

Q3 The crystalliser discharge runs hot. Is the standard version adequate?

A It depends on the measuring point. The medium temperature range of the standard version in the manual is 0 ~ 80 ℃; this project runs from ambient to 80 ℃, exactly at the limit. If the actual temperature at a given point may exceed 80 ℃, the high-temperature custom version must be selected and the exact limit confirmed at the selection stage, rather than ordering the standard version. Temperature also affects the measurement signal in its own right — the manual requires a steady temperature during calibration, and the instrument provides a temperature compensation function (a linear correction of the primary measured value against temperature, disabled by default at the factory). We recommend calibrating at the actual working temperature, and enabling temperature compensation and tuning the compensation slope only at points with a wide temperature span.

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.