I. Process Background
Heavy‑media coal separation is currently the mainstream process in large‑scale coal preparation plants across China. Raw coal enters a three‑product hydrocyclone (or a heavy‑media inclined wheel separator) and is mixed with a heavy‑media suspension prepared by blending magnetite powder with water. Based on the density differences among coal, gangue, and pyrite, three products are separated: clean coal (the light product), middlings, and gangue (the heavy product). The density of the heavy‑media suspension is the decisive parameter for separation efficiency—typically operating within the range of 1.30 to 1.85 g/cm³. A density deviation of only ±0.02 can alter the ash content of the clean‑coal product by more than 0.5 percentage points, thereby affecting its contractual price.
The suspension density is subject to multiple disturbances: changes in the particle size distribution of the raw coal, fluctuations in the slime content of the feed, variations in magnetic separation recovery efficiency, and adjustments to the water addition rate and the feed rate of the media screw. To ensure long‑term stability of the separation process, it is essential to rely on high‑frequency, stable density data, enabling the DCS to automatically close the loop and regulate the water valve and the media screw.
II. Pain Points of Traditional Density Measurement Solutions
• Nuclear (gamma‑ray) concentration meters have long been the industry’s default choice; however, in recent years, environmental approval has become increasingly stringent. In many regions, new coal‑preparation projects can no longer obtain radiation safety permits, and existing installations face pressures related to annual inspections and decommissioning.
• Vibration‑tuning‑fork density meters, when used in suspensions containing magnetite powder and coal slime, suffer from frequent fouling and wear of the fork, requiring shutdowns for cleaning every 1–2 months and leading to frequent measurement interruptions.
• Differential‑pressure density meters rely on diaphragms that come into direct contact with the suspension. Over time, magnetite particles abrade the diaphragm, creating micro‑pores and causing significant zero‑point drift.
• The density of the circulating medium changes rapidly (in response times ranging from milliseconds to seconds); if the instrument responds too slowly, it cannot keep pace, resulting in loss of control over the separation process.
III. PS7000 Solution
The PS7000 is installed on the main pipeline of the heavy‑media suspension loop (at the outlet of the pressurizing pump, just upstream of the hydrocyclone inlet), using a flanged direct‑insertion mounting method that requires only 5D of straight pipe upstream and 2D downstream. The instrument continuously outputs the real‑time density of the suspension, feeding this information back to the plant’s DCS to automatically control the water valve (to reduce density) and the media screw (to increase density), thereby achieving high‑frequency closed‑loop stabilization of the operating density.
Figure 2 Schematic of the density control process for the heavy‑media suspension in a coal preparation plant
Core Value of the PS7000 in the Coal Washing Industry ▸ Completely replaces nuclear density meters—no radiation, no licensing required, and zero hurdles for environmental approval in new projects; ▸ The non-contact probe is immune to wear and entanglement by magnetite particles; no shutdowns are needed for cleaning, ensuring an availability of ≥ 99%; ▸ The Chirp algorithm delivers a response time of < 1 second, enabling it to track rapid fluctuations in slurry density and significantly improving the accuracy of DCS closed-loop control; ▸ By installing a second PS7000 on the discharge line of the fine-coal medium‑removal screen or centrifuge, the concentration of the final product slurry can be monitored simultaneously, ensuring product quality. |
IV. Customer Value
Comparison Dimensions | Original nuclear density meter | PS7000 Solution |
Radiation Safety | License required · Annual inspection training | Intrinsically safe · No radiation whatsoever |
Maintenance workload | Tuning fork body cleaning / source decommissioning is cumbersome | Essentially maintenance-free · Long service life |
Density tracking | Slow response / lag | < 1-second response · Real-time tracking |
Stability of the Clean Coal Product | Density fluctuation → Ash content fluctuation | Density precision control · Stable ash content meeting standards |
Medium consumption | High dielectric loss (loose control) | Medium consumption decreases by 5%–10% |
In an engineering retrofit project at a large thermal‑coal preparation plant in Shanxi (with an annual throughput of 6 million tons), three PS7000 units replaced the original nuclear concentration meters, being deployed across two heavy‑media circulation loops and the clean‑coal desliming line. Six months of operational data show that density fluctuations narrowed from the previous ±0.03 g/cm³ to ±0.008 g/cm³, the average ash content of the clean coal decreased by 0.4 percentage points, and annual additional revenue exceeded ten million yuan.
