Industrial mineral sorting

Quality control of rock
Quality control of rock

Minerals and rock

The production of high-quality mineral products and building materials demands high-grade raw materials as well. Expensive and time-consuming laboratory methods are usually used for the analysis of raw materials. Inline analysis is commonly realised by RGB cameras. For a successful separation by RGB cameras, the colour of the valuable minerals and the waste rock fraction must differ significantly. If both fractions show a similar colour, the application of NIR-process technology will enable a successful separation nevertheless. Each NIR active mineral shows a unique pattern in the NIR spectrum. These spectral features are used as basis for the identification routines in mineral sorting. The identification routines are developed specifically for each sorting task, depending on the mineral composition. The routines can be adjusted easily by the customer.

The imaging NIR process spectrometers of the uniSPECx.xHSI series are ideally suited for such identification tasks. Due to the high spatial resolution, even small particles are detected at a conveyor belt speed of up to
3 m/s. The multiplexed NIR-spectrometer uniSPEC2.2MPL is used for analysis of large particles. All NIR-systems are easy to integrate into existing plants of the process industry or in the quarry. For the separation of valuable minerals from waste rock, several identification routines are available. An overview for standard applications is given in the table below.

Application uniSPEC2.2MPL uniSPEC1.7HSI uniSPEC1.9HSI uniSPEC2.2HSI
Calcite / Dolomite + - - ++
Talc / Waste rock + + + ++
Borate / Waste rock + ++ - -
Bauxite / Waste rock + - - +
Gypsum / Anhydrite + + ++ -

++ particularly suitable  + suitable  - limited suitability

In addition, NIR-spectroscopy is utilised for quantitative determination of the mineral composition in NIR active minerals. For these applications the portable uniSPEC2.2P is suitable. The data is compared to database information and the result is displayed subsequently. The NIR-process spectrometers uniSPECx.xHSI and uniSPEC2.2MPL can be used for semi-quantitative determination of minerals in rock. The results can be used to comply with product specifications and limits.

Qualitative separation of calcite and dolomite

Sorting of limestone by NIR technology
Sorting of limestone by NIR technology

Limestone has a global annual production rate of 1000 mega tonnes and is therefore one of the main products in the mining industry next to sand and grovel. Limestone must contain a preferably low amount of magnesium oxide (MgO) for usage in cement production or as additive in smelting. The amount of MgO often originates from dolomite impurities in the raw material.

The NIR application idMgO was developed for the identification of MgO phases in limestone. This application significantly reduces the amount of MgO in the resulting product and is customised for specific on-site conditions of the natural mineral deposit. A RGB-camera can be combined with the NIR-camera uniSPEC2.2HSI for additional identification of differently coloured impurities.

Identification of swellable minerals in stone

Diabase is commonly used as high-quality split for road construction. Impurities of swellable clay minerals in diabase are problematic. If water permeates into the split layer, the clay minerals start to swell, resulting in additional stress for the asphalt and finally damage to the road surface. To avoid damage such as potholes, the amount of clay minerals in diabase must be monitored.

For random control purpose by the NIR-spectrometers uniSPEC2.2S and uniSPEC2.2P, the application idDiabase was developed. The identification routine is based on spectral differences between diabase and clay minerals and enables an early identification of batches exceeding the clay mineral limit.

Quantitative determination of MgO in limestone

Quantitative determination of MgO in limestone
Quantitative determination of MgO in limestone

The MgO content in limestone must be determined regularly during cement production. For this purpose, XRF measurements are performed routinely on powdered samples, although the sample preparation is costly and time consuming. In contrast, quantitative NIR-measurements of the MgO content using the NIR-spectrometer uniSPEC2.2S are possible without further sample preparation.

The quantitative analysis of the MgO content is based on the identification routine idMgOquant. This routine is available also for the hand scanner of the uniSPEC2.2MPL process spectrometer for at-line analysation. Measurements on reference samples with known MgO content show a high correlation of NIR-analysis results and XRF data.