Crystallization process instrumentation continuous crystallizers

Chemically pure reagents were used. Cadmium was added as its sulfate salt in concentrations of about 50 ppm. Lanthanides were added as nitrates. For the experiments with other metal ions so-called "black acid from a Nissan-H process was used. In this acid a large number of metal ions were present. To achieve calcium sulfate precipitation two solutions, one consisting of calcium phosphate in phosphoric acid and the other of a phosphoric acid/sulfuric acid mixture, were fed simultaneously in the 1 liter MSMPR crystallizer. The power input by the turbine stirrer was 1 kW/m. The solid content was about 10%. Each experiment was conducted for at least 8 residence times to obtain a steady state. During the experiments lic iid and solid samples were taken for analysis by ICP (Inductively Coupled Plasma spectrometry, based on atomic emission) and/or INAA (Instrumental Neutron Activation Analysis). The solid samples were washed with saturated gypsum solution (3x) and with acetone (3x), and subsequently dried at 30 C. The details of the continuous crystallization experiments are given in ref. [5]. 

It should be noted that development of the crystallization processes in most of the examples presented in later chapters occurred before the availability of many of the online measurement and control methods that are now available. Utilization of these methods would have aided both the process development and the manufacturing operations. The literature that describes these methods—for example, feedback control of supersaturation for crystallization (Nonoyama et al. 2006 Zhou et al. 2006)—is now extensive, and the instrumentation to carry out the measurements and control continues to be improved. 

Direct geometry instruments use choppers or crystal monochromators to fix the incident energy and they are found on both continuous and pulsed sources. To compensate for the low incident flux resulting from the monochromation process, direct geometry instruments have a large detector area. This makes the instruments expensive, they are generally twice the price of a crystal analyser instrument. At present, they are used infrequently for the study of hydrogenous materials, so we will limit our discussion to a chopper spectrometer at a pulsed source and a crystal monochromator at a continuous source. 

When the nature and composition of the sample is not well known, it is necessary to use influence correction methods, of which there are three primary types fundamental, derived, and regression. In the fundamental approach, the intensity of fluorescence can be calculated for each element in a standard sample from variables such as the source spectrum, the fundamental eiiuations for absorption and fluorescence, matrix effects the crystal reflectivity (in a WDXRF instrument), instrument aperture, the detector efficiency, and so forth. The XRF spectrum of the standard is measured, and in an iterative process the instrument variables are refined and combined with the fundamental variables to obtain a calibration function for the analysis. Then the spectrum for an unknown sample is measured, and the iterative process is repeated using initial estimates of the concentrations of the analytes. Iteration continues until the calculated spectrum matches the unknown spectrum according to appropriate statistical criteria. This method gives good results with accuracies on the order of I %-4% but is generally considered to be less accurate than derived... 

Fluorescence spectrometers are widely used in the metal industry. Frequently, parallel spectrometers are employed. Such an instrument actually consists of a number of crystal spectrometers, each set for a particular emission line. The spectrometers are arranged around the sample, which is irradiated by an X-ray tube. One of the spectrometers is set for a standard sample that is contained in the sample holder. In this way the intensity of the X-ray tube can be monitored. Frequently, a measurement is terminated when a preset number of comits for the reference sample has been obtained. The corresponding number of counts from the other detectors can then be directly used for a pai allel assessment of the elemental composition of the sample. With a sequential spectrometer, a number of selected elements are measured sequentially by turning the crystal and the detector to preset positions. With computer steering the measurement process is automatic. This type of instrument is well suited for varying types of analysis, whereas parallel spectrometers are more suited to continuous control operation of, for example, a steel mill in near-real time. 

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