Measurements of the refractive index, its temperature dependence, and the optical transmission of black liquors produced during sulfate pulping are reported for soluble solid residue concentrations up to k 60%. The design features of a commercial refractometer for monitoring the concentration of black liquor are examined briefly. A procedure is proposed for laboratory calibration of commercial sensors that employs black liquor solutions in highly refractive organic liquids as reference samples.Introduction. Refractometric methods and instrumentation are used quite successfully for determining the concentration of soluble solids in black liquors produced during sulfate pulping [1][2][3][4][5]. After several stages of thermal evaporation, these alkalis are fired in coregeneration boilers at most plants at solid residue concentrations of up to k = 70-75% or above [6] with partial recovery of the cooking residue in the form of green lye. Commercial refractometric sensors can monitor the solid residue in the black liquors at any stage of the evaporation process if their optical properties are reliably known. Only limited amounts of the required data can be found in the available scientific and technical literature, and, at best, only at a qualitative level [7]. Thus, the purpose of this article is to study the optical properties of black liquors at levels corresponding to the specifications for refractometric technologies for solid residue monitoring.Experiments and Discussion. Ten reference samples of black liquor from the Segezhskii cellulose plant with concentrations up to k = 60% (as determined by weighing the dry residue, i.e., by gravimetry) were used for measuring the refractive index n of the test samples. The refractive index of the test solutions was measured with an Expert pro digital laboratory refractometer at a wavelength of λ = 589 nm for residue concentrations up to k 60%, as well as with an Abbe refractometer at λ = 633 nm for k up to 32%, where measuring the visually observable light-dark interface did not significantly reduce the accuracy of the measurements: the absolute error Δn in measuring n for the test samples with both instruments was within ±0.0005. The measured concentration dependence k(n) at t = 20

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We have studied diffuse reflectance in the spectral range λ = 380-760 nm of sulfate cellulose pulp with initial hardness G = 30-70 after it was treated with a chlorine-containing bleaching agent with active chlorine concentration C = 0%-10% for different time intervals. We determined the general behavior and basic features of the concentration and time dependences of the brightness B and the diffuse reflectance spectral ratio (R 437 /R 650 )⋅100% at λ = 457 nm and 650 nm. Based on the data obtained, we propose an optimal algorithm for using optical spectral technologies for metered addition of chlorine-containing bleaching agents. Introduction.Optical spectral technologies are of considerable interest for bleaching plants in the paper and pulp industry, even today using chlorine-containing bleaching agents such as chlorine, chlorine dioxide, and sodium hypochlorite [1]. Optical spectral sensors can rather effectively be used to monitor metered addition and consumption of chlorine-containing bleaching agents [2-5]. As we know, the diffuse reflection coefficient R of a dry pulp handsheet in the λ = 440-480 nm band of an optical filter, with transmission maximum at λ = 457 nm, is determined by the lignin content in the analyte sample [6]. When chlorinated, lignins are converted to soluble chlorolignins, which leads to a decrease in the diffuse reflection coefficient in the blue-violet region of the spectrum and gives the chlorinated cellulose an orange-red tinge [2][3][4][5]. The chlorolignins can be removed in subsequent washing, as a result of which the whiteness of the cellulose increases relative to its initial value. In connection with known environmental and occupational safety problems, optimization of consumption of chlorine-containing bleaching agents under production conditions remains an important issue.The Experiment. Algorithms for metered addition of a chlorine-containing bleaching agent that are used in known optical spectral sensors for paper and pulp plants [2][3][4][5] call for measurement of the brightness (B) at λ = 457 nm or the diffuse reflectance spectral ratio R 457 /R 650 at λ = 457 nm and λ = 650 nm, where reflection weakly depends on the consumption of the bleaching agent. These are essentially photometric sensors. Therefore their readings can depend on such influencing factors as the change in the concentration of the cellulose pulp to be bleached, "occlusion" of the entrance optical window of the sensor in the process stream, etc., or the need arises in routine recalibration of the sensor when the process parameters of the pulp to be bleached change.In this connection, in this work we have studied the effect of chlorination on the diffuse reflectance spectra of cellulose in the wavelength region λ = 380-760 nm with regard to optimization of the algorithm for metered addition of the bleaching agent. In the experiments, we used samples of sulfate (soda) cellulose pulp with initial hardness G (Bjo .. rkman permanganate number) within the range 30-70, the brightness in this case was B = 62%-...

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