Optical properties of black liquor and refractometric methods for monitoring the solid residue concentration in sulfate cellulose production

Abstract

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

Synchronous silicon removal and viscosity reduction in the soda-oxygen pulping of wheat straw

“…Recycling of BL during the cooking process could increase the solid contents in the BL. However, while these strategies can decrease the silica in the BL by more than 70%, there are still some silicon remained in the BL (Belov et al 2012;Mohan et al 2006). Moreover, alkali recovery in the BL is fairly challenging.…”

Pyrolysis Behavior of the Black Liquor Derived from Soda–Anthraquinone and Soda–Oxygen Pulping of Rice Straw at Different Reaction End Points

“…However, growing environmental policy pressure has led to a decrease of the proportion of the straw used for clean pulping in recent years so that only about 20 million tons of agricultural residuals are being used to produce paper products, and rice and wheat pulp account for only 4% currently. The inherent disadvantages, high content of ash and silicon accompanied with parenchyma cells, not only make the black liquor (BL) hard to extract and concentrate but also bring about the silicon disturbance in the recovery system . As a result, the BL obtained from straw pulping is hard to apply in the traditional recovery system, which becomes the bottleneck for development of straw pulping.…”

“…The inherent disadvantages, high content of ash and silicon accompanied with parenchyma cells, 9 not only make the black liquor (BL) hard to extract and concentrate but also bring about the silicon disturbance in the recovery system. 10 As a result, the BL obtained from straw pulping is hard to apply in the traditional recovery system, which becomes the bottleneck for development of straw pulping.…”

Effects of pH and Suspended Matter on the Physico-Chemical Properties of Black Liquor from Alkali-Oxygen Pulping of Rice Straw

“…For instance, rice straw black liquor (BL) has a higher viscosity and a higher concentration of silica, leading to unsmooth jetting of the BL into the recovery furnace at the threshold value of BL viscosity. Unlike the concentration of 65% or more for wood BL (Belov et al 2012) maintaining higher thermal capacity, the biggest obstacle is the extremely low solids concentration, commonly below 50%. Therefore, it is difficult to employ an alkali recovery process in the pulping of rice straw.…”