Scales and deposits in the pulp and paper industry

Scales and deposits in black liquor evaporation

Scaling in black liquor evaporators counts as one of the most notorious problems in the modern kraft pulp mills. The deposits commonly known as insoluble scales (mostly CaCO3) require a complicated removal procedure, usually acid or high-pressure washing. The soluble deposits are usually composed of double salts of Na-CO3-SO4 type or of sodium oxalate. Although easier to wash out, they may form in such large amounts that their sheer volume may effectively block the entire evaporation unit. In both cases, the scales may greatly reduce the heat transfer, increase the energy consumption and worsen the steam economy, leading eventually to expensive shutdowns. It is therefore important to understand how a deposit is formed and thus find a way to avoid it.

Evaporation experiments at RISE

RISE has a long experience in describing the solubility of salts in the evaporation plant of kraft pulp mills. A reactor, with a pressurized stirred 3.75 L vessel unit, makes it possible to study the solubility limits and precipitation behaviour of black liquor during laboratory evaporation. The experiments can be conducted using black liquor from our laboratory cook or from your own mill. Samples of the liquor and the equilibrium solid precipitate can be obtained at different dry solid levels and thoroughly analysed. Our experimental setting is especially suitable for investigating the precipitation of double salts of Na-CO3-SO4 type. Depending on the black liquor composition, this salt system produces solids with different composition, crystalline structure and scaling behaviour, the most known being burkeite and (sodium sulphate) dicarbonate. 

RISE has a good experience in modelling this type of solids. The evaporation experiments can be combined with the analysis of particle formation using FBRM® (Focused Beam Reflectance Measurement), which makes it possible to study nucleation and crystal growth. With the reactor, different process strategies, such as choosing an optimal position for the addition of ESP-dust from the recovery boiler or spent liquor from the tall oil plant can be investigated. The reactor is also suitable for other types of experiments, especially those involving controlled precipitation (e.g. of lignin) or high-temperature treatment, for instance upgrading of biosludge.

Scales and deposits in the fibre line

Precipitation of sparingly soluble salts, such as calcium oxalate, barium sulphate and calcium carbonate can severely reduce the availability of process equipment in the fibre line. The risk for the scale formation depends mostly on the total input of non-process elements, but also on the flow strategy and the degree of system closure. Recycling of process water reduces the water consumption but may also lead to an accumulation of non-process elements and thus increase the scaling risk. It is therefore very important to understand the technical conditions under which precipitation occurs.

Solubility expertise at RISE

A filtrate from the kraft pulp fibre line is a very special chemical system where metal ions interact with complex organic compounds. RISE has many years of experience in studying such systems, during which we have developed a chemical equilibrium model for the formation of sparingly soluble salts. This model can be used in connection with process simulation for estimating the risk for scaling in the bleach plant and testing various methods to minimize it. 

RISE can also conduct solubility experiments using filtrates from your mill in order to assess the risk for deposit formation in your process streams. The typical studies include the precipitation of the most common sparingly soluble salts found in the fibre line: calcium carbonate, calcium oxalate and barium sulphate, but even other compounds can be investigated, if necessary. Temperature, pH, ionic strength and COD levels are but a few of the parameters whose effect can be evaluated. The results can subsequently be compared with RISE database and implemented into a simulation model.

Analysis of unknown precipitates

MoRe is part of RISE and has extensive experience of helping mills with analyzes of, for example, unknown precipitates. Read more at www.more.se