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.

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.

RISE investigates scaling and precipitation problems

RISE supports pulp mills in investigating and mitigating problems related to scaling and precipitation through four complementary approaches:

Scale analysis
Analysing samples of existing scale deposits is often the first step in understanding a scaling problem. By identifying exactly what has precipitated, it is possible to form hypotheses about root causes and assess which mitigation measures are feasible. In addition to performing the analyses, RISE provides expert support in interpreting the results and advising on potential process measures at the mill.

Modelling
Process modifications typically alter the chemical composition of mill streams, potentially introducing or aggravating problems related to scaling and precipitation. At the same time, the chemistry is complex and difficult to predict. RISE has many years of experience studying such systems and has developed a chemical equilibrium model for the formation of sparingly soluble salts. This model can be applied already at the planning stage of a rebuild, using process simulation to assess precipitation risks and evaluate different mitigation strategies. We can also perform direct solubility simulations using OLI Studio software.

Solubility experiments
RISE can perform solubility experiments on specific process streams to assess precipitation risks under realistic conditions. A typical study includes precipitation of the most common sparingly soluble salts found in the fiber line, such as calcium carbonate, calcium oxalate, and barium sulfate, but other compounds can be investigated as needed. The effects of parameters such as temperature, pH, ionic strength, and COD levels can be evaluated. The results can then be compared with the RISE database and implemented in simulation models or used as supporting data in investigations.

Precipitation experiments
RISE has access to a unique, dedicated experimental setup for studying scaling and precipitation phenomena. The equipment enables studies both in laboratory environments and under full-scale industrial conditions.
Read more about this pilot facility.