The 2013 TAPPI PEERS Conference in Green Bay, Wis., USA, September 15-18, focuses on advances and emerging developments in the kraft pulping and bleaching arena as well as chemical recovery. In the chemical recovery island specifically, Session 18 on Tuesday, September 17, from 1:30-3:00 p.m., explores some of the industry's latest developments and breakthroughs in the recovery cycle, focusing on operating floor applications at several pulp mills.
In the first of three papers in this session, Wei Ren and Honghi Tran of the University of Toronto look at the free lime content in lime mud. As the authors point out during this presentation, in the causticizing plant of kraft pulp mills, lime (CaO) is added to the slaker to convert green liquor into white liquor through slaking and causticizing reactions that precipitate lime mud (CaCO3). If the lime dosage exceeds the maximum amount required for the causticizing reaction to reach equilibrium, the precipitated lime mud will contain unreacted Ca(OH)2 or free lime. Such a causticizing condition is conventionally referred to as "overliming," which can cause many problems including poor mud settling, low mud solids content, high soda content, poor kiln thermal performance, high TRS emissions, and ring formation. To avoid overliming, many mills monitor and control their free lime content in lime mud below 2 wt% Ca(OH)2. The free lime content is typically determined in a mill laboratory using the conventional ammonium chloride (NH4Cl) method.
The authors explain that at their laboratory, the free lime contents of mud samples from several mills, with and without problems of low mud solids and/or high TRS emissions, were determined using a thermogravimetric analyzer (TGA). They all were found consistently to have Ca(OH)2 below 2 wt%. Why the values were low and did not change between different mills, and how could such low free lime content have a large impact on lime mud solids and TRS emissions were puzzling. A systematic study was therefore conducted both in the laboratory and at a mill site to examine the effect of overliming on the free lime content and the validity of the TGA and NH4Cl methods used for free lime determination. The results show that the free lime content of a given mud sample was different depending on whether or not the sample was washed before the analysis. For unwashed mud samples, increasing the liming ratio, as expected, resulted in a higher free lime content. For mud samples that had been washed with water, however, the free lime content was low and relatively constant even when they were prepared at a high liming ratio. The low free lime content of the washed mud was suspected to be caused by the continued causticizing reaction between the residual Na2CO3 and Ca(OH)2 and the dissolution of Ca(OH)2 during the washing process.
In pulp mills, the authors note, the free lime analysis is typically performed on mud samples collected from the precoat filter before the lime kiln. Since the samples have already been washed to some extent, the resulting low free lime value cannot be used to indicate the extent of overliming in the causticizing plant. On the other hand, if the analysis is performed on unwashed mud before the precoat filter using the standard NH4Cl method, the result is not correct either. In this case, the unwashed alkali (in the residual white liquor) in the mud will neutralize the NH4Cl reactions, resulting in low free Ca(OH)2 content. This study suggests that there is a need for developing a new method for analyzing the free lime content that truly indicates the degree of overliming, Ren and Tran conclude.
In the second paper of Session 18, Greg Ashton Busby of MeadWestvaco describes a continuous digester that experienced severe erosion-corrosion of the shell wall inside the internal flow collection headers of the extraction and MCC zones. Not surprisingly, the worst damage was located where high velocity liquor exits the screen orifices and enters the collection headers. With erosion-corrosion rates as high as 200 mpy, the damage had effectively reduced the wall thickness in half in the worst areas. Also affected were the horizontal backing rings that form the bottom of the flow channels.
Busby encourages owner-users to inspect these locations in their digesters to ensure that erosion-corrosion has not caused accelerated shell thinning. This, he says, is a location that is not readily accessible for internal visual inspection without removal of the flow collection header covers. External, on-stream UT at the proper collection header elevations is an effective way to detect this erosion-corrosion phenomena. Busby also discusses considerations to make on-stream UT thickness testing as accurate as possible.
In this session's third presentation, Jean-Noel Cloutier of Quebec Hydro examines "Electrolytic Treatment of Black Liquor with Caustic Recovery." According to Cloutier, the technical feasibility of electrolytic acidification of black liquor to precipitate lignin was demonstrated at Quebec Hydro's pilot plant. The electrolytic process was tested in the 4.5 – 7.0 pH range. The best energetic performance was obtained at pH 7.0 while converting 63% of the recovered black liquor sodium into a 10% caustic solution using 3 920 kWh/MT. At pH 4.5, the sodium recovery could reach 70% but at the expense of higher energy consumption partially due to the excessive formation of carbon dioxide. A cyclic caustic rinse of the anode compartment helps to remove the fine lignin particle deposits and restore the cell voltage.
The electrochemical option Cloutier proposes leading to lignin precipitation requires no acidifying agents and generates no additionnal effluent. Moreover, he says, it represents an advantage compared with the chemical approach using CO2 for kraft mills limited by the recausticizing system. .
More information on the 2013 PEERS Conference, and more information on the International Bioenergy and Bioproducts Conference being held September 18-20 in conjunction with PEERS are available online.
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