Written by Roxare on May 13th, 2007
Barium sulphate is another problematic scale in the bleach plant. Barium enters the mill with wood/bark (especially hardwood) and water. Barium scaling depends on the kappa number, the pH and the concentration of Ba and SO4 in the bleaching stage. The problem of BaSO4 scaling has been observed when switching from chlorine to ECF bleaching. In the D0 stage, pH is controlled by the addition of H2SO4 or generator waste acid, thereby increasing the concentration of SO4. Scaling normally takes place in acid stage washers. Some BaSO4 scales contain radium sulphate; radium is a radioactive element which can be found in wood and ground water.
The solution to inorganic scaling is to reduce its source; i.e., have a wood supply low in metals (good debarking), control pH in the D0 stage, have good white liquor clarification, use good quality chemical make-up, use chelant or scale inhibitors and use treated water for showers. If scales have formed, the required removal measures are hydroblasting or chemical boil-outs.
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Posted in Bleach plant scaling | No Responses »
Written by Roxare on April 29th, 2007
In a kraft mill, large amounts of oxalate ions [C2O4]-2, are formed in oxygen delignification, as well as in the D, Z and, to a lesser extent, in the E and P stages in bleaching. In the bleach plant, CaC2O4 scales can form in the acid stage. CaC2O4 solubility depends on the concentration of the Ca, and the pH range, and it increases with temperature. Upset conditions, such as high lime mud carry-over with white liquor, can cause CaC2O4 scaling in the bleach plant. While CaC2O4 scaling was not a problem in chlorine bleaching at low pH, chlorine dioxide stages with a higher pH have encountered scaling problems. Oxalate scaling is also associated with the closure of the bleach plant, and has been reported in various bleaching stages, including the Q stage.
CaC2O4 is not expected to precipitate in the black liquor evaporators, since CaCO3 has a lower solubility in the evaporator environment and will precipitate instead. However, in a closed mill, where bleach plant filtrates are recycled to the recovery cycle, the possibility of Na2C2O4 scaling exists. Solubility data are available for Na2C2O4 and show that the solubility in black liquor depends on the temperature and the total Na concentration*. The recycle of bleaching effluent could double the amount of oxalate in the black liquor. Under normal operating conditions, when the dry solids content of black liquor are between 50-55%, burkeite (2Na2SO4.Na2CO3) starts to precipitate, reducing the concentration of Na and preventing the formation of sodium oxalate. However, when the amount of oxalate is increased due to the recycle of bleaching filtrates, the risk of Na2C2O4 scaling is increased. Since the solubility of Na2C2O4 increases with temperature, crystallization is expected to take place in the bulk solution and not on heat transfer surfaces. This could cause plugging problems in the evaporator tubes.
* Deposition of sodium oxalate in black liquor evaporation, P. Ulmgren and R. Radestrom, 2001, Int. Chem. Rec. Conf.
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Posted in Bleach plant scaling, Evaporator scaling | No Responses »
Written by Roxare on April 14th, 2007
Calcium contributes to scaling in various operations in a pulp and paper mill. CaCO3 is water insoluble, and has inverse solubility with temperature. Consequently, it forms on heat transfer surfaces such as digester heaters and black liquor evaporators. CaCO3 scales are also seasonal, and not easy to deal with. Calcium enters the mill through wood/ bark, chemical make-up and water. Depending on the type of wood, the method of pulping used and the efficiency of clarification for the white liquor, the calcium concentration can be high in the digester. Scaling is initiated when the concentration of free Ca and free carbonate exceed the solubility limit, to form calcite (CaCO3). In green liquor processing, where large amounts of Na2CO3 are present, pirssonite (Na2CO3.CaCO3.2H2O) forms.
Calcium can form water soluble compounds with components of black liquor. Many of these compounds are not thermally stable and break-up at high temperatures, resulting in supersaturation of CaCO3 in black liquor, which leads to scaling. The scaling tendency is high in the temperature range 110-140 C, where most evaporators operate. There is a patented method for calcium deactivation, in which black liquor is heated and stored at high temperatures to break up the calcium compounds and reduce scaling tendency
Calcium is purged with tall oil soap, the dregs, the grits and the pulp in the recovery cycle. CaCO3 scaling can also occur in bleaching in stages employing high pH. In the bleach plant, the acid sewer is rich in calcium and other metals. If acidic effluents are recycled to the recovery cycle, a metals removal system is required.
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Posted in Bleach plant scaling, Evaporator scaling | No Responses »
Written by Roxare on April 1st, 2007
Another area in a kraft mill which suffers from solubility problems is the evaporation of black liquor. Evaporator scaling problems are mostly due to the precipitation of sodium salts. The two important components are sodium sulphate and sodium carbonate; they precipitate together to form a range of double salts, including burkeite (2Na2SO4.Na2CO3). Sodium carbonate is another component of the scales formed. The solubility limit depends on the solids content, the total sodium content, the ratio of carbonate to sulphate, as well as the soluble organics and the operating temperature. Sodium carbonate content is influenced by the efficiency of causticizing, and the sodium sulphate content depends on the reduction efficiency in the recovery boiler and the sodium/sulphur make up. Other components such as Ca, Al, Si, fibre and soap also contribute to the fouling problem. For example, if the soap removal efficiency is poor, Ca scaling can be a problem.
Exessive scaling and frequent cleaning reduce evaporator capacity, and place a limit on the recovery boiler capacity. To prevent scaling, it is important to achieve a steady operation, to make sure that the evaporators always operate below the solubility limit of burkeite. To prevent precipitation on heat transfer surfaces, homogenous crystal nucleation in the bulk liquor is required.
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Posted in Evaporator scaling | No Responses »
Written by Roxare on March 17th, 2007
As mills move towards system closure and reduce water consumption and purge points, the levels of non-process elements increase, causing problems in the operation of the recovery cycle. Two such elements are aluminum (Al) and silicon (Si). These elements enter the mill mostly from contaminated wood, and from chemical make-up, and also from non-wood fibre sources. Al and Si form a range of aluminosilicate compounds such as sodalite, cancrinite, natrolite and hydrotalcite (in the presence of magnesium) in the alkaline pulping liquor, with their solubility being dependant on the hydroxide concentration, the ionic strength of the solution, temperature, the concentration of other species present and the initial quantities of Al and Si*.
Al and Si are most soluble in white liquor, and least soluble in black liquor. At constant temperature, their solubility increases as black liquor is concentrated. In mills where aluminosilicate deposits are a problem in black liquor evaporators, they are usually present as tenacious glassy scales which have to be removed by hydroblasting. Cancrinite limits the solubility of Al and Si in green liquor, and possibly in black liquor as well. To prevent aluminosilicate deposit formation, the input (i.e., from chemical make-up and wood) has to be low in these elements. The use of high purity chemical make-up may have to be considered when the input of Al and Si exceeds the solubility limit in black liquor. The normal purge points are the grits and the dregs.
* Wannenmacher, P.N., et al., Application of solubility data to predicting the accumulation of aluminum and silicon in alkaline pulp mills, pp. 303-308, 1996, TAPPI Minimum Effluent Mills Symposium.
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Posted in Scaling in alkaline liquors | No Responses »
Written by Roxare on March 3rd, 2007
Pirssonite, a double salt of sodium and calcium carbonate (Na2CO3.CaCO3.2H2O) often deposits in smelt dissolving tanks, green liquor lines, clarifiers and storage tanks; the scale has a stratified appearance. The scaling tendency depends on the concentration of total titratable alkali (TTA) in the green liquor, the liquor temperature, the calcium concentration, and the efficiency of mixing in the dissolving tank. Pirssonite forms at temperatures above 35 C in solutions containing high concentrations of Na2CO3, other sodium salts and calcium (i.e., CaCO3). The solubility of pirssonite decreases as the NaOH concentration increases. Other sodium salts have the same effect as NaOH. The solubility also decreases as green liquor cools. The change in Na2CO3 solubility is about 0.5 lb/ft3 (8 g/L) as Na2O per 10 C change in temperature*.
Pirssonite solubility curves (Na2CO3 concentration versus other Na salts concentration) are available at different temperatures. These curves can be used to determine the maximum TTA level at which the pirssonite will precipitate in green liquor (TTA sat). This level will vary from mill to mill because of differences in the composition of the green liquors. To find out the TTA sat, the mill operating line has to be defined. The intersection of this line with the solubility curves determines the upper limit for green liquor concentration. It is recommended that the operating TTA max should be 5-10% below TTA sat.
* Frederick, Jr., W.J. et al, “Pirssonite Deposits in Green Liquor Processing”, Tappi J., 73(2): 135-140, 1990.
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Posted in Causticizing, Scaling in alkaline liquors | No Responses »
Written by Roxare on February 17th, 2007
The biorefinery has been the subject of much discussion in the past few years. Biorefinery technologies are divided into biochemical and thermochemical processes. In the biochemical route, wood chips are pre-treated to extract acetic acid and hemicellulose. The separated hemicellulose is then fermented to ethanol and/or converted into specialty products. Identifying products that can be economically produced is a critical step in the biorefinery process. The thermochemical process is based on gasification/pyrolysis of the spent liquor. There are a small number of mills in the USA and Canada that have biorefinery projects (biomass or black liquor gasification). A sulphite mill in Canada has been converting its spent cooking liquor (by fermentation of sugars) into ethanol and specialty chemicals for some years. The mill has also recently started anaerobic biogas production.
In this competitive world, where pulp and paper can be produced much cheaper elsewhere, the biorefinery could provide a much needed edge for the North American industry. However, the biorefinery solution requires careful study of technical and economic issues to have a positive outcome.
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Posted in Biorefinery | No Responses »
Written by Roxare on January 28th, 2007
I’d like to start with an environmental theme.
System closure in pulp and paper mills has been practiced in varying degrees over the past two decades. System closure means a reduction in the mill water use and the recycle of mill effluents. In a bleached kraft mill, the closure generally implies the partial or full recycle of the bleaching effluents. However, several steps have to be taken to prepare the mill for closure. Complete water, energy and process/non-process element balances are required to establish a base-case scenario. Process modelling can do this, and predict the effect of process changes before a mill trial is carried out. In preparing the mill for the recycle of bleaching effluent, water reduction and process optimization can provide tangible benefits in terms of water treatment cost, and fiber, energy and chemical savings even before the recycle is attempted.
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Posted in System Closure | No Responses »
