General description of the LignoBoost process
The LignoBoost process separates lignin from pulping black liquor and is based on precipitation and separation by filtration. The LignoBoost process concept is today owned and commercilized by
Metso.
The black liquor is evaporated to about 30% and the soap is separated. Lignin is precipitated by lowering the pH to about 10 by injecting CO2. At this pH, small amounts of H2S are released, collected and recirculated to the kraft process. The main concept uses purchased CO2 in a pure form which is easy to inject into the black liquor. Since the cost of CO2 is high, the possibility of using CO2-rich flue gases from the lime kiln is an interesting opportunity.
Background
The traditional process for lignin precipitation and separation from kraft black liquors results in severe problems with complete or partial plugging of the filter cake and/or the filter media. The case of more or less complete plugging results in an extremely low flow of wash liquor through the cake (virtually zero). A partial plugging of the filter cakes, on the other hand, results in very high levels of impurities in the lignin.
These negative filtering effects are shown to be caused by changes in lignin solubility, caused by excessive pH and ionic strength gradients in the cake during the washing process (
Figure 1). These changes can be the restructuring of the particles, e.g. peptisation (returning to a colloidal state), or the dissolution of lignin. Tell-tale evidence that changes are actually occurring is a peak in lignin concentration in the spent wash water just after the breakthrough (
Figure 1). In this region, the pH is still high, while the ionic strength (measured in the form of sodium concentration in the liquor) has decreased to very low values compared to the initial concentration of the entrained liquor in the cake. A high pH combined with a low ionic strength can, according to the theory presented above, lead to the dissolution of the lignin.
Figure 1. The pH profile of the wash water (black line, experimental conditions: wash water pH 1.05, wash water temperature 20 ºC and precipitation pH 10). Sodium and lignin concentrations from another experiment performed at the same conditions are also shown (black dots and white circles respectively, right hand side axis). Figure from Öhman and Theliander (2006).
The LignoBoost method
If the problems encountered with lignin solubility previously mentioned are caused by large gradients in pH and ionic strength (see
Figure 1), it would be desirable to even out these profiles during washing. A process that will realize this is shown in
Figure 2, where lignin is precipitated by acidification and filtered, just as before. Instead of washing the lignin directly after filtration, however, the filter cake is re-dispersed once again. The new slurry is filtered and, finally, washed using displacement washing. If the filter cake is re-dispersed in liquor where the pH and temperature are controlled to approximately that of the final wash liquor, the gradients during the washing stage will be small. The pH change and most of the change in ionic strength, and thus the resulting change in lignin solubility, will then take place in the re-suspension stage instead of in the filter cake during washing.
Figure 2. The modified method, LignoBoost, for washing lignin precipitated from black liquor, with the “new” components marked within the box.
There are several factors that should be taken into account in order to render this process as effective and economically sound as possible. The efficiency of the first dewatering stage is very important, since a smaller amount of entrained black liquor in the filter cake leads to a lower consumption of acid for pH reduction. The dry solids content of the filter cake can be increased by applying mechanical pressure, followed by air-blowing. For the re-slurry liquor, recycling spent wash water to this stage is an attractive option that would reduce the amount of fresh liquid added to the process. The result is a product with low water content, low ash and sodium content which is possible to use as a bio fuel or raw material for production of chemicals. This means a possibility to replace mineral oil in several applications.
Production in pilot scale
A pilot scale filter press (filter area 1.7 m²) was tested in the Bäckhammar kraft mill in western Sweden during the fall of 2004. During the trial runs about 8 tonnes of lignin was produced. The lignin comes out of the LignoBoost process as a compact, powderish cake as shown in
Figure 3. Some of the lignin (200 kg) was then pelletized in a sawdust pelletizer (
Figure 4).
Figure 3. High quality kraft lignin, as produced in the mill trials in Bäckhammar 2004.
Figure 4. Pellets directly from the pieces of a kraft lignin cake according to Figure 3. The pellets have density of about 700 kg/m³.
Typical properties of the lignin from the trials in Bäckhmmar are given in
Table 1. The heat values for mineral oil and wood with a dry content of 50 % are 40 and 12 MJ per kg respectvely. It should be noted that the ash and sodium content of the lignin is very low making the lignin very attractive for use in high performance incinerators. The content of ash and sodium can be lowered significantly further if needed.
Table 1. Typical properties of lignin produced in the Bäckhammar mill trials 2004.
| Parameter |
Value |
| Dry solid content (before drying), % |
70 |
| Ash content, % on dry weight |
0.2 |
| Effective heat value (dry) |
25.4 MJ/kg |
| Effective heat value (30% moisture) |
17.1 MJ/kg |
| Element |
Elemental composition,
% on dry weight |
| - Carbon |
64.0 |
| - Oxygen |
26.4 |
| - Hydrogen |
5.7 |
| - Nitrogen |
0.1 |
| - Chloride |
0.005 |
| - Sodium |
0.03 |