6. SUMMARY AND DISCUSSIONS
6.1. Further discussion on the findings
6.1.1. Discussion based on PI
Figure 6.1. Lignin precipitation and centrifugal washing and recovery method.
Table 6.1. Comparison between the centrifugal washing and recovery method with filtration straight after precipitation.
Recovery Method
Lignin Conc.
in BL (g/L)
Recovered Lignin Conc.
(g/L)
Yield %
Filtration time
(h) Centrifuge 49.90 41.40 82.97 0.5 – 2
Filtration 46.90 93.99 > 7
At first, the centrifugal process seemed not viable due to the lignin yield obtained being lower than that obtained from the straightforward filtration: 83% and 93%, respectively. Two sources of error were identified during the centrifugal process investigation, viz.
- Sample losses during transfer between centrifuge tubes during the washing process.
- Organic matter that absorbs at the same wavelength as lignin when measuring the yield in UV-Vis.
For this investigation, it must be noted that the centrifuge that was utilised had a maximum speed of 4500 rpm, and only accommodated 4x15 mL centrifuge tubes, which then made the recovery process a bit tedious. Even though great care was taken, the major source of error for this investigation was due to sample losses during transfer at the washing stages.
However, the shorter recovery times achieved from the centrifugal method still compensated for the lower yield in terms of the power input into both processes.
Furthermore, there was an improvement in the physical quality of the lignin sample that was observed at the final filtration stage after the centrifugal washing. Comparing the lignin obtained by immediate filtration after precipitation, the centrifugal method lignin was easily filterable. This was due to a decrease of the colloidal nature of the lignin suspension during the washing stages.
The centrifugal process was optimised and up-scaled upon acquiring a bigger centrifuge, where the yields between the two processes were observed to be equivalent. At this stage, new batches of black liquor were obtained from the mill, viz. hardwood (HW) and softwood (SW) black liquor.
The optimisation studies included investigation of precipitation of lignin from both wood species separately, with sulphuric acid and three organic acids. Organic acids in this study and the studies reported in Papers II and III were introduced on the basis that they could serve as
‘greener’ alternatives to H2SO4, as well as be more effective than utilising CO2.
Precipitation of the lignin with organic acids (also sulphuric acid) first commenced with the optimisation and upscaling of the centrifugal method of recovery to increase the lignin yield observed in the work performed in Paper I. A similar precipitation, washing, and recovery method was followed in this study. Instead of the 100 mL of black liquor utilised above, 500 mL was utilised for precipitation. As reported in Paper I, there was a significant amount of sample that was lost and unaccounted for during the recovery process (~10%).
Therefore, careful measures were taken during the recovery process to ensure maximum sample transfer. From these studies, the lignin yield increased to values ranging from 90 to 96%, which was comparable with utilising filtration straight after precipitation (Table 6.2). This further confirmed that the centrifugal method of recovery was effective.
Table 6.2. Comparison between sulphuric and organic acid data obtained for precipitation of lignin from hardwood (HW) and softwood (SW) kraft mill black liquor
Precipitating Agent
Amount of acid utilised
(mL)
Lignin conc in BL
(g/L)
Total recovery
time (h)
Lignin yield %
Cost of acid per 500 mL
(ZAR)
HW SW HW SW HW/SW HW SW
SA 48.50 30.00
78.4 57.8
~ > 7 91.14 96.08 200
FA 114.00 62.00 ~2 – 2.5 96.67 86.47 277
CA 130.00 64.00 ~2.5 – 3.5 93.83 93.33 323 /500g
AA 70.00 45.50 ~2.5 – 3 91.14 97.16 264
It is important to consider whether the centrifugal process could be viable on pilot and industrial scales as the centrifugal process has not been previously used for lignin recovery. However, there are a number of other known industrial processes that utilise centrifugal recovery (discussed in section 2.5.1), and it is envisaged that similar methods can be applied to lignin recovery.
The colloidal suspension nature of the lignin solution that forms during precipitation also makes it a good candidate for centrifugal recovery. Centrifugation would ensure faster separation of the solid component (lignin) from the spent liquors, as shown in the process of this work. Three typical industrial centrifuges are predicted to have good applications during lignin recovery, viz.
decanter centrifuges, filter centrifuges, as well as hydro-cyclones.
For example, industrial decanter centrifuges have process capabilities that can allow 3–50%
(w/w) feed slurry, with a 40–99% (w/w) solid recovery. The maximum rotational speed that can be achieved is 3500 rpm, with power consumptions ranging from 25–450 KW. More importantly, the temperature range of operation is 69–180 °C, which will suit higher temperatures experienced in the mill. Generally, in a typical kraft pulping process, 2 t of biomass input results in 10 t of black liquor as well as 1 t of pulp. However, in a 1000 t per day pulp producing kraft mill, the recovery boiler can handle 1500–1700 t of black liquor to produce 25–35 MW of energy. Total energy production in a pulp mill is known to exceed the mill needs, whereby this excess is sold or utilised in other processes. Thus, the inclusion of the centrifuge such as a decanter centrifuge in the process would not have any additional energy expenses.
As mentioned, since the recovery boiler has limited capacity for black liquor, directing the black liquor towards precipitation of lignin would be viable in this case. The cost that would be incurred by integration of a centrifugal process into mill processes would be the capital for
setting up the process. An all-encompassing precipitation, washing and recovery unit is suggested below.
Further research from this study will look into modifications of a filter centrifuge for an all- encompassing reactor that could have the precipitation reactions, washing, and recovery all occurring in the same system. If incorporated in the pulp mill, this process would involve continuous pumping of the excess black liquor from the pulping unit into a reactor that would feed the precipitating agent for precipitation to commence. Stirring of the solution could occur either by stirrers and/or slow rotation of the centrifugal unit.
Upon completion of the precipitation reaction, a filter would be introduced, and the system would be spun down to separate the mother liquor from the solids. Washing of the lignin could also occur in the same system, where spray nozzles could be fitted in the centrifuge. On completion of washing, compressed air could be introduced to partially dry the lignin, or, the lignin can be removed. In-depth studies of such a system would look into whether the process could be made a batch or continuous. Moreover, if not used as a recovery unit, hydro-cyclones can also be used to concentrate the colloidal lignin suspension which can then be fed into a filter centrifuge for easy separation.
The mother liquor produced from the centrifugation unit would still contain dissolved inorganic material, as well as dissolved sugars (organics). This liquor can be redirected towards the evaporators followed by the recovery boiler to add to the organic material that is burnt, and recovery of the inorganic matter as cooking chemicals. The wash water in the centrifugal unit could be treated in the kraft mill’s waste water treatment plants, which are already in place.
In Chapter 3 FTIR and NMR spectroscopies were used to characterise the lignin sample, obtained by precipitation with sulphuric acid and recovered with the centrifugal method, in an attempt to identify the wood species that constituted the black liquor sample from the mill. A molecular weight distribution of the lignin sample was obtained with SEC. And finally, bearing in mind the ultimate objective of the overall study, that is, valorisation of the lignin, Py-GC/MS was utilised to identify typical compounds of value that can be obtained by pyrolysis of lignin.
FTIR analysis of the sample showed more syringyl type vibrational absorption bands than guaiacyl, indicating that the wood species from which the black liquor sample originated contained hardwood – as softwoods do not contain any syringyl groups. NMR showed the general lignin backbone mostly observed in the literature. However, there were chemical shifts
in the 1H spectrum that represented syringyl unit protons by comparison with spectra observed in the literature. The syringyl protons observed further corroborated the information provided by FTIR that there were more syringyl moieties in the lignin sample. From these observations, it was concluded that the unidentified black liquor sample that was obtained from the mill for this work, was a mixture of hardwood and softwood species.
SEC showed two peaks representing higher and lower molecular weight lignins. Dispersity of the sample was calculated to be Ð = 1.036 cm-3 g-1, which signified uniformity in the polymer, as well as the polymer falling in the narrow molecular weight distribution. Uniformity in the polymer and a narrow molecular weight distribution rendered the lignin in this study a good candidate for valorisation.
Py-GC/MS was utilised to predict the kind of products that could be obtained from lignin, with the main phenolic products obtained after 15-30 minutes. There were more guaiacol type compounds than syringol observed, with the former getting released before the latter. The observation of more guaiacol compounds was attributed to the probability that the syringol compounds were further broken down, resulting in the loss of the methoxy group. Identification of the pyrolysis products provided a scope for the valorisation of lignin into chemicals.