Jimmy Pauck1 and Prof. Jeremy Marsh2

Organisations and addresses

1Dept. of Pulp and Paper Technology, Durban Institute of Technology, PO Box 953, Durban 4000, South Africa
2School of Pure and Applied Chemistry, University of Natal, Durban, South Africa



newsprint, deinking, sodium silicate, fatty acid soaps, calcium ions


The previous work by Pauck & Marsh (2002) suggested that sodium silicate is a poor disperser of hydrophobic ink particles when compared to traditional dispersants such as fatty acid soaps. On the other hand, there was some evidence to suggest that the presence of sodium silicate in a deinking system could lead to an increase in the concentration of sodium soap. It was postulated that this would result in better dispersion of ink particles and better removal of ink by flotation.

A model system consisting of the sodium salt of a collector soap, calcium ion and sodium silicate was studied by Pauck & Marsh (2002) under the conditions that prevail in a typical newsprint deinking pulper. It was found that the soap and the sodium silicate compete for the calcium ions, and increasing levels of sodium silicate lead to an increase in the concentration of sodium soap in solution. Sodium silicate thus had a measurable chelating effect on calcium ions.

This chelating effect was tested in laboratory deinking studies. Samples of newsprint were pulped in a 25l Lamort laboratory pulper using a range of pulper water compositions. The pulps were floated in a 20l flotation cell. The brightness and colour of the unfloated and floated pulps were measured. The level of the final brightness after flotation was taken as a measure of deinking efficiency.

The highest final brightness was achieved when there was an excess of soap in the pulper. The lowest final brightness occurred in the presence of an excess of calcium in the pulper. Calcium in the pulper in the presence of sodium silicate did not result in a significantly lower final brightness.

The results support the contention that sodium silicate sequesters the soluble calcium in a pulping system, thereby increasing the sodium soap concentration and the resultant deinking performance. Apart from sodium silicate's chief role as a peroxide stabiliser, the sequestering action on calcium is its main mechanism of action in a deinking system. An appreciation of this role will facilitate the optimisation of deinking systems with respect to calcium hardness, silicate and sodium soap addition levels and their optimum points of addition.


1.1 The Mondi Merebank deinking process
The recycled fibre plant at Mondi Paper's Merebank mill was built in 1990. The plant consists of equipment supplied mainly by LaMort and Voith. The plant recycles 85 000 tons per annum of waste paper and produces about 70 000 tons per annum of deinked newsprint pulp. This constitutes about 25% of the newsprint furnish of the two newsprint machines at the Merebank mill. The waste paper is collected and sorted off-site by Mondi Paper Waste and delivered to the mill, baled as "flat news" and "magazine".

A wide variety of printing inks can be encountered in a de-inking plant. In South Africa, all newspapers are printed by offset lithography, using cold-set or heat-set inks (Watson 2001).

The layout of the plant is illustrated in Figure 1 below. For a larger, clearer diagram, please click here.

Figure 1

Figure 1. Schematic of the Merebank deinking plant, showing stock and water flows

The process is essentially a single stage alkaline flotation process, followed by a washing stage, disperser and hydrosulphite bleaching.  A 70/30 blend of newsprint and magazine paper is fed into the pulper, together with 10.0 kg/ton (kg active chemical per ton of dry pulp) of sodium silicate, 4.8 kg/ton hydrogen peroxide, a trace of enzyme scavenger and enough caustic soda to bring the pH into the range 9.8 - 10.2. The waste is pulped at a consistency of 10-15% and at a temperature of 45 oC for about 15 minutes. After pulping, the stock is diluted to a consistency of approximately 7% before the heavy media separator, 4% before the HD cleaners and 3% before the coarse screens. After screening further dilution to about 1% takes place, and 4 kg/ton of collector soap and 2.0-3.5 kg/ton of calcium chloride are added just before the flotation cell. The target calcium hardness in the flotation cell is 250 ppm as calcium carbonate. The ink is floated off in a two-stage Voith flotation cell, at a pH of 8.2 - 8.5. Thereafter the pulp is again cleaned, screened, thickened and washed, before it is bleached at medium consistency with sodium hydrosulphite, to a brightness of over 62.

At the washing stage sulphuric acid is used to adjust the pH of the pulp to 6-6.5. The water recovered in the disc filter thickener (decker) is split into two streams, designated cloudy and clear. The cloudy water is heated to 50 oC  with steam and used for dilution in the pulper and the process steps up to the flotation cell. From the flotation cell forward the clear water is used for dilution. It has been found that the disc filter filtrate contains residual soap and calcium ions, which are circulated back into the pulper. A number of samples were taken, and the levels of calcium hardness and soap in solution are reported in Table 1 below. After the disc filter, white water from the paper machine is used for washing and dilution of the pulp.


Ca hardness, ppm Ca2+

Residual soap

1. Cloudy filtrate (3/2000)



2. Cloudy filtrate (4/2000)



3. Cloudy filtrate (4/2000)



Table 1  Calcium hardness and residual soap in the recirculated water, in ppm

1.2  The role of sodium silicate in newsprint deinking
The role that sodium silicate plays in a newsprint deinking system has been studied or reviewed by a number of workers (Ali et. al. 1994, Borchard 1995 & 1997, Ferguson 1991, 1992a & 1992b, Mahagaonkar et. al. 1996 & 1997, Mathur 1991, McCormick 1990 & 1991, Pauck & Marsh 2002, Read 1986 & 1991, Renders et. al. 1996, Santos et. al. 1996). It is apparent that sodium silicate's role seems to be multi-faceted, and can be summarised as follows: sodium silicate's main function is to stabilise the hydrogen peroxide used to bleach the pulp in the pulper. In addition sodium silicate has buffering and saponification properties (Ferguson 1991 & 1992a). Sodium silicate has been reported to assist in the dispersion of the ink particles and influence their size (Ali et. al. 1991, Mahagaonkar et. al. 1997, Read 1986). Also, it appears to act as an ink collector (Santos et. al. 1996), and it reduces fibre losses and suppresses the flotation of fillers (Turvey 1990, Liphard et. al. 1991, Mathur 1994). However, a number of authors refute these findings and claim that sodium silicate has no influence on the final brightness (Zabala & McCool 1988, Mathur 1994, Mak & Stevens 1993. The Merebank mill's own experience has shown that the sodium silicate plays a vital role in the deinking process. The elimination of sodium silicate from the process resulted in an immediate deterioration in deinking performance (Crosby 1999).

An attempt was made to understand the multiple roles of sodium silicate in a deinking system by investigating the dispersing power of sodium silicate for a typical newsprint ink, in the presence of the other main non-fibrous components of a newsprint deinking system viz: calcium ions and fatty acid soap. This work was performed using model ink systems. It was found that sodium silicate had a poor dispersing action for a typical hydrophobic newsprint ink, but the effects of calcium on ink particle size and morphology can be considerably modified by the presence of sodium silicate. This was demonstrated to be due to the chelation of the calcium ion by sodium silicate, which resulted in increased levels of soap in solution. It was postulated that this would lead to increased dispersion of ink particles and thus better ink removal from the fibre surface during pulping. This would have as a consequence better flotation performance. This lead to the hypothesis that the chelating effect that sodium silicate has on calcium has an indirect but significant effect on the performance of a deinking system. (Pauck & Marsh 2002, Pauck 2002).

1.3  Objectives
A series of experiments were devised to study the effect of various water qualities on the pulping efficiency in a newsprint deinking system. The experimental conditions were chosen to approximate the process conditions of the recycling plant at Mondi Ltd.'s Merebank mill, as described in Section 1.1 above. It was intended to verify the postulated effect of sodium silicate on a newsprint deinking system, in the presence and absence of calcium ions and fatty acid soap, as discussed in Section 1.2 above. In addition, it was intended to make recommendations to optimise the Merebank recycled fibre plant process with respect to the addition of pulping chemicals.


2.1 Experimental conditions
In order to determine the effect of different calcium and soap levels in the pulper on deinking performance, a number of experiments were devised. These are detailed in Table 2 below. The concentrations of calcium and soap used in these experiments were solution concentrations, and not based on the amount of pulp. This was done to relate the experiments to the concentrations of calcium and soap in the disc filter filtrate water, as detailed in Table 1 above. In the deinking experiments, potable or tap water was used in all cases. The local potable water is very soft, with an average calcium concentration of ca. 7 mg/l and a magnesium concentration of ca. 3 mg/l. (Umgeni Water Report 2000). Thus, the hardness introduced by the potable water can be regarded as negligible. Although the Mondi Merebank deinking plant recycles a blend of newsprint and magazine, only newsprint was used in this series of experiments. This was done to simplify the experimental conditions and remove some possible interfering factors that could be introduced by the magazine paper, such as fillers like clay or calcium carbonate, different inks and coating latexes.


Ca and soap addition to pulper water (see Sect. 2.2(3) below)



Water + 100 ppm calcium + 200 ppm sodium soap

This simulates normal pulper water. Refer to Table 1.



To simulate the use of ''fresh'' process water i.e. to not recycle the disc filter filtrate.


Water + 200 ppm soap

To simulate the addition of extra soap in the pulper.


Water + 100 ppm calcium

To simulate pulping in a high hardness environment.


Water + 100 ppm calcium, no sodium silicate

To simulate pulping in a high hardness environment in the absence of sodium silicate.

Table 2: Experimental conditions for pulping concentration in parts per million parts of water

A standard laboratory method for pulping and flotation has been developed and used at the Mondi Merebank mill for flotation investigations (Addis & Kathi 1992). This laboratory method produces comparable results to that produced by the recycling plant, although slightly different amounts of chemical reagents are used. These methods are detailed below. The experiments in each series were repeated at least five times in order to obtain statistically significant results.

2.2 Laboratory pulping method
Based on the work of Addis & Kathi (1992), the following laboratory pulping method was used.

1. Newspaper samples were drawn from the deinking plant warehouse at regular intervals. Due to practical constraints, it was not possible to use exactly the same types of newspaper for each pulping and flotation experiment. Nevertheless, no newsprint older than 3 months was used. It is well known that ageing deteriorates the newsprint and adversely affects the deinking result (Haynes 2000). The samples generally contained editions of some or all of the following South African newspapers: The Cape Times, Die Burger, The Daily News, The Tribune, Ilanga, Rapport, The Citizen, The Sowetan and The Cape Argus. This inconsistent sample produced some variability in the results, but represents a good cross section of the papers and printing inks used in South Africa. The conclusions drawn from this work would thus be applicable to the local industry.

2. A LaMort 25 litre laboratory pulper was used to pulp the newsprint. This pulper has a working volume of 15 litres and contains a helical rotor.

3. The following chemicals were added in order into the pulper:

    Potable water at 55 oC  13.5 litres

    Caustic soda lye 45% 36g  (Equivalent to 14 kg/ton dry pulp)

    Sodium silicate 38%(Na20:SiO2 3.3:1)  34g (Equivalent to 10kg/ton dry pulp)

    Enzyme scavenger 0.5 ml                    

In addition to these chemicals, calcium (as calcium chloride) and sodium soap were added as outlined in Table 2 above. In experiment 5, the sodium silicate was omitted. With the exception of sodium silicate, the commercial chemicals in use in the Merebank deinking plant were used in these experiments. No chelating agent has been in use in the deinking plant for many years, and so none was added in the laboratory work. However, an enzyme scavenger is being used in the plant, and was added in the laboratory work. The enzyme scavenger is used to deactivate catalase enzymes, which can rapidly decompose hydrogen peroxide (Sundblad & Mattila 2001). A water temperature of 55oC ensured a final pulping temperature of 50oC.

4. The chemicals were mixed briefly. Thereafter 1.4 kg of air-dry newsprint (ca. 10% moisture) was torn into strips and introduced into the pulper. Unprinted edges from each newspaper used were torn off and retained. The brightness of the unprinted edges was measured according to Tappi T452 om-92: Brightness of pulp, paper and paperboard (Directional reflectance at 457 nm.), to determine the unprinted, or edge brightness. Newspapers were sampled at random from the deinking plant warehouse. It was not possible to use exactly the same type of newspaper for each experiment, but no newspapers older than three months were used. The paper, water and chemicals were mixed until the paper had started to disintegrate (about 30 seconds) and then 22 g of hydrogen peroxide 30% (equivalent to ca. 5 kg/ton dry pulp) was added and the paper was pulped for 15 minutes at 50oC.

5. A sample was drawn and the temperature, consistency, pH, hardness and residual peroxide were determined according to standard Mondi Merebank methods. The addition of hydrogen peroxide was such that a small residual peroxide concentration at least 0.35 kg/ton dry pulp was maintained, to ensure complete bleaching.

6. Handsheets were made on a Standard British Sheet Machine according to Tappi method T205 om-88: Forming handsheets for physical tests of pulp. A minimum of six handsheets were made, and the brightness and colour were determined on an Elrepho 3000 reflectometer , according to Tappi T452: Brightness of pulp, paper and paperboard (Directional reflectance at 457 nm.). The average brightness of the six handsheets is reported.

2.3 Laboratory flotation method
Immediately after the paper had been pulped and tested, the flotation was commenced, according to the method below. A Denver type laboratory flotation cell with a working volume of 20 litres was used.

1. The equivalent of 0.200 kg of dry pulp (calculated as 0.2/consistency x 100) was weighed out for flotation.

2. The pulp and most of the flotation water were mixed in the flotation cell and a 250 ml. sample was taken for a hardness test. The hardness was determined by EDTA titration against a Calcon indicator.

3. Collector soap was pre-mixed with some hot water and added at a rate of 7.2 g/kg of dry pulp. The current addition rate in the Merebank deinking plant was 4 kg/ton dry pulp, but it was necessary to add more soap in the laboratory flotation cell to achieve a good head of foam and comparable flotation results to the plant (Addis & Kathi 1992). In experiments 1 and 3  (see Table 2), where 200 ppm of soap was added into the pulper, the amount of soap added at the flotation stage was decreased accordingly, so that the total soap addition of 7.2 g/kg dry pulp was maintained.

4. Based on the result of the hardness test, calcium chloride was added to achieve a hardness of 250 ppm as CaCO3, according to the following calculation:   

ml of calcium chloride = (250 ppm calcium hardness) x 18
                                 Calcium chloride conc (mg/ml)

250 ppm of calcium hardness as CaCO3 corresponds to 100 ppm of calcium hardness as Ca.

5. The flotation cell was topped up with water. The temperature of the water was such that the final flotation temperature was 40oC.

6. Air was introduced into the flotation cell, and the pulp was floated for 20 minutes. The ink and foam were scooped off manually. The flotation period of 20 minutes was excessively long, but the objective was to achieve ''infinite'' or ''hyperflotation'' (McCool 1993), to ensure complete removal of all ink by flotation. The objective of this experiment was to determine the effect of various pulping conditions on deinking efficiency. Incomplete flotation would interfere with the interpretation of the results. Addis & Kathi (1992) used a flotation time of 15 minutes, and some preliminary flotation work (Figure 2) indicated that a flotation time of only 10 minutes would have been enough to remove virtually all the ink. 

Figure 2

Figure 2: Final brightness as a function of flotation time

7. After flotation, a 10-litre sample of floated pulp was taken and tested for temperature, pH and hardness. Six handsheets were made up according to Tappi T205 and the brightness and colour were determined according to Tappi T452 on an Elrepho 3000 spectrometer.  The average brightness of the six handsheets is reported.

8. For each pulper batch, the flotation was done in duplicate. The average of the duplicate results was taken.

2.4 Assessment of deinking efficiency
This experimental work was intended to investigate the effect of various pulping conditions on the overall deinking result. Consequently, all the detached ink was removed by hyperflotation (as discussed in Section 2.3(6) above), and the final brightness was measured, in order to determine only the unfloatable ink that was still adhering to the fibres.

There are a number of techniques to determine the amount of ink in a pulp: brightness measurement, colour measurement, image analysis and ERIK measurements. Of these, only the brightness and colour measurement techniques are regularly used at the Merebank mill, and thus were available to measure deinking efficiency. 

It was found that the top side of the handsheets generally yielded a higher brightness, by about 2 points for the floated pulps and 3-4 points for the unfloated pulps. The higher brightness is a result of ink washout during the formation of the sheet in the sheet maker. The method of preparation of the handsheet prior to the brightness measurement, and the side that is measured, have a large influence on the final result (Dorris 1999a & 1999b). Dorris (1999a & 1999b) found that handsheets formed on a Standard British Sheet Machine according to Tappi T205 showed the greatest loss of ink by washout. This normally distorts the results in an assessment of deinking efficiency, but in this investigation it was desirable to measure only the non-removable ink. Hence the method employed to make the handsheets is appropriate to the final objectives of this work.


3.1 Brightness measurements
The flotation results are summarised pictorially in Figures 3 to 5 below. As a result of the considerations in Section 2.4 above, only the topside sheet brightness was used in the deinking assessment. Figure 3 depicts the results of the brightness measurements in the course of the pulping and flotation experiments.

Figure 3

Figure 3 Brightness results in deinking. The error bars indicate the standard error

The edge brightness, or brightness of the unprinted paper, represents the starting point for each series of experiments. An ANOVA test applied to the edge brightness demonstrated that there was no significant difference between the series at the 95% level of confidence.

The significance of the brightness before flotation, or after pulping, is difficult to interpret, but has been related to the amount of ink fragmentation in the pulper. The more the ink fragments, the finer the particle size and the lower the brightness. If the ink fragmentation is too great, the efficiency of flotation could be affected (Pirttinen & Stenius 2000). In this series of experiments, the presence of calcium in the pulper (experiments 1, 4 and 5) seems to coincide with lower pulper brightness and by implication greater ink fragmentation.

The efficiency of deinking is often assessed by quoting the brightness increase achieved in flotation. Whilst this method might be valid for comparison of flotation results where the same pulp is floated, in this work this can be misleading. Therefore, the final brightness after flotation was taken as the measure of deinking efficiency.

There was no significant difference between the final brightness of the normal pulping conditions (experiment 1), pulping with fresh water (experiment 2) and pulping with hard water (experiment 4). However, when pulping was carried out in the presence of an excess of free and available fatty acid soap (experiment 3), a statistically significant (at 95% confidence) improvement in final brightness was achieved.

3.2 L* measurements
The greyness or lightness of a piece of paper is measured by the L* value. In this work the L* value was found to correlate closely with the brightness. This correlation for the initial edge brightness and the final floated brightness is shown in Figure 4. Because of this close correlation, the L* values for the pulping and flotation experiments were not analysed further.

Figure 4

Figure 4 - Correlation between brightness and L* (Lightness)

The difference between the two lines is due to the yellowing of the fibres during pulping and flotation, which is discussed below.

3.3 The yellowness, or b* value
Figure 5 depicts an analysis of the b* value, or yellowness of the fibre before and after pulping and flotation.

Figure 5

Figure 5 - Changes in yellowness during pulping and flotation. The error bars indicate the standard error

It is apparent that the greatest yellowing has occurred in experiments 2 and 3, and the least in experiment 5. The yellowing is a result of exposure of the fibres to alkaline conditions, and is the reason for the inclusion of hydrogen peroxide in the deinking pulper. Yellowing also results in a decrease in brightness, and offsets the brightness gains made by removing the ink from the paper. Notwithstanding the yellowing in experiment 5.3, the highest brightness was also achieved in this experiment, which suggests that the ink removal was even greater than indicated by the final brightness alone.


The highest final brightness of 53.1% was achieved in experiment 3, in which an excess of soap was present in the pulper. The lowest final brightness of 48.8% was attained in experiment 5, which contained no silicate and an excess of hardness in the pulper. The presence of calcium in the pulper, as long as sodium silicate was present, did not negatively affect the final brightness. This can be seen in the brightness' of 51.5%, 51.4% and 51.5% obtained in experiments 1, 2 and 4 respectively. This can be understood by the fact that the calcium is being sequestered by the silicate, rendering it inert in the form of calcium silicate. However, once the sodium silicate is removed (experiment 5), a lower brightness (48.8%) is achieved. It could be argued that the absence of sodium silicate from the pulper in experiment 5 negatively affected the bleaching and resulted in a lower brightness. However, the yellowing results (see Figure 5) suggest that the bleaching was not impaired by the absence of silicate. The b* value obtained in experiment 5 was 7.17, as opposed to a b* value of 7.96 for experiment 3. Another factor in support of this is that an excess of hydrogen peroxide after bleaching was maintained.  This suggests that, as long as sodium silicate is present in a pulper, the calcium will be sequestered and the soap would be ''protected'', enabling it to launder the ink off the fibre. In the absence of silicate, the calcium precipitates the soap and renders it ineffective.

From the point of view of optimising a deinking process, the above results would suggest that the use of fresh water alone in the repulping stage would not be enough to achieve a significant improvement in deinking. However, if a portion of the collector soap is added into the pulper, a significant improvement in deinking performance could be gained.


The study of the interaction of sodium silicate, soap and calcium ions under various conditions of concentration and pH has suggested that it is undesirable to have calcium ions present during pulping. This is because any calcium present will precipitate free soap in solution as well as react with sodium silicate to affect its ability to act as a peroxide bleach stabiliser.

Laboratory scale pulping and flotation experiments failed to show a significant improvement when soft water rather than recirculated water was used for pulping. This should, however, be tested on a plant scale, as it may be possible to marginally reduce the amount of sodium silicate used without adversely affecting pulp brightness. It will still be necessary to add calcium chloride before flotation because it is needed to convert all soap present from the sodium soap to the calcium soap, which is the required ink collector.

Laboratory pulping and flotation experiments have conclusively shown that a significantly improved brightness can be achieved when fresh water instead of recirculated water is used for pulping, with some soap added. It is recommended that a plant trial be carried out to confirm this and to ascertain the optimum dosage of soap and sodium silicate required. The level of soap added during pulping should be compensated by an equivalent reduction in the amount of soap added before flotation, to avoid increased reagent costs.


I would like to thank the management of Mondi Ltd. Merebank mill for their financial support for this work.


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