The role of sodium silicate in newsprint deinking.
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Date
2003
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Abstract
Mondi Ltd. operates a deinking plant at its Merebank mill. The plant recycles 85 000
tons per annum of flat news and magazine to produce a furnish for its newsprint
papermachines. A review of the relevant pulping and flotation chemistry literature
revealed that the role played by sodium silicate appears to be multi-faceted and in
some dispute. Sodium silicate has an undisputed role in pH buffering, hydrogen
peroxide stabilisation and the prevention of fibre yellowing. However, its role in
deinking is said to be that of an ink collector or alternatively an ink dispersant. The
mill's own experience has shown that the sodium silicate plays a vital role in the
deinking process.
Sodium silicate's ability to disperse ink, both alone and in the presence of calcium ions
and fatty acid soaps, was investigated using a model ink system. A representative
newsprint ink base was dispersed in the laboratory under conditions similar to those
encountered in a deinking pulper. The resultant dispersions were studied using
turbidity and particle size analysis. The morphology of the ink particles was determined
using a scanning electron microscope. Sodium silicate proved to be a poor disperser of
ink particles, but nevertheless appeared to greatly influence the dispersing properties
of the soap in the presence of calcium ions.
The nature of the interactions between sodium silicate, calcium ions and the collector
soap were studied in an attempt to elucidate the role of sodium silicate. A model
system consisting of the sodium salt of collector soap, calcium ions and sodium silicate
was studied 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
sodium silicate showed a measurable chelating e:ffect on calcium ions. Thus,
increasing levels of sodium silicate lead to an increase in the concentration of sodium
soap in solution. It was hypothesised that this effect would lead to better dispersion of
ink particles and improved deinking performance.
This chelating effect was evaluated in laboratory deinking studies. Samples of
newsprint were pulped in a 251 Lamort laboratory pulper under a variety of conditions,
viz. with fresh water, with an excess of soap, with an excess of calcium, with and
without sodium silicate. The pulps were floated in a 201 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 sodium soap
and a low Ca hardness in the pulper. Softening the water used in pulping without
adding excess sodium soap did not significantly improve pulp brightness. 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 hypothesis 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 appears to be 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 and silicate concentration. To this end it was
recommended that the Merebank deinking plant should evaluate the use of water with
a low calcium ion concentration and the addition of some soap into the pulper to
improve their deinking plant performance.
Description
Thesis (M.Sc.)-University of Natal, Durban, 2003.
Keywords
Waste paper--Recycling--South Africa., Sodium silicate., Theses--Chemistry.