END USER FOCUS: Perfecting paper loading

By Jessica Roberts
Published: Tuesday, 25 May 2010

GCC, presently the most widely used mineral filler in paper production, looks set to increase its paper market share now that modern paper machines can handle over 50% mineral loading

The general shift from an acid-based process to an alkaline one has transformed the use of fillers in papermaking, creating an opening for ground calcium carbonate (GCC) and precipitated calcium carbonate (PCC). This has allowed these minerals to increasingly replace wood pulp and other pigments in the paper manufacturing process.

GCC and PCC improve paper quality in terms of its brightness, bulk, opacity and porosity. These mineral additives are widely used in the alkaline process, which produces paper types such as fine printing and writing paper. In some cases, however, paper is still manufactured in an acid environment, and it is here that traditional paper minerals such as kaolin are still required.

Loading paper rolls for transport at one of UPM-Kymmene’s
factories in Finland. UPM is one of the largest paper producers
in the world. UPM-Kymmene Oyj

World paper scene

China has emerged as the world’s leading paper and board producer and its global share continues to increase year on year. In 2009 the country produced an estimated 90m. tonnes of paper and board (P&B), up from 78m. tonnes in 2008, and some projections indicate that China’s P&B share in 2010 could increase again to 98m. tonnes.

This emergence has been encouraged by the Chinese government, which has ensured that investment by large domestic and international companies has created a modern Chinese paper industry.

In this year alone global engineering and process solutions provider, Metso Corp., has won three contracts to provide or renovate new and existing paper manufacturing equipment in Chinese paper factories.

The group also won a significant, €170m. order last year to supply a coated fine paper production line to Shandong Huatai Paper Co. Ltd, member of Huatai Group, in Dong Ying City, Shandong province. The state-of-the-art production line will produce close to 500,000 tpa of coated and uncoated fine papers, within the basis weight range of 80-200g/m2. The paper machine design speed is 1,800 m/min and the coater design speed is 2,000 m/min.

Asian paper minerals

China has a shortage of high quality kaolin suitable for paper products. It does, however, hold significant deposits of high quality marble near to the main papermaking areas, and it has successfully developed these for paper grade GCC.

GCC is now the principal industrial mineral for paper production (Figure 1), where it can be used as a precoat, coating pigment and as filler. As consultant Ian Wilson discussed in his presentation at IM’s 20th Industrial Mineral Congress & Exhibition, March 2010, topcoats previously comprised around 70% GCC and 30% kaolin but this ratio has swung in favour of GCC and the topcoat ratios today are around 85-90% GCC and 10-15% kaolin.

This increased proportion of GCC, particularly in the higher coating colour solids, saves on the energy and costs associated with drying.

Not only has the ratio of GCC used in topcoats increased, so has the actual pigment loading: especially in Chinese Printing and Writing paper (P&W). As Wilson explained, pigment loadings of 40-50% are now common and new paper machines will run, for the first time anywhere in the world, with up to 55% pigment loading the pigment consisting mainly of GCC.

Growth of GCC production in China from 1992-2008

Mineral loading

One company pioneering this increased pigment loading is Asia Pulp & Paper Co. Ltd (APP), which plans to bring on stream a new paper machine at its Hainan Island, China plant, that will see total loading levels of mineral pigments (such as GCC, talc, and titanium dioxide) overtaking those of fibre for the first time.

APP’s imminent start up of Paper Mill 2 (PM2) at the Hainan Island plant will result in increased volume consumption of minerals in paper. Some 600,000 tpa GCC is anticipated in the first phase of production, possibly increasing to 800,000 tpa GCC. The precoat and filler will consist of GCC while the topcoat will comprise 90% GCC and 10% kaolin.

Supplied by Germany’s Voith Paper GmbH, PM2 will have a production capacity of 1.45m. tpa, is 430 metres in length, and produces paper at 2,000 metres/min.

The PM2 will have reduction in freshwater consumption used in papermaking and minimal use of fresh fibres; mainly assisted through increased use of GCC . PM2’s water use will be around 5 l/kg paper produced: the maximum allowable in China is 10.5 l/kg with comparable fine paper machines in Europe consuming on average 8 l/kg.

APP’s planned increase of mineral pigment loading, in excess of 50% and even up to 60%, is a significant increase from the usual filler loading levels of 30-40%. Not only does the increased GCC loading save costs on energy for drying purposes, it also saves significant money on raw material costs.

The price of Northern Bleached Softwood Kraft pulp in the USA is about $875/tonne while the price of Chinese pulp is close to $900/tonne. Chinese precoat GCC, meanwhile, can command price ranges of $80-160/tonne while Chinese filler grade PCC costs about $80-90/tonne. Comparing the mineral prices to pulp prices, it is clear that increasing mineral loading and decreasing pulp content is certainly advantageous for producers.

Acknowledgement: world paper and board production figures taken from the paper by Ian Wilson and Larry Lai, “Why Chinese paper is so competitive”, given at IM’s 20th Industrial Minerals Congress & Exhibition, March 2010.

Kaolin’s paper properties

Since the early 1990s the paper-grade kaolin market has suffered increased competition from ground calcium carbonate (GCC) and precipitated calcium carbonate (PCC) products. At present, numerous kaolin grades exist on the market, with much of this diversity targeting the changing demands of the paper industry.

The growth of GCC in applications such as paper coating has driven the development of specialised kaolin grades, such as Hydragloss¨ 90 by US kaolin producer KaMin LLC (formerly Huber), an example of a fine glossing, high brightness clay.

Paper grade kaolin must undergo a rigorous manufacturing process, and some products may go through as many as six processing steps. One of the principal grades of hydrous kaolin is a water-washed product which is taken through beneficiation processes to remove grit and increase brightness.

This can include blunging (kaolin is mixed with water and chemicals to form a slurry), de-gritting, centrifuging (fine and coarse particles are separated), brightness enhancement (bleaching, magnetic separation, flocculation, ozonation, flotation, and oxidation), and filtering and drying.

Advantages of water-washed kaolin include easy dispersion, chemical inertness, neutral pH, range of particle sizes and brightnesses, low Mohs hardness which minimises machinery wear, rheology aid, and extender for titanium dioxide and resins. Other kaolin types include:

Delaminated: coarse kaolin, the particles of which resemble booklets when magnified, is milled to separate the booklets into individual plates. This extra processing step produces an engineered kaolin, suitable for lightweight coating applications, which has additional benefits to a water-washed product. Benefits include a high aspect ratio (HAR), improved barrier properties, neutral pH, low machinery wear, controlled penetration, and bridging.

Calcined: hydrous kaolin is heat treated to convert the mineral into an amorphous structure. During calcination, air voids are incorporated into the structure. The advantages to using calcined kaolin include chemical inertness, increased brightness, structured particles, low moisture content, increased oil absorption, improved reinforcement, and better optical properties.

Surface treated: kaolin is chemically modified for enhanced coupling with polymer systems. This affects the kaolin’s chemical compatibility by improving dispersion and rheology. Also affected is the chemical coupling of the kaolin, as surface treated kaolin grades have improved physical and mechanical properties in addition to increased cross-link density.