At the request of the National Audit Office, an audit has been performed on the manufacturing process of calcium carbonate. A number of problems or issues have been found in the various stages of manufacturing process, some of which may become serious issues if not overlooked.
For each audit we identify and critically evaluate the controls in place and highlight in our report those potential weaknesses that become apparent as a result of our work. We obtain comments from appropriate staff for each weakness identified.
Our approach is to discuss with relevant personnel and develop an understanding of the processes and procedures established; document and evaluate whether the associated systems and procedures are appropriately designed to achieve the organizations objectives.
The spoilage of paper, pulp, mineral slurries, coatings and other additives is generally due to a mixed population of bacteria and to a lesser extent fungi. The main types of microorganism found in slurries include species of the following genera: Pseudomonas, Alcaligenes, Aeromonas, Bacillus, Klebsiella, Serratia and Erwinia. Ground and precipitated calcium carbonate have an alkaline pH ranging from 7 to 10 with a water content of 25-50% v/v; the presence of various salts is sufficient to sustain growth of microorganisms in such an environment (Rohleder and Huwald, 2001). Therefore, biocides play an important role in preserving calcium carbonate so as to maintain high quality requirements, such as brightness, rheological parameters, and odour neutrality (Schwarzentruber, 2003).
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When high quality precipitated calcium carbonate is being produced, it is especially important to remove the calcium carbonate waste (i.e. PCC waste) from the process, because it contains impurities originating from the limestone, as well as sand, calcium hydroxides and calcium oxide. The slurry contains undissolved calcium hydroxide, calcium ions (Ca2+) and hydroxide ions (OH-). The Ca2+ concentration in the slurry is dependent on the solubility, which decreases as the temperature increases.
These conditions, together with the operating temperatures, which are normally above 30°C, provide the ideal environment for the growth of microorganisms, mainly bacteria. Contamination may originate from the air, fresh water, virgin and recycled pulp or waste water. In the manufacturing process, this can result in the build up of slime at key points in the process. If uncontrolled, the slime can accumulate and eventually break away, leading to problems such as paper breaks, spray nozzle blockage and discolouration of the finished product. Problem organisms are the aerobic slime-forming bacteria such as Pseudomonas, Klebsiella, Enterobacter and Bacillus spp. Anaerobic organisms such as the Desulphovibrio spp. can also accumulate under slime layers and other deposits leading to the production of foul odours and corrosive by-products.
Recommendations on the type of biocide that can be used to avoid the key issues are given taking into account the product specific parameters, plant hygiene and legislation. Two biocides have been recommended for the manufacturing, storage and transportation. Protectol HT, premium quality Triazine biocide which is a 76% solution of Hexahydrotriazine, a formaldehyde-release biocide that is particularly suitable for use as a preservative in mineral slurries especially works well as a preservative in kaolin and calcium carbonate slurries. Because of its toxicity to many microorganisms, formaldehyde is a widely used biocide in various industrial applications such as the production of paper, resins, glue, and in wood processing (Glancer- Soljan et al., 2001). The toxic effect of formaldehyde on many microorganisms has been well studied and can be traced back to its reaction with nucleic acids and different amino acids in proteins.
Formaldehyde-releasing compounds offer an alternative source without any disadvantages such as increased viscosity and an insufficiently balanced range of activity. (Ethylenedioxy)dimethanol (EDDM) and Tetramethylolacetylene diurea (TMAD) are two formaldehyde releasers widely used as preservatives in industrial aqueous systems (Paulus, 2005; Pfuhler and Wolf, 2002; Selvarajuet al., 2005). It can be used as a stand-alone preservative or in conjunction with other biocides. It is stable for up to 12 months when stored in sealed containers at temperatures up to 30°C.
The pH of calcium carbonate slurry is normally between 9.0 -10.0 and it is recommended that formaldehyde can be used for the preservation of calcium carbonate slurries at concentrations between 250 and 1000ppm according to the in-house study to demonstrate the efficacy of formaldehyde. It is readily biodegradable in the environment and should not cause problems in sewage treatment plants and surface water when used as recommended.
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The other biocide is Myacide AS biocide, the industrial grade of 2-Bromo-2-nitropropane-1,3-diol or Bronopol. It provides highly effective antimicrobial activity for use in diverse and demanding industrial biocide applications, combining well-proven efficacy with important environmental safety characteristics. The main benefits of Myacide AS in the paper industry are its proven performance as a slimicide active ingredient in mill process water and as an effective preservative for mill additives.
Bronopol is highly effective, particularly against aerobic slime-forming bacteria such as Pseudomonas spp., Klebsiella spp., Bacillus spp. and the Enterobacter spp. The following table compares the Minimum Inhibitory Concentrations (M.I.C.) of Bronopol against six key microorganisms. There have been no reported cases of resistance to Bronopol in over twenty years of use. Bronopol is readily water-soluble and solutions containing up to 28% w/v are possible at ambient temperature. Bronopol's stability is one of the most important factors in the paper industry for long term preservation. It shows optimum pH stability in the acidic range with excellent efficacy. At alkaline pH, Bronopol is less stable but still provides effective control. In these systems, Bronopol reaches equilibrium with its breakdown products, which are also microbiologically active. This self-stabilisation means that preservative efficacy can be maintained over an extended shelf life even within aggressive, more alkaline matrices.
Bronopol is non-carcinogenic, nonmutagenic, non-embryotoxic and non-teratogenic. It is also non-phototoxic. Bronopol is biodegradable and does not accumulate in the environment. Due to its rapid photolysis and hydrolysis, Bronopol will not, under normal use conditions, cause problems to effluent treatment plants or to the environment following discharge into wastewaters. Bronopol is classified in the EU Council Directive -67/548 EEC as harmful, irritant and dangerous for the environment.
Fig 2: The figure demonstrates the efficacy of Bronopol in preserving a typical paper mill additive based on calcium carbonate. Myacide AS at a level of >20ppm was able to control Pseudomonas aeruginosa, Enterobacter cloacae and Klebsiella aerogenes for a period of six weeks following a single initial challenge of 106cfu/ml and a reinoculation at 21 days.
The Minimum Inhibitory Concentration (M.I.C.) against a range of test organisms are displayed in the following table:
Fig 3: M.I.C for formaldehyde biocide Fig 4: M.I.C for Bronopal biocide
In liquid cultures, commercial biocide formulation was dosed in ppm (w/w) commodity based on the total weight of the solution. In a different way, the biocide amount dosed in calcium carbonate slurry is reported in ppm (w/w) commodity based on the weight of the water in calcium carbonate slurry with 75% solid content.
(M.I.C. data is generated according to in-house methodology.)
The occurrence of biocide-resistant bacteria, technical limitations in the use of biocides, as well as a more rigorous regulatory situation created by the BPD (Biocidal Products Directive 98/8/EC), are together increasing the demand for
new biocide research to ensure continuing effective WMD preservation.
An overdosing due to the poor hygiene is the major contributor to the increased costs of prevention. With a rigorous hygiene system in place annual biocide spend can be reduced through optimum dosing.
It is also important to recognize that the amount of CO2 captured in PCC manufacture is equal to the amount of fossil CO2 released from CaCO3 in the manufacture of lime used to produce the precipitated calcium carbonate. Compared to the off-site manufacture and shipping of PCC slurry, however, the on-site manufacturing of PCC is somewhat less CO2 intensive because it enables dry lime to be shipped to the mill rather than PCC slurry, thus reducing
trasportation-related CO2 emissions (Miner and Upton2002; Teir et al. 2005).