Gypsum plasterboard
Gypsum plasterboard
Description
The data set covers all relevant process steps / technologies over the supply chain of the represented cradle to gate inventory with a good overall data quality. The inventory is mainly based on industry data and is completed, where necessary, by secondary data. End-of-life recycling stage is not included in the cradle-to-gate inventory. The recycled content of the products is 40% taking into account recovered paper as well as post-consumer recycling of plasterboard, FGD gypsum (fluegas desulphurisation gypsum) and titanogypsum, the three of them replacing natural gypsum stone. This indicator specifies the advantages of the use of recycled or recovered material on resource depletion and land use not integrated in the mandatory assessments under the ISO 14040 series.
Technology
Technology description [Report on Production Process parameters]:
1. PLASTERBOARD MANUFACTURING PROCESS STATE OF THE ART
Gypsum plasterboards are flat rectangular building boards consisting of a plaster core whose surfaces and longitudinal edges are paper-covered and profiled to suit the intended application. The paper-covered plaster core can contain additives to achieve certain properties. Plasterboards are used in a broad range of building applications, especially as wall and ceiling linings, as well as for the manufacturing of prefabricated building components. [Gips-Datenbuch, 2006;Life Cycle Assessment of Plasterboard]
Plasterboards are manufactured in a two-step process. The first steps generic stages include pre-processing of the gypsum feedstock (size reduction and pre-drying depending on feedstock type and properties), followed by the thermal process of calcination. The intermediate product is stucco, a partly dehydrated form of gypsum, which is then mixed with water and additives to form the plaster slurry. The slurry is fed to the board line where it is encased between two layers of special strong paper and gradually sets while it is conveyed along the line at an appropriate speed. When set, the continuous length of plasterboard is cut to individual uniformly sized boards, which proceed to a large multi-zone drier to remove the excess free water and exit as the finished product.
1.1 Raw Materials
The feedstock mix for the production of stucco may consist of one or more types of gypsum from conventional sources (natural and/or synthetic). Traditionally, most plants that introduced synthetic gypsum into their process used it in a mixture with natural gypsum stone, but today there are many modern plants that manufacture plasterboard exclusively from synthetic gypsum [Gypsum Association].
Feedstock can also contain a percentage of recycled gypsum derived from production waste and/or post-consumer gypsum-based waste from construction and demolition/deconstruction jobsites.
In any case, each individual gypsum type as well as its source must be assessed regarding its particular suitability for plasterboard manufacturing, which may vary depending on purity or other important characteristics and properties of the material.
1.1.1 Natural Gypsum
Natural gypsum is a soft rock-like sulphate mineral predominantly composed of calcium sulfate dihydrate (chemical formula CaSO4.2H2O), formed geologically from the evaporation of super-saturated aqueous solutions resulting in the sedimentary deposition of calcium salts in large basins (former shallow seas). The earliest deposits date to around 200 million years ago [Factsheet on what is gypsum].
Gypsum rocks differ in their degree of purity (CaSO4.2H2O % w/w content), colour and structure based on their geological history [Gips-Datenbuch, 2006]. Depending on the deposit, purity varies between 75-95% [Factsheet on what is gypsum], the remainder being other generally inert minerals such as clays, sand, anhydrite, dolomite and limestone [Gips-Datenbuch, 2006]. The mineral anhydrite (chemical formula CaSO4) is the un-hydrated form of gypsum. It has a very different structure which makes it comparatively limited in its technical applications [Gips-Datenbuch, 2006; Life Cycle Assessment of Plasterboard; Factsheet on what is gypsum] and its presence in the rock as a raw material for plasterboard production is mainly considered undesirable.
The color of gypsum rock is usually white, but it can also be colourless, grey or shades of red, brown and yellow, being naturally influenced by the types of impurities contained [Gips-Datenbuch, 2006; DA.1 Report Inventory of current practices]. Rock size may reach up to 50 cm in diameter.
Gypsum is a common mineral abundantly found in many countries of the world. The top three worldwide crude gypsum producers are China, Iran and USA, while the principal gypsum deposits in Europe are located in Spain, Italy, Russia, France, Germany, Poland, the UK, Romania, and Ukraine [DA.1 Report Inventory of current practices]. It is extracted from open-cast which is primarily the case in Europe or underground mines, using specific drilling machinery and non-polluting explosives [Life Cycle Assessment of Plasterboard; DA.1 Report Inventory of current practices].
1.1.2 Flue Gas Desulphurization Gypsum
Flue Gas Desulphurization gypsum or FGD is the most widely used type of synthetic gypsum in the gypsum industry. It is obtained as a by-product from the wet flue gas desulphurization process typically used for cleaning the emissions of power stations fired with fossil fuel (e.g. coal). The combustion flue gases are washed in scrubbing towers in counterflow with an aqueous suspension of finely powdered limestone or lime. The contained SO2 is removed from the flue gas by the water and reacts with the alkaline suspension to calcium sulphite, which is subsequently oxidized with atmospheric oxygen and precipitates as calcium sulphate dihydrate (CaSO4.2H2O) crystals, i.e. gypsum. The gypsum is separated from the suspension, washed with clean water to remove the water soluble impurities and finally dewatered with the aid of centrifuges or vacuum filters. [Factsheet on what is gypsum; Life Cycle Analysis of Gypsum Board and Associated Finishing Products]
FGD gypsum is a wet material in finely grained powder form, with a purity of >95%, considerably higher than that of most natural gypsums (typically ~80%) [Gips-Datenbuch, 2006; DA.1 Report Inventory of current practices], the remainder being mainly unreacted calcium carbonate and traces of fly ash from the flue gases as impurities [Gypsum Association]. FGD gypsum is a commercial product and a directly usable raw material serving either as an alternative or as supplement to natural gypsum feedstock.
FGD gypsum is produced in most Western European countries, with around half the output coming from Germany [DA.1 Report Inventory of current practices].
1.1.3 Other Types of Synthetic Gypsum
Apart from flue-gas desulphurization, there are certain other industrial processes that produce gypsum as a by-product, obtained when calcium compounds react with sulphates or sulphuric acid. The principal, other than FGD, synthetic gypsum types potentially suitable for use in plasterboard manufacturing include titanogypsum (by-product of the sulphate process for titanium oxide production) and, to a lesser extent, citrogypsum (by-product of citric acid production process). Phosphogypsum (from phosphoric acid production) presents a higher level of natural radioactivity which is an important limiting factor for its extensive use [Gypsum Association; Factsheet on what is gypsum].
The use of substitutes to natural gypsum reinforces the environmental-friendly profile of the gypsum industry since it both reduces pressure on natural resources and promotes the utilization of valuable materials that would otherwise end up in landfills. However, the potential suitability and usage of these types of synthetic gypsum for specific manufacturing applications highly depends on quality (impurities, structure, consistency etc.) as well as financial issues and in practice the quantities used are low [DA.1 Report Inventory of current practices].
1.1.4 Recycled Gypsum
Recycled gypsum is derived from gypsum-based waste (plasterboard, gypsum blocks, moulds etc.) generated from the manufacturing process and from construction and demolition/deconstruction jobsites. It is produced by controlled processing of these wastes to separate the gypsum, paper and any contaminants, so that it can be used as a substitute to natural or synthetic gypsum [Life Cycle Assessment of Plasterboard]. In fact, gypsum is amongst the few construction materials where closed loop recycling is possible, i.e. gypsum waste can be used to reproduce the same product [Factsheet on what is gypsum].
Once collected, the waste is recycled by specialized companies (i.e. recyclers). The recycling process includes crushing, mechanical separation of paper from the gypsum core of plasterboard and fine grinding of gypsum (Figure 1-1). The removed paper can be used in agriculture for fertilizers, mulch etc. For gypsum waste arising from demolition/deconstruction works which may contain a certain amount of physical contaminations such as metallic and wooden parts, coatings, coverings, insulation, etc. decontamination carried out either manually during sorting of waste and/or mechanically during processing is necessary in order to achieve a high quality, pure recycled gypsum product. Modern recycling units are mechanically equipped to remove most of the impurities and foreign objects from the gypsum core.
Recycled gypsum is usually in the form of a fine or sandy powder, or a small aggregate-type material [Life Cycle Assessment of Plasterboard]. Currently the quality requirements for recycled gypsum are defined either by national specifications that have been issued in some European countries, or by individual commercial agreements between manufacturers and recyclers, the latter being mostly the case.
Plasterboard waste from the manufacturing process are often recycled directly at the manufacturing plant and, given the relatively small percentage of production recycled gypsum incorporated in the feedstock mix, the waste may be simply crushed and ground without any paper separation taking place. Many plants equipped with a recycling line accept and process production waste from other plants and potentially post-consumer gypsum waste from jobsites, depending on their equipments capacity and specifications. Manufacturing plants without recycling systems send their production waste to recyclers, who in some cases may operate recycling facilities located within manufacturing plant sites.
The recycling of production waste is traditionally a common practice and the inclusion of post-consumer recycled gypsum in the process is currently increasing, prompted by the need for compliance with legislative dictates as well as by potential economic benefits, given, of course, that it is not hindered due to technical reasons. In any case, recycled gypsum is introduced in a controlled blend into the manufacturing process as one single stream and not as separate streams depending on their sources. As an indicative example, this stream may consist of gypsum from internally recycled production waste and recycled gypsum received from a recycler.
1.2 Pre-processing of Raw Materials
The pre-processing of raw materials refers to the size-reduction (crushing and grinding) and pre-drying operations potentially carried out before calcination.
1.3 Calcination
In the gypsum industry calcination is the thermal processing of gypsum to change the hydration state of its dihydrate content (calcium sulphate dihydrate, CaSO4.2H2O) by partly or completely removing its chemically bound (i.e. crystal) water in order to produce hemihydrate (CaSO4.1/2H2O) or anhydrite (CaSO4) respectively. It is a reversible process that can be repeated almost indefinitely, which is why gypsum has been characterized as eternally recyclable. Specifically, when the calcined (i.e. dehydrated) material is mixed with water it rehydrates into its original state and sets, obtaining a relatively high strength by forming a crystalline structure.
Four different basic plaster products are commercially produced by calcination of gypsum: - and -hemihydrate, soluble anhydrite and insoluble anhydrite. All types are called stucco in the industry and the hemihydrates are also commonly known as plaster of Paris.
In the manufacture of plasterboards calcination refers to the production of -hemihydrate, whose crystals have a micro-porous structure and high specific surface. The produced stucco is more soluble than the -type and when it sets after rehydration it has high porosity, but low mechanical properties and is therefore mainly used in lightweight building applications such as plasterboard or moulds [DA.1 Report Inventory of current practices].
1.3.1 Process Chemistry
Stucco for plasterboard manufacturing is produced by the Beta Process of calcination that results in the formation of -hemihydrate. The ground gypsum feedstock is heated under regular ambient pressure at the temperature range of 120 to 165oC and the contained calcium sulphate dihydrate releases 75% of its crystal water as steam and converts to hemihydrate according to the equation:
CaSO4.2H2O + energy CaSO4.1/2H2O + 3/2H2O
1.3.2 Calcination Equipment
The kettle calciner is the most widely used calcination unit, available in several designs. Rotary kilns are the second most commonly used type of gypsum calciner for -hemihydrate production, but in plasterboard manufacturing they have been largely replaced by continuous kettles or by more modern single-unit grinding and calcining equipment.
1.4 Plasterboard Production
Gypsum plasterboards are produced on large highly automated board lines in a continuous operation shown in Figure 1-2. The main sections are analyzed below.
1.4.1 Blending of the Stucco Slurry
The slurry that comprises the boards plaster core is produced by mixing stucco with water and appropriate dry and liquid additives and admixtures in defined amounts according to the so-called recipe followed in each plant.
Stucco is accurately metered and blended with the dry additives mix in a mixing screw conveyor and the dry ingredients are fed to a continuous mixer where water with premixed liquid additives is added. The resulting slurry is deposited on the bottom sheet of paper at the forming station.
To achieve proper consistency and fluidity of the slurry and to ensure complete rehydration of stucco back to gypsum, the added water is in excess of the stoichiometrically required amount for the rehydration reaction. This excess water is later driven off by drying the boards.
The specific recipe and most importantly the types and quantities of additives used determine the particular properties of the board and therefore depend on the type of plasterboard produced. Generally, stucco makes up for at least 95% of the material used prior to mixing with water, and additives include at least starch, fibres and an accelerator, among others [Gypsum Plasters and Wallboards]. Table 1-1 shows some commonly used additives in plasterboard production and their respective attributes. However, it should be noted that exact recipes are essentially technical and commercial exclusivities of each manufacturing company.
1.4.2 Board Forming Station
At the forming station the paper is unrolled on racks that run below and above the mixer so that the stucco slurry can be sandwiched in between. The slurry from the mixer is poured on the bottom sheet of paper moving on a conveyor belt and covered with the top sheet. Two small mixers may be used to deposit slurry of higher density along the boards edges in order to improve their strength and facilitate handling [Life Cycle Analysis of Gypsum Board and Associated Finishing Products]. The roughly formed board passes between edge guides and moves on under a roller or a forming plate to obtain the specified thickness. Finally an adhesive is added to seal the paper along the edges. It should be noted that the bond between the paper and the boards plaster core is achieved by the growth of gypsum crystals into the fibrous pores of the paper during setting and not by the use of adhesives.
The paper used in plasterboard manufacturing, called facing or lining paper or plasterboard liner, is a special multi-ply couched cardboard, typically recycled, and gives the board most of its tensile strength. The face of the board is usually ivory-coloured and the back side gray.
1.4.3 Setting and Cutting
After it is formed, the long continuous sheet of plasterboard travels along on a conveyor belt of about 100 m to several hundred meters length, while setting. The length of the board line is designed in conjunction with the belts speed and capacity to allow the required time for the plaster core to set. Setting time usually varies between 0.5 to 5 min. The speed of the line depends on the design of the machine and the type and thickness of the board manufactured, but it usually ranges between 0.5 and 3.0 m/sec, a typical line speed being around 1 m/s [Life Cycle Assessment of Plasterboard; Gypsum Plasters and Wallboards].
The actual setting process refers to the completion of the rehydration reaction of the hemihydrate contained in the stucco which converts it back into interlocking dihydrate crystals. The basic reaction is the reverse of calcination, but the newly formed dihydrate is more solid and stiff than the one originally calcined:
CaSO4.1/2H2O + 3/2H2O CaSO4.2H2O
Due to the above reaction the core of the plasterboard sets and bonds to the paper facing, and hardens enough so that the board can be cut by the time it approaches the end of the line. At that point an automatic cutter cuts the continuous board at specific intervals to produce individual uniform panels of the proper length. As they continue to move, the boards pass by an inspection point where out-of-specification boards (i.e. wet rejects) are diverted from the process and the rest are turned over and proceed to the dryer.
1.4.4 Drying and Finishing Process
At their final production stage, the boards stacked in layers slowly enter a continuous multi-deck drying kiln where the excess free moisture (i.e. the excess water added at mixing) is evaporated. This is accomplished by direct heat transfer with hot air streams, while the boards move through three or more drying sections (zones) depending on the kiln/process design, where they are exposed to gradually decreasing levels of heat. Drying time ranges between 35-60 minutes and the boards exit with less than 0.5% residual free moisture. The temperature and humidity are closely controlled during the drying process to prevent recalcination of the gypsum core.
At the exit of the dryer the boards are once more inspected and off-specs (dry rejects) are removed. The finished plasterboards are conveyed to a machine that trims their ends to produce accurate lengths and then are bundled in two, stacked and taken to storage.
1.5 Energy Use and Efficiency
Plasterboards are among the most environmentally friendly construction products and have very low embodied energy value based on cradle-to-gate values due to four main reasons; the long established use of recycled lining paper, the large substitution of natural gypsum resources with synthetic FGD, the essentially 100% recycling of plasterboard waste arising from production and the increasing incorporation of post-consumer recycled gypsum in the manufacturing process.
Plasterboard manufacturing is nonetheless quite energy intensive, particularly regarding the calcining and board drying operations. The plasterboard dryer kiln consumes more energy than all the other stages of the process combined, while calcination represents the second most energy-intensive stage, followed by the drying and grinding of gypsum raw materials [Life Cycle Analysis of Gypsum Board and Associated Finishing Products]. Estimates on the total energy required per square meter of standard 12.5 mm thickness board range between 23.6-28.4 MJ [Life Cycle Assessment of Plasterboard; Life Cycle Analysis of Gypsum Board and Associated Finishing Products; Gypsum Plasters and Wallboards].
The use of natural gas for covering thermal energy demands is widespread. Calcination and drying processes are entirely fueled with natural gas in the largest part of the industry, as the use of coal, oil and LPG is being increasingly abandoned. In addition to the financial and environmental benefits of natural gas, this shift was also prompted by the use of directly fired drying equipment, in which clean-burning fuels are highly preferable to prevent contamination of the raw materials and/or of the plasterboard product [Gypsum Plasters and Wallboards]. The supplementary use of alternative fuel (e.g. waste fuel) is not uncommon, especially in calcination, however fuel flexibility is considered limited within such product quality related restraints.
The use of alternative energy sources has not been generally favoured despite the gypsum industrys efforts to introduce them in the energy mix, due to availability issues, technological impediments and cost considerations.
However, in the last 50 years major strides have been made to reduce energy consumption and improve energy efficiency, which have led the industry close to the theoretical optimum energy consumption value. The rise in energy costs over time encouraged the investigation and development of process technology to reach lower energy requirements. The most important advancements include the shift from batch to continuous kettles, the submerged combustion kettles that offer higher energy efficiency than indirectly heated units and the gradual replacement of rotary kilns with kettles of improved design.
Heated mills and directly fired grinding/calcination units rendered the stucco production process more compact and continuous innovation in calcination technologies and equipment further improved its energy efficiency. New cutting-edge technologies allow higher hot gas inlet temperatures and lower gas volumes in direct heat transfer systems that lead to optimal energy consumption and offer a more accurate temperature control regime.
The minimization of the water/stucco ratio in the slurry by using more sophisticated additives decreased considerably the plasterboard drying energy demands. Extensively implemented energy-saving practices also include judicious fuel selection and use and insulation measures.
The remaining gap between theoretical barrier and current thermal energy use levels is mainly due to heat losses, however latest generation heat recovery systems are increasingly being adopted. Heat recovery systems allow considerable energy savings by preheating air streams needed in several stages of the process with collected heat from calciners and dryers flue gas recirculation, thus achieving to recapture a large part of the conditioned temperatures that would otherwise be lost. Dryers and calciners of latest technology have integrated such systems, but these can also be added on the existing units.
Regarding electrical energy, consumption has also approached the feasible minimum due to the widespread adoption of variable speed drive technologies that regulate power input, thus avoiding over-consumption of electricity. Variable speed drive systems match the speed of motor-driven equipment to the process requirements and lead to significant electrical energy savings.
Based on the above there is no scope for further reduction of energy consumption and improvements in energy efficiency in the plasterboard manufacturing industry.
Most major EU plasterboard manufacturing plants have both heat recovery and variable speed drive systems installed and perform energy audits; hence there is little room for low or medium cost energy saving measures, as long as "house tiding" practices are implemented as requested by energy audits. High capital cost measures such as upgrades in new calcination/drying equipment or energy production technologies could have some positive impact, but, given that plaster is a commodity facing a fall in demand and an increasing pressure on prices, the adverse market situation in Europe and the decreased share of European production in the global picture hinder short term investments in this direction.
Taking into account the increasing energy prices, this leaves fuel flexibility (i.e. the use of different fuels according to price and availability) as perhaps the only energy saving and overall cost reduction issue in plasterboard manufacturing, although within the restraints of ensuring product quality.
1.6 Process Waste Streams and Atmospheric Emissions
1.6.1 Atmospheric Emissions
Crushing, grinding and handling of gypsum raw materials and stucco at the plant, including the calcination step of the process, the blending of dry raw materials for plasterboard production, as well as the final trimming of the finished board result in dust emissions, which are minimized by particulate emission control systems. High efficiency baghouses and/or electrostatic precipitators are installed and used in all modern plasterboard manufacturing plants.
Fuel combustion for covering the process thermal energy demands results in the common atmospheric emissions (CO2, CO, SO2, NOx, CH4 and VOCs) as in any process where fuels are used. Both gypsum calcination and plasterboard drying process require relatively low temperatures where no significant NOx¬ is generated [Life Cycle Analysis of Gypsum Board and Associated Finishing Products]. Moreover, the extensive switch from heavy fuels to natural gas has practically eliminated SO2 emissions and the use of high combustion efficiency burners, as well as the whole set of energy efficiency widely adopted measures have minimized the release of all other pollutants.
Regarding CO2, the process involves only fuel CO2 generation (i.e. there is no chemical release of CO2). As a consequence of continuous process innovation that led to energy efficiency improvements and significant overall reduction in energy consumption over the last 50 years, both direct and indirect carbon emissions of the plasterboard industry have been minimized close to the theoretical benchmark. In fact, fuel CO2 emissions have proportionally decreased more than the respective reduction of thermal energy consumption thanks to the shift to natural gas.
1.6.2 Liquid Waste
The plasterboard manufacturing process itself generates hardly any liquid effluents. However some liquid waste are indirectly generated from plasterboard plants from the washing of equipment and spaces and the rainwater that washes away gypsum dust from yards and open storage areas. These runoff waters are typically drawn into containment areas and the resulting sludge after settling is disposed to landfills.
1.6.3 Solid Waste
A small amount of plasterboard waste, typically around 5% of total production, is generated from the manufacturing process as wet and dry out-of-specification boards. Some solid waste also is also produced during the final trimming of the boards. As already noted, 100% of production waste is recycled, at least as far as standard plasterboards of Type A are concerned. However, some special types of plasterboard are currently considered unfitted for recycling and they are disposed to landfill, although the quantities generated are relatively low.
Background system:
Electricity: Electricity is modelled according to the individual country-specific situations. The country-specific modelling is achieved on multiple levels. Firstly, individual energy carrier specific power plants and plants for renewable energy sources are modelled according to the current national electricity grid mix. Modelling the electricity consumption mix includes transmission / distribution losses and the own use by energy producers (own consumption of power plants and "other" own consumption e.g. due to pumped storage hydro power etc.), as well as imported electricity. Secondly, the national emission and efficiency standards of the power plants are modelled as well as the share of electricity plants and combined heat and power plants (CHP). Thirdly, the country-specific energy carrier supply (share of imports and / or domestic supply) including the country-specific energy carrier properties (e.g. element and energy content) are accounted for. Fourthly, the exploration, mining/production, processing and transport processes of the energy carrier supply chains are modelled according to the specific situation of each electricity producing country. The different production and processing techniques (emissions and efficiencies) in the different energy producing countries are considered, e.g. different crude oil production technologies or different flaring rates at the oil platforms.
Thermal energy, process steam: The thermal energy and process steam supply is modelled according to the individual country-specific situation with regard to emission standards and considered energy carriers. The thermal energy and process steam are produced at heat plants. Efficiencies for thermal energy production are by definition 100% in relation to the corresponding energy carrier input. For process steam the efficiency ranges from 85%, 90% to 95%. The energy carriers used for the generation of thermal energy and process steam are modelled according to the specific import situation (see electricity above).
Transports: All relevant and known transport processes are included. Ocean-going and inland ship transport as well as rail, truck and pipeline transport of bulk commodities are considered.
Energy carriers: The energy carriers are modelled according to the specific supply situation (see electricity above).
Refinery products: Diesel fuel, gasoline, technical gases, fuel oils, lubricants and residues such as bitumen are modelled with a parameterised country-specific refinery model. The refinery model represents the current national standard in refining techniques (e.g. emission level, internal energy consumption, etc.) as well as the individual country-specific product output spectrum, which can be quite different from country to country. The supply of crude oil is modelled, again, according to the country-specific situation with the respective properties of the resources.
Completeness
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