Tile Manufacturing: Zircon Sand Lifecycle Assessment
A recent assessment shows that tile production using zircon generates significantly lower overall environmental impacts over a range of environmental indicators.
Zircon has been used by tile manufacturers as a raw material in the production of ceramic tile for many decades, principally as an opacifier to obtain lasting brilliant whiteness. Its high hardness helps produce a resilient tile capable of resisting mechanical damage, as well as increasing resistance to water, heat and chemicals, while making tile surfaces more suitable for the latest digital printing techniques.
The environmental impacts of a lifecycle assessment (LCA) of zircon sand were recently assessed in a study commissioned by the Zircon Industry Association (ZIA). The LCA study examined the environmental impacts associated with the production of 1 kg of zircon sand. Because of the comparatively low environmental impact of the downstream manufacturing process, the impact relates overwhelmingly to local electricity consumption associated with upstream mining processes, much like the mining of other natural minerals.
“The ability to quantify environmental impact is increasingly important in our industry,” said Keven Harlow, Ph.D., executive director of the ZIA. “We now have a reliable benchmark for zircon sand production, which confirms that the majority of its environmental impact—albeit low—is related to upstream electricity consumption.”
The study went on to compare the environmental impacts when using zircon as a whitener in ceramic tile production with the main alternative product, alumina. It found that tile production using zircon generates significantly lower overall environmental impacts over a range of environmental indicators selected for the LCA study, according to international standards.
In collaboration with Centro Ceramico di Bologna, the study included an evaluation of different tile mixtures, using alumina as an alternative to zircon as a whitener. The LCA was reviewed by an independent panel of three experts to ensure conformity to ISO 14040/44 standards.
The systems under study are termed “cradle-to-gate.” The zircon sand LCA includes mining and mineral separation only. For the comparison step, zircon flour production (mining, separation and milling), alumina flour production (bauxite mining, refining, milling) and the tile body mixture preparation (i.e., production of feldspar, clay, etc.) were all considered. Centro Ceramico developed six mixture recipes, three each for zircon and alumina based on a comparable whiteness level.
In this LCA, two functional units were used:
- Zircon LCA (zircon production average)—1 kg of zircon sand
- Comparison (alumina vs. zircon tile mixture)—1 kg of mix used for porcelain stoneware ceramic tile production, with a specific and measurable whiteness level called super-white; Centro Ceramico consulted multiple tile manufacturers to identify the super-white grade level as that with a mean value of L > 85
The reference year used was 2015. The geographical scope for the zircon LCI is global, where the zircon mining companies involved in this study represent > 77% of worldwide zircon sand production. The geographical scope for the comparison (zircon/alumina milling and mixture preparation) considered production in Europe, where the milling plants involved in this study represent approximately 64% of European capacity. The alumina primary data, used for the comparison and obtained from the European Aluminium Association (EAA) environmental profile, represents the European context and covers 84% of European production.
Company data (“primary data”) were obtained using questionnaire templates that were sent to data providers at participating companies. Each returned questionnaire was cross-checked for completeness and plausibility. The LCA model was then created using the GaBi 8 Software system for lifecycle engineering, developed by thinkstep AG. The GaBi LCI database provided the lifecycle inventory data for the raw and process materials necessary for modelling of the product systems: zircon, alumina and the tile application. The impact assessment was based on the CML2001 (January 2016) impact assessment methodology framework.
Zircon Sand Production Results
Overall lifecycle results per kg of global average zircon sand are presented in Table 1, while Figure 1 shows the contribution of single processes. The main energy conversion process, in terms of impact, is electricity production (85-99%). The importance of electricity, considering every indicator, is directly linked to each country’s own grid mix. The main emissions contributing to impacts are:
- Global Warming Potential (GWP)—carbon dioxide (91%) and methane (8%)
- Acidification Potential (AP)—nitrogen oxides (22%) and sulphur dioxide (77%)
- Eutrophication Potential (EP)—mainly from nitrogen oxides (97%)
- Photochemical Ozone Creation Potential (POCP)—carbon monoxide represents only 3% of POCP potential impact connected with energy processes (CO is 20% of the entire POCP when combustion processes are used during mining and MSP operations); POCP for energy production is also driven by nitrogen oxides (24%) and sulphur dioxide (60%)
- Abiotic Depletion Elements (ADPe)—mainly driven by auxiliary materials’ consumption (tires and batteries), with high consumption of elementary resources and elements like lead (82%), silver (12%) and zinc (4%); End of Life (EoL) credits relate to the disposal and recovery processes of these kinds of auxiliary materials
- Transport (for haulage of HMC to the MSP, and for zircon sand to the port)—minor contribution (< 6%)
Figure 2 describes the relative contribution of the mining and MSP/concentration processes. Mining is the most relevant process for all potential impacts, particularly for ADPe, EP, Ozone Depletion Potential (ODP) and POCP.
Results demonstrate that the environmental profile of zircon sand production is driven by energy-related processes, particularly during mining and heavy mineral concentration. Other impacts relate to the generation of thermal energy or combustion of fuel for drying processes and transportation.
Tile Mixture Comparison
The three mixture comparisons are shown in Table 2, while overall lifecycle results for 1 kg of ceramic tile mixture are presented and compared in Figures 3-5. In each case, the alumina-containing mixtures have higher environmental burdens, some significant, when compared to the zircon-containing mixtures for the impact categories assessed. The difference for ODP is less only because ODP is driven by electricity consumption at the zircon milling process.
Alumina has much higher impacts in the categories studied due to the primary refining (Bayer) of the bauxite ore. Moreover, in the case of alumina use, the mixture to be atomized needs more feldspar to reach the same degree of vitrification because of the alumina grain size. Alumina milling is not a relevant stage for potential impacts related to the mixture preparation, whereas milling impacts are more relevant for zircon due to electricity consumption at the mill, plus thermal energy for the drying process when wet milling is used.
Electricity consumption creates 76% of ODP, driven by those EU countries that consume electricity from nuclear power sources, and 54% of ADPe. Transport from the zircon mine to milling plants has an impact on AP (26%), EP (29%) and POCP (11%). Transoceanic shipping is the main contribution due to emissions from fuel combustion. Carbon monoxide, NMVOC (non-methane volatile organic compounds) and sulphur oxides are emissions that contribute to POCP, nitrogen oxides and NMVOC to EP, and nitrogen oxides and sulphur oxides to AP.
The ADPe was approximately 50% less when using zircon-containing tile mixes, while the Abiotic Depletion Fossil (ADPf), POCP and Primary Energy Demand (PED) were all found to be about 20% lower when using zircon-containing tile mixes. The impact for ODP was similar for both products.
Scenario analysis was performed to compare different sets of assumptions or modelling choices (e.g., thermal energy and electricity consumption and type of fuel). The analyses showed that, even considering the best case for an alumina mixture and worst case for a zircon mixture, the zircon-containing mix has a GWP 16% lower than the alumina mix, AP 21% lower, and EP 23% lower. Only ODP is at a similar order of magnitude for the two scenarios.
The zircon LCA demonstrates that the environmental profile of zircon sand production is mainly driven by energy-related processes, particularly during mining and heavy mineral concentration. When compared to alumina in super-white tile mixes, zircon-containing mixes show significantly lower environmental burdens in all but one of the potential impacts assessed in this study. Differences of approximately 20% for GWP, EP, AP, POCP, ADPf and PED are identified, and around 50% for ADPe. ODP is similar in both zircon- and alumina-containing mixes.
Milling affects all the impact categories in this study, mainly due to the higher consumption of electrical and thermal energy used in zircon milling (particularly wet milling) and for the generally finer grain size of zircon used in tile mixtures. The LCA demonstrates that the main differences between zircon and alumina mixtures are closely tied to the environmental performance of regional grid electricity production.
“This LCA demonstrates our commitment to environmental transparency and will allow our members to develop meaningful targets as they continually strive to reduce environmental impact,” said Harlow. “The study also demonstrates that tile manufacturers and users can lower their environmental footprint by using zircon as the opacifier instead of other whiteners.”
For more information, visit www.zircon-association.org.
Note: The study has two limitations. The impact of radiation from zircon as a norm is not assessed due to insufficient data to generate a reliable average for zircon production. This is also not considered for alumina production (especially the residual red mud waste stream). In addition, the reference year of alumina production data is 2010 (EAA), which is compared to 2015 zircon sand data in the LCA.