Editing Gypsum (section)

==Synthesis==
[[Chemical synthesis|Synthetic]] gypsum is produced as a waste product or by-product in a range of industrial processes.

=== Desulfurization ===
[[Flue gas desulfurization]] gypsum (FGDG) is recovered at some coal-fired power plants. The main contaminants are Mg, K, Cl, F, B, Al, Fe, Si, and Se. They come both from the limestone used in desulfurization and from the coal burned. This product is pure enough to replace natural gypsum in a wide variety of fields including drywalls, water treatment, and cement set retarder. Improvements in flue gas desulfurization have greatly reduced the amount of toxic elements present.<ref name="pmid31561139">{{cite journal |last1=Koralegedara |first1=NH |last2=Pinto |first2=PX |last3=Dionysiou |first3=DD |last4=Al-Abed |first4=SR |title=Recent advances in flue gas desulfurization gypsum processes and applications – A review. |journal=Journal of Environmental Management |date=1 December 2019 |volume=251 |pages=109572 |doi=10.1016/j.jenvman.2019.109572 |pmid=31561139 |pmc=7396127}}</ref>

=== Desalination ===
Gypsum precipitates onto brackish water [[membrane]]s, a phenomenon known as mineral salt [[Fouling|scaling]], such as during [[brackish]] water [[desalination]] of water with high concentrations of [[calcium]] and [[sulfate]]. Scaling decreases membrane life and productivity.<ref>{{cite journal |last1=Uchymiak |first1=Michal |last2=Lyster |first2=Eric |last3=Glater |first3=Julius |last4=Cohen |first4=Yoram |title=Kinetics of gypsum crystal growth on a reverse osmosis membrane |journal=Journal of Membrane Science |date=April 2008 |volume=314 |issue=1–2 |pages=163–172 |doi=10.1016/j.memsci.2008.01.041}}</ref> This is one of the main obstacles in brackish water membrane desalination processes, such as [[reverse osmosis]] or [[nanofiltration]]. Other forms of scaling, such as [[calcite]] scaling, depending on the water source, can also be important considerations in [[distillation]], as well as in [[heat exchanger]]s, where either the salt [[solubility]] or [[concentration]] can change rapidly.

A new study has suggested that the formation of gypsum starts as tiny crystals of a mineral called [[bassanite]] (2CaSO<sub>4</sub>·H<sub>2</sub>O).<ref>{{Cite journal| last1 = Van Driessche| first1 = A.E.S.| first2 = L. G. | last2= Benning | first3= J. D. | last3= Rodriguez-Blanco| first4= M. | last4= Ossorio| first5= P. | last5= Bots | first6= J. M. | last6=  García-Ruiz| year = 2012| title = The role and implications of bassanite as a stable precursor phase to gypsum precipitation| journal = [[Science (journal)|Science]]| volume = 336| issue = 6077| pages = 69–72 | doi = 10.1126/science.1215648|bibcode = 2012Sci...336...69V| pmid=22491851| s2cid = 9355745}}</ref> This process occurs via a three-stage pathway: 
# homogeneous nucleation of nanocrystalline bassanite; 
# self-assembly of bassanite into aggregates, and 
# transformation of bassanite into gypsum.

=== Refinery waste ===
The production of [[phosphate]] fertilizers requires breaking down calcium-containing [[phosphate rock]] with acid, producing calcium sulfate waste known as [[phosphogypsum]] (PG). This form of gypsum is contaminated by impurities found in the rock, namely [[fluoride]], [[silica]], radioactive elements such as [[radium]], and heavy metal elements such as [[cadmium]].<ref name=Taylor>{{cite journal|doi=10.1016/j.jenvman.2009.03.007|pmid=19406560|title=Environmental Impact and Management of Phosphogypsum|journal=Journal of Environmental Management|volume=90|pages=2377–2386|year=2009|last1=Tayibi|first1= Hanan|last2=Choura|first2=Mohamed|last3=López|first3=Félix A.|last4=Alguacil|first4=Francisco J.|last5=López-Delgado|first5=Aurora|issue=8|bibcode=2009JEnvM..90.2377T |hdl=10261/45241|hdl-access=free}}</ref> Similarly, production of [[titanium dioxide]] produces titanium gypsum (TG) due to neutralization of excess acid with [[calcium hydroxide|lime]]. The product is contaminated with silica, fluorides, organic matters, and alkalis.<ref name="pmid26495867">{{cite journal |last1=Zhang |first1=Y |last2=Wang |first2=F |last3=Huang |first3=H |last4=Guo |first4=Y |last5=Li |first5=B |last6=Liu |first6=Y |last7=Chu |first7=PK |title=Gypsum blocks produced from TiO2 production by-products. |journal=Environmental Technology |date=2016 |volume=37 |issue=9 |pages=1094–100 |doi=10.1080/09593330.2015.1102329 |url=https://www1.cugb.edu.cn/uploadCms/file/20600/papers_upload/20161008164505966187.pdf |archive-url=https://web.archive.org/web/20220325023932/https://www1.cugb.edu.cn/uploadCms/file/20600/papers_upload/20161008164505966187.pdf |archive-date=2022-03-25 |url-status=live |pmid=26495867|bibcode=2016EnvTe..37.1094Z |s2cid=28458281 }}</ref>

Impurities in refinery gypsum waste have, in many cases, prevented them from being used as normal gypsum in fields such as construction. As a result, waste gypsum is stored in stacks indefinitely, with significant risk of leaching their contaminants into water and soil.<ref name=Taylor/> To reduce the accumulation and ultimately clear out these stacks, research is underway to find more applications for such waste products.<ref name="pmid26495867"/>