Towards Sustainable Batik
Published in the Batik Guild Magazine, March 2008
Soda Ash. Na2CO3. Sodium Carbonate. Washing Soda
Call it what you will, soda ash is a traditional chemical of our dyeing craft. As an alkali it produces optimal dyeing conditions by raising the pH (ie alkalinity) of fibre-reactive dye enabling cellulose fibre (eg cotton) to form a permanent bond with the dye. Cellulosics will not dye at low pH (ie acidic conditions), though silk can be dyed at high pH with fibre-reactive dyes or low pH with acid dyes.
But soda ash is no newfangled chemical - it has history. Before soda ash, potash (potassium carbonate) was the European alkali of choice, extracted from wood fire ash with water. From the second millennium BCE ashes of certain salt-tolerant plants were used to produce alkali, including saltworts, glassworts, mangroves and seaweeds. Plants were harvested, dried and burned. An alkali solution formed by washing the ashes with water was then filtered and boiled dry to produce impure soda ash. Sodium carbonate concentration varied greatly from around 3% in kelp ash to 30% in the best quality Spanish saltwort ash.
Soda ash, like potash, was used predominately in the making of glass and soap. It was in use in dyeing in Europe by AD 50. Spain was Europe's main source until the nineteenth century - its soda ash was known as barilla (from Spanish for 'saltwort'). The saltwort Salsola soda was a major cultivated species, hence the name 'soda ash'. In the eighteenth century Scotland, France, Ireland and Norway had major industries producing soda ash from seaweed (known as kelp) - at its peak Orkney was exporting 3000 tonnes of kelp ash annually. Glasswort was also used in France. But plant-derived soda ash couldn't keep up with eighteenth-century European demand, and chemists began researching artificial production.
Differences between potash and soda ash were recognised but not fully comprehended until 1807, when Cornish chemist Humphry Davy isolated a metal from molten wood ash (potash) which he called potassium, and another metal from molten soda ash which he named sodium. Sodium and potassium do not exist in a free state in nature but only in combination with another element - they are extremely reactive elements that react vigorously with water. Their relatively simple atomic structures give them strong bonding reactions but also they create molecules which can separate into ionic mode in water. Sodium ions can interact with other molecules and affect their chemical behaviour. With Procion MX dyes, sodium ions interfere in hydrolysis of the dye (dye bonding to water) and with chemical bonds on the cellulose fibres, encouraging the two to link up - to dye the cellulose.
LeBlanc Process
In 1791 French chemist Nicolas LeBlanc developed a process to make soda ash using sulphuric acid, limestone, salt and coal. Despite pollution problems created by the by-product hydrochloric acid the LeBlanc process remained the main production method until the late 1880s.
Solvay Process
French physicist A J Fresnel discovered in 1811 that carbon dioxide bubbled through brine containing ammonia created sodium bicarbonate (NaHCO3, baking soda) and from this sodium carbonate can be derived. The process was commercialised in 1864 by Belgian Ernest Solvay and by 1900 the Solvay process provided 90% of soda ash production. It remains the core procedure of synthetically-derived soda ash worldwide, including with Britain's sole producer Brunner Mond.
Intermediate chemical reactions and a flow diagram of the process shown below are on pages 21 and 23 respectively at www.cefic.be/files/Publications/ESAPA_Soda_Ash_Process_BREF3.pdf
CaCO3 → CO2 + CaO (1)
Limestone (calcium carbonate) is burned in a kiln to produce carbon dioxide and lime (calcium oxide).
CO2 + H2O + NaCl + NH3 → NaHCO3 + NH4Cl (2)
The carbon dioxide bubbles through brine (sodium chloride (salt) solution) containing dissolved ammonia. Sodium bicarbonate is precipitated, leaving ammonium chloride.
2 NaHCO3 → Na2CO3 + H2O +CO2 (3)
The sodium bicarbonate is heated and decomposes to soda ash (sodium carbonate), water and carbon dioxide. Carbon dioxide is captured and recycled through the ammoniacal brine solution (stage 2).
CaO + H2O → Ca(OH)2 (4)
The lime (calcium oxide) from stage 1 is hydrated in slakers to form milk of lime (calcium hydroxide).
2 NH4Cl + Ca(OH)2 → 2 NH3 + CaCl2 + 2 H2O (5)
Ammonium chloride is mixed with milk of lime to produce ammonia for recycling in stage (2). Calcium chloride is recovered from its dissolved state in wastewater using energy (mainly steam).
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The quality of the raw products has a major bearing on composition, volume and treatment of wastes. Each tonne (t) of soda ash requires 1.7 t salt, 1.4 t limestone, 2.8 t steam, 0.1 t coke (for limestone decomposition in kilns), and up to 0.8 t coal or energy source equivalent (for boilers and dryers); each tonne results in 1.7 t waste products including sodium chloride and calcium chloride. Ammonia is recoverable and recycled within the process with less than 1% ammonia being lost. More carbon dioxide is required than recovered - the extra is (usually) generated through combustion of coke to heat the limestone. Any surplus at this stage is lost to the atmosphere (or can be captured for localised sodium bicarbonate purification). Minor gaseous wastes are carbon monoxide and dust.
Brine purification and the distillation processes produce wastewater with chloride and other salt impurities. Where possible these are dispersed to the sea or high flow rivers where natural sediments are of a similar composition, otherwise the solids and liquid need physical separation. The liquid is discharged to a local watercourse (with any necessary pH balancing) while solids build up in a settling basin. Alternatively solids are deposited underground if production plants use locally-extracted salt and the system enables it. Recovery and re-use of the salt content consumes large quantities of both water for washing and energy for drying the material. The costs are considered prohibitively high, and the market for the main by-product calcium chloride is small. Calcium chloride has uses in oil exploration and drilling and in alginate production, as well as the refrigerant, food, pharmaceutical and chemical industries.
Trials of recovery of coarse solids in the distillation wastewater and experimentation for future uses such as soil amendment and cement manufacture have been made but not at commercial levels. Other waste outputs are impure solids from raw limestone and milk of lime. Limestone fines (small particles) can be used for road filling, cement manufacturing and civil engineering works as their composition is close to or the same as raw limestone. Grits from the slaking process can be recycled to the lime kiln or re-used as concrete filler or soil conditioner.
Hou or Dual Process
In the 1930s Chinese chemist Hou Debang developed an alternate version of the Solvay process, also known as the Dual process, that co-produces ammonium chloride. No limestone is used and no calcium chloride produced. The main raw material is salt (sodium chloride), with ammonia and carbon dioxide introduced rather than being recycled. After the resultant ammonium chloride is precipitated out, the remaining liquor is recycled to produce more sodium carbonate. The advantages of this process are lower effluent generation (no liquid is wasted) and better utilisation of salt, but ammonium chloride has a limited market even as an agricultural fertiliser. The Dual process is mostly used in Asia, close to fertiliser plants for availability of ammonia and carbon dioxide and where limestone sources are distant.
Natural Soda Ash - Natron and Trona
Natron is a naturally-occurring form of soda ash - sodium carbonate decahydrate (Na2CO3 · 10 H2O) - found in saline lake beds of arid areas. Sodium's chemical symbol Na and Latin name natrium are derived from 'natron', which itself is named after Wadi El Natrun in Egypt. Deposits here were harvested by Egyptians from the fourth millennium BCE for use in the production of steatite beads, and in glass production from the first millennium BCE. It was used as cleaner for home and body, and as drying and antibacterial agent in mummification. The Bible describes its use as scouring agent by fullers, who usually doubled as dyers. Natron is found and mined in several locations across the world.
Trona is another naturally-occurring mineral - sodium sesquicarbonate (Na3HCO3CO3 · 2 H2O) - found either underground or in dry lakes. The US has the world's largest resource in Wyoming, enough to supply the world for several hundred years. It was laid down 50 million years ago as a large lake in the Green River Basin, and has been mined since 1938 as a source of soda ash. Beypazari in Turkey has the world's second largest trona reserve allowing production for up to 100 years. Lake Magadi in the Kenyan Rift Valley is a 100 sq km saline, alkaline lake that is 80% covered by soda in the dry season. Along with neighbouring Lake Natron it's also famous for its flamingos and other waders. The Magadi Soda Factory has been dredging soda here for around 100 years. The source is considered renewable as the lake is recharged by saline hot springs flowing into it.
Soda ash production from trona is comparatively less costly than synthetic production. About 1.8 t of trona ore is needed to make 1.0 t of soda ash. One of two techniques used to refine soda ash, the monohydrate process is the most used today. Trona is crushed and calcined with heat to remove water and carbon dioxide, reducing the ore's weight by 27%. The remaining product is 85% sodium carbonate and 15% insoluble impurities. It is dissolved in hot water and then crystallised - sodium carbonate monohydrate crystals precipitate out at 40-100° C. Insolubles are filtered out and washed to recover additional alkali, before being piped to tailing ponds or used to fill abandoned sections of mines. The crystals are then dehydrated into anhydrous soda ash.
Brunner Mond
Brunner Mond (BM) was formed to produce soda ash in Northwich, Cheshire in 1873 by John Brunner and Ludwig Mond. The business grew, and in 1924 bought Magadi Soda Company of Kenya. It merged with three other companies to form ICI in 1926, but was re-created as Brunner Mond in 1991 through acquisition of UK and Kenyan businesses. The soda ash business of Dutch company Azko Nobel was added in 1998. BM was bought by Tata Chemicals of India in 2006. The combined company is now the world's third largest producer of soda ash.
Brunner Mond is ideally located for one of its raw materials - brine from Cheshire's salt deposits. Limestone is brought in by rail from Derbyshire. The UK's only remaining merchant coke producer, the Monckton Coke and Chemical Company in Royston, Yorkshire, is BM's sole coke supplier. Of BM's two Northwich soda ash plants, one takes out calcium chloride in the end process (to make a saleable product), while the other sends effluents to drain in a settling lagoon. Depleted salt extraction cavities also provide a source of disposal of treated residual sludge.
Both BM's Northwich sites are certified to ISO 14001 (2004) standards (Environmental Management Systems). These international standards indicate BM meets the requirements for "objective evidence which can be audited to demonstrate that the environmental management system is operating effectively in conformity to the standard" (www.iso.org/iso/iso_14000_essentials). (Their UK, Dutch and Kenyan plants are also certified to ISO 9001 (2000) standards (Quality Management).) Without doubt the cost of raw materials, landfill and waste taxes, and competition with lower-cost production from Eastern Europe, China and the US impels BM to make their plants as energy-efficient as possible. They have invested £130m in a combined heat and power facility in partnership with E.ON-UK serving both Northwich sites with steam and electricity, powered by natural gas. They have improved rail facilities for limestone transportation, and implemented computer control systems to enhance plant efficiency. The Netherlands and Kenya plants have similar investments.
Soda Ash Today
Around 0.9m tonnes of soda ash are manufactured annually in Britain by BM. In 2000, 42m tonnes were produced worldwide, more than half by the Solvay process and over a quarter from natural sources. Western European demand was around 18 kg per person (global average 7 kg). Half of all soda ash use is for glass manufacture. Other major users are detergent, chemical, pharmaceuticals, paper and pulp, metal refining, food and water treatment industries. Its widespread importance means its monthly production data is included in Federal Reserve Board monitoring of the state of the US economy.
Soda ash is a white odourless powder. It is stable, not flammable, explosive or toxic. Dust can irritate eyes, mucous membranes and respiratory tract - ensure adequate ventilation when using it. Three grades are available: BM's light and dense soda ash (both anhydrous), and soda crystals/washing soda (hydrous) made by Dri-Pak. Addition of water to light soda ash results in larger crystals. For dense soda ash the water is later driven off resulting in a more granular product, while soda crystals (sodium carbonate decahydrate) retain their water of crystallisation (37% soda ash, 63% water). For dyeing purposes, to achieve the same alkalinity value use the same weight or half the volume of dense compared to light soda ash, or around 2.7 times the weight of soda crystals compared to light soda ash. Fibrecrafts and Kemtex sell Brunner Mond's light soda ash.
Many thanks for comments on earlier drafts to Isabella Whitworth and especially to Ian Bowers of Fibrecrafts, and John McClean and Steve McNamara of Brunner Mond for scientific advice.
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Robin Paris, February 2008
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