History of chlorine production and use
Chlorine bleach has a history that dates back to the late 1700’s.
In 1774, Swedish chemist Karl Wilhelm Scheele discovered the chemical element chlorine, a, greenyellow gas. Just over a decade later, the French scientist Claude Berthollet found that chlorine was an excellent whitening agent for fabrics.
Berthollet began a small facility in Javel, a town near Paris, for the manufacture of a new product called 'Eau de Javelle.' The product was a bleaching powder that consisted of potash (soda ash) which had absorbed chlorine gas.
In 1799, a slightly different bleaching powder was invented by Scottish chemist Charles Tennant, made by passing chlorine into a mixture of lime and water. Lime (composed of calcium oxides and calcium hydroxide) and water were spread thinly over the concrete or lead floor of a large room. Chlorine gas was pumped into the room to be absorbed by the lime. Though an effective whitener, the powder was chemically unstable. However, this product had the advantage of being cheaper than the one generally used at the time because it substituted lime for potash.
In the early years of the Industrial Revolution, this lime powder was widely used to whiten a variety of fabrics and paper products.
This was common bleach until around World War I, when liquid chlorine and sodium hypochlorite solutions—the forerunners of modern household bleach—were introduced.
Today, cal-hypo is produced in much the same way as it has been since 1799. On an industrial scale cal-hypo is made via the reaction of chlorine gas with calcium hydroxide.
2Ca(OH) 2 + 2Cl2 → Ca(ClO) 2
+ H2O + CaCl2
In 1890, a more effective bleach production method was invented by E.S. Smith which involved the electrolysis of salt solution to produce NaOH and Cl2 gas, which was then mixed together to form NaOCl. Smith found that injecting salt water with electrical current broke down the salt (sodium chloride) molecules and produced a compound called sodium hypochlorite. This discovery enabled the mass production of sodium hypochlorite, or chlorine, bleach.
Nowadays, the only large-scale industrial method for production of NaOCl is called the Hooker process. This is an improved version of Smith's original electrolysis process. In the Hooker process, chlorine gas is passed into cold dilute NaOH solution, forming NaOCl, with NaCl as the main byproduct. The reaction is driven to completion by electrolysis, and the mixture must be kept below 40°C to prevent the undesired formation of sodium chlorate.
Cl2 + 2 NaOH → NaCl + NaOCl + H2O Specifics of Sodium Hypochlorite Production Raw Materials
The raw materials for making household bleach are chlorine, caustic soda and water. The chlorine and caustic soda are produced by putting direct current electricity through a sodium chloride salt solution in a process called electrolysis. Sodium chloride, common table salt, comes from either mines or underground wells. The salt is dissolved in hot water to form a salt solution, which is then treated for impurities before it is reacted in the electrolytic cell.
The Manufacturing Process
Manufacturing sodium hypochlorite bleach requires several steps. All these steps can be carried out at one large manufacturing facility, or the chlorine and caustic soda can be shipped from different plants to the reactor site.
Preparing the components
Caustic soda is usually produced and shipped as a concentrated 50% solution. At its destination, this concentrated solution is diluted with water to form a new 25% solution.
When water dilutes the strong caustic soda solution, heat is created, so the diluted caustic soda is cooled before it is reacted.
If the chlorine is manufactured outside the reactor facility, it travels in liquid form in specially designed railroad tank cars with double walls that will not rupture in the event of a derailment. Upon arrival at the plant, the liquid chlorine is pumped from the railroad cars to a holding vat.
Inside the facility, chlorine vats are housed in an enclosed area called a car barn. This enclosed room is equipped with air 'scrubbers' to eliminate any escaped chlorine gas, which is harmful to humans and the environment. The vacuum-like scrubbers take any chlorine gas from the enclosed area and injects it with caustic soda. This turns it into bleach, which is incorporated into the manufacturing process.
The chemical reaction
Chlorine and the caustic soda solution are mixed to form sodium hypochlorite bleach. To make sodium hypochlorite, liquid or gaseous chlorine is circulated through the caustic soda solution. The reaction of chlorine and caustic soda is nearly immediate.
Cooling and purifying
The bleach solution is then cooled to help prevent decomposition.
This cooled bleach is filtered to remove impurities that can discolor the bleach or cause its decomposition.
The finished sodium hypochlorite bleach is shipped to a bottling plant or bottled on-site. It is also tested to make certain it contains the correct percentage of sodium hypochlorite. Householdstrength bleach is typically 5.25% sodium hypochlorite in an aqueous solution, while swimming pool bleach is typically 10-12%.
Sodium hypochlorite bleach was originally sold in steel containers and then in glass bottles.
The 1960s brought plastic jugs - a cheaper, lighter and non-breakable alternative. Furthermore, the thick plastic did not permit ultraviolet light to reach the bleach, which improved its chemical stability and effectiveness. Today, however, plastic containers have become an environmental concern because of the time it takes to decompose.
Many companies have begun to reduce the amount of plastic in their packaging or to use recycled plastics. In the early 1990s, Clorox introduced postconsumer resins (PCR) in its packaging. The newer bottles are a blend of virgin high-density polyethylene (HDPE) and 25% recycled plastic, primarily from clear milk jug-type bottles.