Uses
A
major use of formic acid is as a preservative and antibacterial agent
in livestock feed. In Europe, it is applied on silage (including fresh hay) to promote the
fermentation of lactic
acid and to suppress the formation of butyric acid;
it also allows fermentation to occur quickly, and at a lower temperature,
reducing the loss of nutritional value.[4] Formic acid arrests
certain decay processes and causes the feed to retain its nutritive value longer,
and so it is widely used to preserve winter feed for cattle.[ In the poultryindustry,
it is sometimes added to feed to kill E. coli bacteria.[15][16] Use as preservative
for silage and (other) animal feed constituted 30% of the global consumption in
2009.[7]
Formic
acid is also significantly used in the production of leather, including tanning (23%
of the global consumption in 2009[7]), and in dyeing and
finishing of textile (9% of the global consumption in 2009[7]) because of its acidic
nature. Use as a coagulant in the production
of rubber[4] constituted in 2009
6% of the global consumption.[7]
Formic
acid is also used in place of mineral acids for various cleaning products,[4] such as limescale remover
and toilet
bowl cleaner. Some formate esters are artificial flavorings or
perfumes. Beekeepers use
formic acid as a miticide against
the tracheal mite (Acarapis
woodi) and the Varroa mite.[17] The use of formic
acid in fuel cells is also under investigation.[
Formic
acid is a source for a formyl group
for example in the formylation of
methylaniline to N-methylformanilide in toluene.[19] In synthetic organic chemistry,
formic acid is often used as a source of hydride ion.
The Eschweiler-Clarke
reaction and the Leuckart-Wallach
reaction are examples of this application. It,
or more commonly its azeotrope withtriethylamine,
is also used as a source of hydrogen in transfer
hydrogenation.
Like acetic acid and trifluoroacetic
acid, formic acid is commonly used as a volatile
pH modifier in HPLC and capillary
electrophoresis.
As
mentioned below,
formic acid may serve as a convenient source of carbon monoxide by
being readily decomposed by concentrated sulfuric acid.
CH2O2(l) +
H2SO4(l) →
H2SO4(l) +
H2O(l) +
CO(g)
Formic
acid has been shown to be an effective treatment against warts[20] and is marketed for
that purpose by Meda
AB under the trade name Vårtfri in Sweden
and Endwarts internationally.[21]
Prepared by:
M.Temoor Rafiq
M.Rashid shehzad
Applications
Hydrochloric
acid is a strong inorganic acid that is used in many industrial processes such
as refining metal. The application often determines the required product
quality.[7]
Pickling of steel
One
of the most important applications of hydrochloric acid is in the pickling of
steel, to remove rust or iron oxide scale
from iron or steel before subsequent processing, such asextrusion, rolling, galvanizing,
and other techniques.[7][20] Technical quality HCl
at typically 18% concentration is the most commonly used pickling agent for the
pickling of carbon
steel grades.
Fe2O3 +
Fe + 6 HCl → 3 FeCl2 + 3 H2O
The spent acid has
long been re-used as iron(II) chloride (also
known as ferrous chloride) solutions, but high heavy-metal levels
in the pickling liquor have decreased this practice.
The
steel pickling industry has developed hydrochloric acid regeneration processes,
such as the spray roaster or the fluidized bed HCl regeneration process, which
allow the recovery of HCl from spent pickling liquor. The most common
regeneration process is the pyrohydrolysis process, applying the following
formula:[7]
4
FeCl2 +
4 H2O +
O2 →
8 HCl+ 2 Fe2O3
By
recuperation of the spent acid, a closed acid loop is established.[20] The iron(III) oxide by-product
of the regeneration process is valuable, used in a variety of secondary
industries.[7]
Production of organic
compounds
Another
major use of hydrochloric acid is in the production of organic compounds,
such as vinyl
chloride and dichloroethane for PVC.
This is often captive use, consuming locally produced hydrochloric acid that
never actually reaches the open market. Other organic compounds
produced with hydrochloric acid include bisphenol A for polycarbonate,activated carbon,
and ascorbic
acid, as well as numerous pharmaceutical products.[20]
Production of inorganic
compounds
Numerous
products can be produced with hydrochloric acid in normal acid-base
reactions, resulting in inorganic compounds.
These include water treatment chemicals such asiron(III)
chloride and polyaluminium
chloride (PAC).
Both
iron(III) chloride and PAC are used as flocculation and
coagulation agents in sewage treatment, drinking water production,
and paper production.
Other
inorganic compounds produced with hydrochloric acid include road application
salt calcium
chloride, nickel(II)
chloride for electroplating,
and zinc
chloride for thegalvanizing industry
and battery production.[20]
OH− +
HCl → H2O +
Cl−
In
industry demanding purity (food, pharmaceutical, drinking water), high-quality
hydrochloric acid is used to control the pH of process water streams. In
less-demanding industry, technical quality hydrochloric acid suffices for neutralizing waste
streams and swimming pool treatment.[20]
High-quality hydrochloric acid is used in the
regeneration of ion exchange resins. Cation exchange is widely used to remove ions such as Na+ and Ca2+ from aqueous solutions, producing demineralized water. The acid is used to rinse the cations from
the resins.[7] Na+ is
replaced with H+ and
Ca2+ with
2 H+.
Ion exchangers and demineralized water are
used in all chemical industries, drinking water production, and many food
industries.[7]
Hydrochloric acid is used for a large number
of small-scale applications, such as leather processing, purification of common
salt, household cleaning,[29] and building construction.[20] Oil production may be stimulated by injecting hydrochloric acid
into the rock formation of an oil well, dissolving a portion of the rock, and creating a
large-pore structure. Oil well acidizing is a common process in the North Sea oil production industry.[7]
Hydrochloric acid has been used for
dissolving calcium carbonate, i.e. such things as de-scaling kettles and for
cleaning mortar off brickwork, but it is a hazardous liquid which must be used
with care. When used on brickwork the reaction with the mortar only continues
until the acid has all been converted, producing Calcium Chloride, Carbon Dioxideand water:
2HCl + CaCO3 → CaCl2 + CO2 + H2O
2HCl + CaCO3 → CaCl2 + CO2 + H2O
Many chemical reactions involving
hydrochloric acid are applied in the production of food, food ingredients,
and food
additives. Typical products include aspartame, fructose, citric acid, lysine, hydrolyzed vegetable protein as food enhancer, and in gelatin production. Food-grade (extra-pure) hydrochloric
acid can be applied when needed for the final product.[7][20]
Uses
Sodium
hydroxide is a popular strong base used
in the industry. Around 56% of sodium hydroxide produced is used by the
industry, 25% of which is used in paper industry. Sodium hydroxide is also used
in manufacturing of sodium salts and detergents, pH regulation, and organic
synthesis. It is used in the Bayer process of aluminium production.[9] In bulk, it is most
often handled as an aqueous solution,[13]since solutions are cheaper
and easier to handle.
Sodium
hydroxide is used in many scenarios where it is desirable to increase the
alkalinity of a mixture, or to neutralize acids.
For
example, in petroleum industry, sodium hydroxide is used as an additive
in drilling
mud to increase alkalinity in bentonite mud
systems, to increase the mud viscosity,
and to neutralise any acid gas (such
as hydrogen
sulfide and carbon dioxide)
which may be encountered in the geological
formation as drilling progresses.
Poor
quality crude
oil can be treated with sodium hydroxide to
remove sulfurous impurities
in a process known as caustic washing. As above, sodium hydroxide
reacts with weak acids such as hydrogen sulfide and mercaptans to give the
non-volatile sodium salts which can be removed. The waste which is formed is
toxic and difficult to deal with, and the process is banned in many countries
because of this. In 2006, Trafigura used
the process and then dumped the waste in Africa.[14][15]
Sodium
hydroxide is also widely used in pulping of wood for making paper or
regenerated fibers. Along with sodium sulfide,
sodium hydroxide is a key component of the white liquor solution used to
separate lignin from cellulose fibers in the kraft process.
It also plays a key role in several later stages of the process of bleaching
the brown pulp resulting from the pulping process.
These stages include oxygen delignification, oxidative extraction,
and simple extraction, all of which require a strong alkaline environment with
a pH > 10.5 at the end of the stages.
Tissue digestion
In
a similar fashion, sodium hydroxide is used to digest tissues, such as in a
process that was used with farm animals at one time. This process involved
placing a carcass into a sealed chamber, then adding a mixture of sodium
hydroxide and water (which breaks the chemical bonds that keep the flesh
intact). This eventually turns the body into a liquid with coffee-like
appearance,[16][17] and the only solid
that remains are bone hulls, which could be crushed between one's fingertips.[18] Sodium hydroxide is
frequently used in the process of decomposing roadkill dumped
in landfills by animal disposal contractors.[17] Due to its low cost
and availability, it has been used to dispose of corpses by criminals.Italian serial killer Leonarda
Cianciulli used this chemical to turn dead bodies
into soap.[19] In Mexico, a man who
worked for drug cartels admitted disposing over 300 bodies with it.[20]
Strong
bases attack aluminium.
Sodium hydroxide reacts with aluminium and water to release hydrogen gas. The
aluminium takes the oxygen atom from sodium hydroxide, which in turn takes the
oxygen atom from the water, and releases the two hydrogen atoms, The reaction
thus produces hydrogen gas
and sodium
aluminate. In this reaction, sodium hydroxide acts as
an agent to make the solution alkaline, which aluminium can dissolve in. This
reaction can be useful in etching, removing anodizing, or converting a polished
surface to a satin-like finish, but without further passivation such
as anodizing or alodining the
surface may become degraded, either under normal use or in severe atmospheric conditions.
In
the Bayer
process, sodium hydroxide is used in the refining
of alumina containing
ores (bauxite)
to produce alumina (aluminium oxide) which is the raw material used to
produce aluminium metal
via the electrolytic Hall-Héroult
process. Since the alumina is amphoteric,
it dissolves in the sodium hydroxide, leaving impurities less soluble at high
pH such as iron
oxides behind in the form of a highly
alkaline red
mud.
Other
amphoteric metals are zinc and lead which dissolve in concentrated sodium
hydroxide solutions to give sodium zincate and
sodium plumbate respectively.
Esterification and
transesterification reagent
Sodium
hydroxide is traditionally used in soap making (cold process soap, saponification).[21] It was made in the
nineteenth century for a hard surface rather than liquid product because it was
easier to store and transport.
For
the manufacture of biodiesel,
sodium hydroxide is used as a catalyst for
the transesterification of
methanol and triglycerides. This only works with anhydrous sodium
hydroxide, because combined with water the fat would turn into soap, which would be tainted with methanol.
NaOH is used more often than potassium
hydroxide because it is cheaper and a smaller
quantity is needed.
Sodium
hydroxide is also being used experimentally in a new technology to create
synthetic gasoline.[22]
Food preparation
Food
uses of sodium hydroxide include washing or chemical peeling of fruits and vegetables, chocolate and cocoa processing, caramel coloring production, poultry scalding, soft drink processing,
and thickening ice
cream. Olives are often soaked in sodium hydroxide
for softening; Pretzels and German lye rolls are
glazed with a sodium hydroxide solution before baking to make them crisp. Owing
to the difficulty in obtaining food grade sodium hydroxide in small quantities
for home use, sodium carbonate is
often used in place of sodium hydroxide.[23]
Specific
foods processed with sodium hydroxide include:
·
Hominy is
dried maize (corn)
kernels reconstituted by soaking in lye-water. These expand considerably in size and
may be further processed by frying to make corn nuts or
by drying and grinding to make grits. Nixtamal is
similar, but uses calcium hydroxide instead
of sodium hydroxide.
·
Sodium hydroxide is also the chemical that
causes gelling of egg whites in the production of Century eggs.
·
German pretzels are
poached in a boiling sodium carbonate solution
or cold sodium hydroxide solution before baking, which contributes to their
unique crust.
·
Lye-water is an essential ingredient in the
crust of the traditional baked Chinese moon cakes.
·
Most yellow coloured Chinese noodles are
made with lye-water but are commonly
mistaken for containing egg.
·
The Filipino dessert (kakanin)
called kutsinta uses
a bit of lye water to help give the rice flour batter a jelly like consistency.
A similar process is also used in the kakanin known as pitsi-pitsi or pichi-pichi except
that the mixture uses grated cassava instead
of rice flour.
Cleaning agent
Sodium
hydroxide is frequently used as an industrial cleaning agent where
it is often called "caustic". It is added to water, heated, and then
used to clean process equipment, storage tanks, etc. It can dissolve grease, oils, fats and protein based
deposits. It is also used for cleaning waste discharge pipes under sinks and
drains in domestic properties.Surfactants can
be added to the sodium hydroxide solution in order to stabilize dissolved
substances and thus prevent redeposition. A sodium hydroxide soak solution is
used as a powerful degreaser on stainless steel and
glass bakeware. It is also a common ingredient in oven cleaners.
A
common use of sodium hydroxide is in the production of parts washer detergents.
Parts washer detergents based on sodium hydroxide are some of the most
aggressive parts washer cleaning chemicals. The sodium hydroxide based
detergent include surfactants, rust inhibitors and defoamers. A parts washer
heats water and the detergent in a closed cabinet and then sprays the heated
sodium hydroxide and hot water at pressure against dirty parts for degreasing
applications. Sodium hydroxide used in this manner replaced many solvent based
systems in the early 1990s[citation
needed] when trichloroethane was
outlawed by the Montreal Protocol.
Water and sodium hydroxide detergent based parts washers are considered to be
an environmental improvement over the solvent based cleaning methods.
Paint stripping with caustic soda
Sodium
hydroxide is used in the home as a type of drain
opener to unblock clogged drains, usually in
the form of a dry crystal or as a thick liquid gel. The alkali dissolves greases to produce water soluble products.
It alsohydrolyzes the proteins such
as those found in hair which
may block water pipes. These reactions are sped by theheat generated when
sodium hydroxide and the other chemical components of the cleaner dissolve in
water. Suchalkaline
drain cleaners and their acidic
versions are highly corrosive and
should be handled with great caution.
Sodium
hydroxide is used in some relaxers to straighten hair.
However, because of the high incidence and intensity of chemical burns,
manufacturers of chemical relaxers use other alkaline chemicals in preparations
available to average consumers. Sodium hydroxide relaxers are still available,
but they are used mostly by professionals.
A
solution of sodium hydroxide in water was traditionally used as the most common
paint stripper on wooden objects. Its use has become less common, because it
can damage the wood surface, raising the grain and staining the colour.
Historical uses
Sodium
hydroxide has been used for detection of carbon
monoxide poisoning, with blood samples of such patients turning
to a vermilion color
upon the addition of a few drops of sodium hydroxide.[25] Today, carbon
monoxide poisoning can be detected by CO oximetry.
The
standard first aid measures for alkali spills on the skin is, as for other
corrosives, irrigation with large quantities of water. Washing is continued for
at least ten to fifteen minutes.
Sodium
hydroxide is corrosive to several metals, like aluminium which
reacts with the alkali to produce flammable hydrogen gas
on contact:[26]
2
Al + 2 NaOH + 2 H2O → 3 H2 + 2 NaAlO2
2
Al + 6 NaOH + x H2O → 3 H2 + 2 Na3AlO3 +
x H2O
2
Al + 2 NaOH + 6 H2O → 3 H2 + 2 NaAl(OH)4
Sodium
hydroxide is also mildly corrosive to glass, which can cause damage to glazing or
freezing of ground
glass joints. Careful storage is needed.
Uses
Nitric acid in a laboratory.
The
main industrial use of nitric acid is for the production of fertilizers.
Nitric acid is neutralized with ammonia to giveammonium nitrate.
This application consumes 75–80% of the 26M tons produced annually (1987). The
other main applications are for the production of explosives, nylon precursors,
and specialty organic compounds.[13]
Precursor to organic
nitrogen compounds
In organic synthesis,
industrial and otherwise, the nitro group is a versatile functionality. Most
derivatives of aniline are
prepared via nitration of aromatic compounds followed
by reduction. Nitrations entail combining nitric and sulfuric acids to generate
the nitronium
ion, which electrophilically reacts with
aromatic compounds such as benzene.
Many explosives, e.g.TNT, are prepared in this way.
The
precursor to nylon, adipic acid,
is produced on a large scale by oxidation of cyclohexanone and cyclohexanol with
nitric acid.[13]
Rocket propellant
Nitric
acid has been used in various forms as the oxidizer in liquid-fueled
rockets. These forms include red fuming nitric acid,
white fuming nitric acid, mixtures with sulfuric acid, and these forms with HF
inhibitor.[14] IRFNA
(inhibited red
fuming nitric acid) was one of 3 liquid fuel components for
the BOMARC missile.[15]
Woodworking
In
a low concentration (approximately 10%), nitric acid is often used to
artificially age pine and maple. The color produced is a grey-gold very much
like very old wax or oil finished wood (wood finishing).[18]
Etchant and cleaning agent
The
corrosive effects of nitric acid are exploited for a number of specialty
applications, such as pickling stainless steel. A
solution of nitric acid, water and alcohol, Nital, is used for etching of metals to reveal the
microstructure. ISO 14104 is one of the standards detailing this well known
procedure.
Commercially
available aqueous blends of 5–30% nitric acid and 15–40% phosphoric acid are
commonly used for cleaning food and dairy equipment primarily to remove
precipitated calcium and magnesium compounds (either deposited from the process
stream or resulting from the use of hard water during production and cleaning).
The phosphoric acid content helps to passivate ferrous alloys against
corrosion by the dilute nitric acid.[citation
needed]
Nitric
acid can be used as a spot test for alkaloids like LSD, giving a variety of colours depending on
the alkaloid.[19]
Applications
About
25% of produced oxalic acid is used as a mordant in
dyeing processes. It is used in bleaches,
especially for pulpwood.
It is also used in baking powder.[5]
Cleaning
Oxalic
acid's main applications include cleaning or bleaching, especially for the
removal of rust (iron complexing agent). Bar
Keepers Friend is an example of a household cleaner
containing oxalic acid. Its utility in rust removal agents is due to its
forming a stable, water soluble salt with ferric iron, ferrioxalate ion.
Extractive metallurgy
Oxalic
acid is an important reagent in lanthanide chemistry.
Hydrated lanthanide oxalates form readily in strongly acidic solutions in a
densely crystalline,
easily filtered form, largely free of contamination by nonlanthanide elements.
Thermal decomposition of these oxalate gives the oxides, which is the most commonly marketed form of
these elements.
Niche uses
Vaporized
oxalic acid, or a 3.2% solution of oxalic acid in sugar syrup, is used by
some beekeepers as
a miticide against
the parasitic varroa mite.[citation needed]
Oxalic
acid is rubbed onto completed marble sculptures to seal the surface and
introduce a shine. Oxalic acid is also used to clean iron and manganese
deposits from quartzcrystals.[17][18]
Oxalic
acid is used as a bleach for wood, removing black stains caused by water
penetration.
Content in food items
Uses
The
manufacture of glass is
one of the most important uses of sodium carbonate. Sodium carbonate acts as
a flux for silica, lowering the melting point of the mixture
to something achievable without special materials. This "soda glass"
is mildly water soluble, so somecalcium carbonate is
added to the pre-melt mixture to make the glass produced insoluble. This type
of glass is known as soda lime glass:
"soda" for the sodium carbonate and "lime" for the calcium
carbonate. Soda lime glass has been the most common form of glass for
centuries.
Sodium
carbonate is also used as a relatively strong base in
various settings. For example, sodium carbonate is used as a pHregulator to maintain stable alkaline
conditions necessary for the action of the majority of photographic film
developing agents.
It
is a common additive in municipal pools used to neutralize the corrosive
effects of chlorine and
raise the pH.
In
cooking, it is sometimes used in place of sodium hydroxide for lyeing, especially with German pretzels and
lye rolls. These dishes are treated with a solution of an alkaline substance to
change the pH of the surface of the food and improve browning.
In taxidermy,
sodium carbonate added to boiling water will remove flesh from the skull or
bones of trophies to create the "European skull mount" or for
educational display in biological and historical studies.
In
chemistry, it is often used as an electrolyte.
This is because electrolytes are usually salt-based, and sodium carbonate acts
as a very good conductor in the process of electrolysis. In addition, unlike
chloride ions, which form chlorine gas, carbonate ions are not corrosive to the
anodes. It is also used as a primary standard for acid-base titrations because
it is solid and air-stable, making it easy to weigh accurately.
Domestic use
In
domestic use, it is used as a water softener in laundering. It competes with
the magnesium and calcium ions in hard water and prevents them from bonding
with the detergent being used. Sodium carbonate can be used to remove grease,
oil and wine stains. Sodium carbonate is also used as a descaling agent
in boilers such as those found in coffee pots and espresso machines.
In dyeing with fiber-reactive dyes, sodium
carbonate (often under a name such as soda ash fixative or soda ash activator)
is used to ensure proper chemical bonding of the dye with cellulose (plant)
fibers, typically before dyeing (for tie dyes), mixed with the dye (for dye
painting), or after dyeing (for immersion dyeing).
Other applications
Sodium
carbonate is a food additive (E500) used as an acidity regulator, anti-caking
agent, raising agent, and stabilizer. It is one of the components of kansui a
solution of alkaline salts used to give ramen noodles their characteristic flavor and
texture. It is also used in the production of snus (Swedish-style snuff)
to stabilize the pH of the final product. In Sweden, snus is
regulated as a food product because it is put into the mouth, requires
pasteurization, and contains only ingredients that are approved as food
additives.
Sodium
carbonate is also used in the production of sherbet powder.
The cooling and fizzing sensation results from the endothermic reaction between
sodium carbonate and a weak acid, commonly citric acid,
releasing carbon dioxide gas, which occurs when the sherbet is moistened by
saliva.
In
China, it is used to replace lye-water in the crust of traditional
Cantonese moon
cakes, and in many other Chinese steamed buns and
noodles.
Sodium
carbonate is used by the brick industry as a wetting agent to reduce the amount
of water needed to extrude the clay.
In
casting, it is referred to as "bonding agent" and is used to allow
wet alginate to
adhere to gelled alginate.
Sodium
carbonate is used in toothpastes, where it acts as a foaming agent and an
abrasive, and to temporarily increase mouth pH.
Sodium
carbonate is used by the cotton industry to neutralize the sulfuric acid need
for acid de-linting of fuzzy cottonseed.
Sodium
carbonate, in a solution with common salt, may be used for cleaning silver. In
a non-reactive container (glass, plastic or ceramic) aluminium foil and the
silver object are immersed in the hot salt solution. The elevated pH dissolves
the aluminium oxide layer on the foil and enables an electrolytic cell to be
established. Hydrogen ions produced by this reaction reduce the sulfide ions on
the silver restoring silver metal. The sulfide can be released as small amounts
of hydrogen sulfide. Rinsing and gently polishing the silver restores a highly
polished condition.
Carbonation
Carbonation or fizz is
the process of dissolving carbon dioxide in a liquid. The process usually involves carbon dioxide under high
pressure. When the pressure is reduced, the carbon dioxide is released from the
solution as small bubbles, which causes the solution to become effervescent, or fizzy. An example of carbonation is the dissolving
of carbon dioxide in water,
resulting in carbonated water.
·
Chemistry
Carbon dioxide is weakly soluble in water, therefore it
separates into a gas when
the pressure is released.
Biochemistry
Carbonation also describes the incorporation of carbon
dioxide into chemical compounds. Our carbon-based life originates from a
carbonation reaction that is most often catalysed by the enzyme RuBisCO. So important is this carbonation process that a
significant fraction of leaf mass
consists of this carbonating enzyme.[1]
Carbonation
of ribulose bisphosphate is the starting point of the incorporation of carbon
dioxide into the biosphere
Drying
Drying is
a mass transfer process
consisting of the removal of water or another solvent[1] by evaporation from a solid, semi-solid or liquid. This process is often used as a final production step
before selling or packaging products. To be considered "dried", the
final product must be solid, in the form of a continuous sheet (e.g., paper),
long pieces (e.g., wood), particles (e.g., cereal grains or corn flakes) or
powder (e.g., sand, salt, washing powder, milk powder). A source of heat and an agent to remove the vapor produced by the
process are often involved. In bioproducts like food, grains, and pharmaceuticals like vaccines, the solvent to be removed is almost invariably water.
In the most common case, a gas stream, e.g., air, applies
the heat by convection and carries away the vapor as humidity. Other possibilities are vacuum drying, where heat is supplied by conduction or radiation (or microwaves), while the vapor thus produced is removed by the vacuum system. Another indirect technique is drum drying (used, for instance, for manufacturing potato
flakes), where a heated surface is used to provide the energy, and aspirators
draw the vapor outside the room. In contrast, the mechanical extraction of the
solvent, e.g., water, by centrifugation, is not considered "drying" but rather
"draining".
Drying mechanism
In some products having a relatively high initial
moisture content, an initial linear reduction of the average product moisture
content as a function of time may be observed for a limited time, often known
as a "constant drying rate period". Usually, in this period, it is
surface moisture outside individual particles that is being removed. The drying
rate during this period is mostly dependent on the rate of heat transfer to the
material being dried. Therefore, the maximum achievable drying rate is
considered to be heat-transfer limited. If drying is continued, the slope of
the curve, the drying rate, becomes less steep (falling rate period) and
eventually tends to nearly horizontal at very long times. The product moisture
content is then constant at the "equilibrium moisture content", where it is, in practice, in equilibrium with the
dehydrating medium. In the falling-rate period, water migration from the
product interior to the surface is mostly by molecular diffusion, i,e. the
water flux is proportional to the moisture content gradient. This means that
water moves from zones with higher moisture content to zones with lower values,
a phenomenon explained by the second law of thermodynamics. If water removal is considerable, the products usually
undergo shrinkage and deformation, except in a well-designed freeze-drying
process. The drying rate in the falling-rate period is controlled by the rate
of removal of moisture or solvent from the interior of the solid being dried
and is referred to as being "mass-transfer limited".
Methods of drying
In a typical phase diagram, the boundary between gas and liquid runs from the
triple point to the critical point. Regular drying is the green arrow, while supercritical drying is the red arrow and freeze drying is the blue.
The following are some general methods of drying:
·
Application
of hot air (convective or
direct drying). Air heating increases the driving force for heat transfer and
accelerates drying. It also reduces air relative
humidity, further increasing the
driving force for drying. In the falling rate period, as moisture content
falls, the solids heat up and the higher temperatures speed up diffusion of
water from the interior of the solid to the surface. However, product quality
considerations limit the applicable rise to air temperature. Excessively hot
air can almost completely dehydrate the solid surface, so that its pores shrink
and almost close, leading to crust formation or "case hardening",
which is usually undesirable. For instance in wood (timber) drying, air is
heated (which speeds up drying) though some steam is also added to it (which
hinders drying rate to a certain extent) in order to avoid excessive surface
dehydration and product deformation owing to high moisture gradients across
timber thickness. Spray drying belongs in this category.
·
Indirect
or contact drying (heating through a hot wall), as drum drying, vacuum drying.
Again, higher wall temperatures will speed up drying but this is limited by
product degradation or case-hardening. Drum drying belongs in this category.
·
Dielectric
drying (radiofrequency or microwaves being absorbed inside the material) is the
focus of intense research nowadays. It may be used to assist air drying or
vacuum drying. Researchers have found that microwave finish drying speeds up
the otherwise very low drying rate at the end of the classical drying methods.
·
Freeze drying or lyophilization is a drying method where the
solvent is frozen prior to drying and is then sublimed, i.e., passed to the gas phase directly from the solid
phase, below the melting point of the solvent. It is increasingly applied to
dry foods, beyond its already classical pharmaceutical or medical applications.
It keeps biological properties of proteins, and retains vitamins and bioactive
compounds. Pressure can be reduced by a high vacuum pump (though freeze drying
at atmospheric pressure is possible in dry air). If using a vacuum pump, the
vapor produced by sublimation is removed from the system by converting it into
ice in a condenser, operating at very low temperatures, outside the freeze
drying chamber.
·
Supercritical drying (superheated steam drying) involves steam drying of
products containing water. This process is feasible because water in the
product is boiled off, and joined with the drying medium, increasing its flow.
It is usually employed in closed circuit and allows a proportion of latent heat
to be recovered by recompression, a feature which is not possible with
conventional air drying, for instance. The process has potential for use in
foods if carried out at reduced pressure, to lower the boiling point.
·
Natural air
drying takes place when
materials are dried with unheated forced air, taking advantage of its natural
drying potential. The process is slow and weather-dependent, so a wise strategy
"fan off-fan on" must be devised considering the following
conditions: Air temperature, relative humidity and moisture content and
temperature of the material being dried. Grains are increasingly dried with
this technique, and the total time (including fan off and on periods) may last
from one week to various months, if a winter rest can be tolerated in cold
areas.
Applications of drying
Foods are dried to inhibit microbial development and
quality decay. However, the extent of drying depends on product end-use.
Cereals and oilseeds are dried after harvest to the moisture content that
allows microbial stability during storage. Vegetables are blanched before
drying to avoid rapid darkening, and drying is not only carried out to inhibit
microbial growth, but also to avoid browning during storage. Concerning dried
fruits, the reduction of moisture acts in combination with its acid and sugar
contents to provide protection against microbial growth. Products such as milk
powder must be dried to very low moisture contents in order to ensure
flowability and avoid caking. This moisture is lower than that required to
ensure inhibition to microbial development. Other products as crackers are
dried beyond the microbial growth threshold to confer a crispy texture, which
is liked by consumers.
Among Non-food products, those that require considerable
drying are wood (as
part of Timber processing), paper and washing powder. The first two, owing to
their organic origins, may develop mold if insufficiently dried. Another
benefit of drying is a reduction in volume and weight.
Evaporation
Evaporation is
a type of vaporization of
a liquid that occurs from the surface of
a liquid into a gaseous phase that is not saturated with the evaporating
substance. The other type of vaporization is boiling, which is characterized by bubbles of saturated vapor
forming in the liquid phase. Steam produced in a boiler is another example of
evaporation occurring in a saturated vapor phase. Evaporation that occurs
directly from the solid phase below the melting point, as commonly observed
with ice at or below freezing or moth crystals (napthalene or
paradichlorobenzine), is called sublimation.
On average, a fraction of the molecules in a glass of
water have enough heat energy to escape from the liquid. Water molecules from
the air enter the water in the glass, but as long as the relative humidity of
the air in contact is less than 100% (saturation), the net transfer of water
molecules will be to the air. The water in the glass will be cooled by the
evaporation until an equilibrium is reached where the air supplies the amount
of heat removed by the evaporating water. In an enclosed environment the water
would evaporate until the air is saturated.
With sufficient temperature, the liquid would turn into
vapor quickly (see boiling point). When the molecules collide, they transfer energy to
each other in varying degrees, based on how they collide. Sometimes the
transfer is so one-sided for a molecule near the surface that it ends up with
enough energy to 'escape'.
Evaporation is an essential part of the water cycle. The sun (solar energy) drives evaporation of water from
oceans, lakes, moisture in
the soil, and other sources of water. In hydrology, evaporation and transpiration (which involves evaporation within plant stomata) are collectively termed evapotranspiration. Evaporation of water occurs when the surface of the
liquid is exposed, allowing molecules to escape and form water vapor; this
vapor can then rise up and form clouds.
Theory
For molecules of a liquid to evaporate, they must be located near
the surface, be moving in the proper direction, and have sufficient kinetic energy to overcome liquid-phase intermolecular forces.[1] When only a small proportion of the molecules meet
these criteria, the rate of evaporation is low. Since the kinetic energy of a
molecule is proportional to its temperature, evaporation proceeds more quickly
at higher temperatures. As the faster-moving molecules escape, the remaining
molecules have lower average kinetic energy, and the temperature of the liquid
decreases. This phenomenon is also called evaporative cooling. This is why evaporating sweat cools the human body. Evaporation also tends to
proceed more quickly with higher flow rates between the gaseous and liquid
phase and in liquids with higher vapor pressure. For example, laundry on a clothes line will dry (by
evaporation) more rapidly on a windy day than on a still day. Three key parts
to evaporation are heat, atmospheric pressure (determines the percent humidity) and air movement.
On a molecular level, there is no strict boundary between
the liquid state and the vapor state. Instead, there is a Knudsen layer, where the phase is undetermined. Because this layer is
only a few molecules thick, at a macroscopic scale a clear phase transition
interface can be seen.
Liquids that do not evaporate visibly at a given
temperature in a given gas (e.g., cooking oil at room temperature) have molecules that do not tend to transfer energy to
each other in a pattern sufficient to frequently give a molecule the heat
energy necessary to turn into vapor. However, these liquids are evaporating.
It is just that the process is much slower and thus significantly less visible.
Evaporative equilibrium
If evaporation takes place in enclosed area, the escaping
molecules accumulate as a vapor above the liquid. Many of the moleculesreturn to the liquid, with returning molecules becoming
more frequent as the density and pressure of the vapor increases. When the process of escape
and return reaches an equilibrium,[1] the vapor is said to be "saturated", and
no further change in either vapor pressure and density or liquid temperature will occur. For a
system consisting of vapor and liquid of a pure substance, this equilibrium
state is directly related to the vapor pressure of the substance, as given by
the Clausius–Clapeyron relation:
where P1, P2 are the vapor pressures at temperatures T1, T2 respectively, ΔHvap is the enthalpy of vaporization, and R is the universal gas constant. The rate of evaporation in an open system is related to the vapor pressure found in a closed system. If a liquid is heated, when the vapor pressure reaches the ambient pressure the liquid will boil.
The ability for a molecule of a liquid to evaporate is
based largely on the amount of kinetic energy an individual particle may possess. Even at lower
temperatures, individual molecules of a liquid can evaporate if they have more
than the minimum amount of kinetic energy required for vaporization.
Factors influencing the rate of evaporation
Note: Air used here is a common example; however, the
vapor phase can be other gasses.
Concentration of the substance evaporating in the air
If the air already has a high concentration of the
substance evaporating, then the given substance will evaporate more slowly.
Concentration of other substances in the air
If the air is already saturated with other substances, it
can have a lower capacity for the substance evaporating.[citation needed]
Flow rate of air
This is in part related to the concentration points
above. If "fresh" air (i.e., air which is neither already saturated
with the substance nor with other substances) is moving over the substance all
the time, then the concentration of the substance in the air is less likely to
go up with time, thus encouraging faster evaporation. This is the result of theboundary layer at the evaporation surface decreasing with flow
velocity, decreasing the diffusion distance in the stagnant layer.
Inter-molecular forces
The stronger the forces keeping the molecules together in
the liquid state, the more energy one must get to escape. This is characterized
by the enthalpy of vaporization.
Evaporation happens faster if there is less exertion on
the surface keeping the molecules from launching themselves.
A substance that has a larger surface area will evaporate
faster, as there are more surface molecules per unit of volume that are
potentially able to escape.
Temperature of the substance
the higher the temperature of the substance the greater
the kinetic energy of the molecules at its surface and therefore the faster the
rate of their evaporation.
In the
US, the National Weather Service measures the actual rate of evaporation from a
standardized "pan" open water surface outdoors, at various locations
nationwide. Others do likewise around the world. The US data is collected and
compiled into an annual evaporation map. The measurements range from under 30
to over 120 inches (3,000 mm) per year.
Thermodynamics
Applications
Industrial
applications include many printing and coating processes;
recovering salts from solutions; and drying a variety of materials such as
lumber, paper, cloth and chemicals.
When
clothes are hung on a laundry line, even though the ambient temperature is
below the boiling point of water, water evaporates. This is accelerated by
factors such as low humidity, heat (from the sun), and wind. In a clothes dryer, hot air is blown through the clothes, allowing water to
evaporate very rapidly.
The Matki/Matka, a traditional Indian porous clay
container used for storing and cooling water and other liquids.
The botijo, a
traditional Spanish porous clay container designed to cool the contained water
by evaporation.
Evaporative coolers, which can
significantly cool a building by simply blowing dry air over a filter saturated
with water.
Combustion
vaporization
Fuel droplets vaporize
as they receive heat by mixing with the hot gases in the combustion chamber.
Heat (energy) can also be received by radiation from any hot refractory wall of
the combustion chamber.
Pre-combustion
vaporization
Internal
combustion engines rely upon the vaporization of the fuel in the cylinders to
form a fuel/air mixture in order to burn well. The chemically correct air/fuel
mixture for total burning of gasoline has been determined to be 15 parts air to
one part gasoline or 15/1 by weight. Changing this to a volume ratio yields 8000
parts air to one part gasoline or 8,000/1 by volume.
Film
deposition
Thin films may be deposited by evaporating a substance and
condensing it onto a substrate, or by dissolving the substance in a solvent,
spreading the resulting solution thinly over a substrate, and evaporating the
solvent.
Filtration
Diagram of simple
filtration: oversize particles in the feed cannot pass through
the lattice structure of the filter, while fluid and small particles pass
through, becoming filtrate.
Filtration is
commonly the mechanical or physical operation which is used for the separation
of solids from fluids (liquids or gases) by interposing a medium through which
only the fluid can pass. The fluid that passes through is called the
filtrate. [1]Oversize solids in the fluid are retained, but the
separation is not complete; solids will be contaminated with some fluid and
filtrate will contain fine particles (depending on the pore size and filter
thickness). Filtration is also used to describe somebiological
processes, especially in water treatment and sewage treatment in which undesirable constituents are removed by
absorption into a biological film grown on or in the filter medium as in slow sand
filtration.
ApplicationsFiltration is used to separate particles and fluid in a
suspension, where the fluid can be a liquid, a gas or
a supercritical fluid. Depending on the application, either one or both of the
components may be isolated.
·
Filtration,
as a physical operation is very important in chemistry for the separation of
materials of different chemical composition. A solvent is chosen which dissolves one component, while not
dissolving the other. By dissolving the mixture in the chosen solvent, one
component will go into the solution and pass through the filter, while the
other will be retained. This is one of the most important techniques used by
chemists to purify compounds.
·
Filtration
is also important and widely used as one of the unit operations of chemical engineering. It may be simultaneously combined with other unit
operations to process the feed stream, as in the biofilter, which is a combined filter and biological digestion
device.
·
Filtration
differs from sieving, where separation occurs at a single perforated layer
(a sieve). In sieving, particles that are too big to pass through
the holes of the sieve are retained (see particle size distribution). In filtration, a multilayer lattice retains those
particles that are unable to follow the tortuous channels of the filter.[2] Oversize particles may form a cake layer on top of
the filter and may also block the filter lattice, preventing the fluid phase
from crossing the filter (blinding). Commercially, the term filter is applied
to membranes where the separation lattice is so thin that the
surface becomes the main zone of particle separation, even though these
products might be described as sieves.
·
Filtration
differs from adsorption,
where it is not the physical size of particles that causes separation but the
effects of surface charge. Some
adsorption devices containingactivated charcoal and ion exchange resin are commercially called filters, although
filtration is not their principal function.
·
Filtration
differs from removal of magnetic contaminants from fluids with magnets (typically lubrication oil, coolants and fuel oils), because there is no filter medium. Commercial devices
called "magnetic filters" are sold, but the name reflects their use,
not their mode of operation.
The remainder of this article focuses primarily on liquid
filtration.
Methods
There are many different methods of filtration; all aim
to attain the separation of substances. Separation is achieved by some form
of interaction between the substance or objects to be removed and the filter.
The substance that is to pass through the filter must be a fluid, i.e. a liquid or gas. Methods of filtration vary depending on the location of
the targeted material, i.e. whether it is dissolved in the fluid phase or
suspended as a solid.
Two main types of filter media are employed in any
chemical laboratory— surface filter, a solid sieve which traps the
solid particles, with or without the aid of filter paper (e.g.Büchner funnel, Belt filter, Rotary vacuum-drum filter, Cross-flow filters, Screen filter), and a depth filter, a bed of granular
material which retains the solid particles as it passes (e.g. sand filter). The first type allows the solid particles, i.e. the
residue, to be collected intact; the second type does not permit this. However,
the second type is less prone to clogging due to the greater surface area where
the particles can be trapped. Also, when the solid particles are very fine, it
is often cheaper and easier to discard the contaminated granules than to clean
the solid sieve.
Filter media can be cleaned by rinsing with solvents or
detergents. Alternatively, in engineering applications, such as swimming pool water treatment plants, they may be cleaned
by backwashing. Self-cleaning screen filters utilize point-of-suction backwashing to clean the
screen without interrupting system flow.
Fluids flow through a filter due to a difference in
pressure — fluid flows from the high pressure side to the low pressure side of
the filter, leaving some material behind. The simplest method to achieve this
is by gravity and can be seen in the coffeemaker example. In the laboratory, pressure in the form of
compressed air on the feed side (or vacuum on the filtrate side) may be applied
to make the filtration process faster, though this may lead to clogging or the
passage of fine particles. Alternatively, the liquid may flow through the
filter by the force exerted by a pump, a method commonly used in industry when a reduced
filtration time is important. In this case, the filter need not be mounted
vertically.
Certain filter aids may be used to aid filtration. These
are often incompressible diatomaceous earth, or kieselguhr, which is composed primarily of silica. Also used are woodcellulose and other inert porous solids such as the cheaper
and safer perlite.
These filter aids can be used in two different ways. They
can be used as a precoat before the slurry is filtered. This will prevent gelatinous-type
solids from plugging the filter medium and also give a clearer filtrate. They
can also be added to the slurry before filtration. This increases the porosity of the cake and reduces resistance of the cake
during filtration. In a rotary filter, the filter aid may be applied as a
precoat; subsequently, thin slices of this layer are sliced off with the cake.
The use of filter aids is usually limited to cases where
the cake is discarded or where the precipitate can be chemically separated from the filter.
Filtration is a more efficient method for the separation of mixtures than decantation, but is much more time consuming. If very small amounts
of solution are involved, most of the solution may be soaked up
by the filter medium.
An alternative to filtration is centrifugation — instead of filtering the mixture of solid and
liquid particles, the mixture is centrifuged to force the (usually) denser
solid to the bottom, where it often forms a firm cake. The liquid above can
then be decanted. This method is especially useful for separating solids which
do not filter well, such as gelatinous or fine particles. These solids can clog
or pass through the filter, respectively.
Filter flask (suction
flask, with sintered glass filter containing sample). Note the almost
colourless filtrate in the receiver flask.
Examples of filtration include
·
Furnaces
use filtration to prevent the furnace elements from fouling with particulates.
·
Pneumatic
conveying systems often employ filtration to stop or slow the flow of material
that is transported, through the use of abaghouse.
·
In the
laboratory, a Büchner funnel is often used, with a filter paper serving as the porous barrier.
An experiment to prove the existence of microscopic
organisms involves the
comparison of water passed through unglazed porcelainand unfiltered water. When left in sealed containers the
filtered water takes longer to go foul, demonstrating that very small items
(such as bacteria) can
be removed from fluids by filtration.
In the kidney, renal filtration is the filtration of blood in the glomerulus, followed by selective reabsorbtion of many substances
essential for the body to maintain homeostasis.
Nitration
Nitration is
a general class of chemical process for the introduction of a nitro group into an organic chemical
compound. More loosely the term
also is applied incorrectly to the different process of forming nitrate esters between alcohols and nitric acid, as occurs in the synthesis
of nitroglycerin. The
difference between the resulting structure of nitro compounds and nitrates is
that the nitrogen atom in nitro compounds is directly bonded to a non-oxygen
atom, typically carbon or another nitrogen atom, whereas in nitrate esters,
also called organic nitrates, the nitrogen is bonded to an oxygen atom that in
turn usually is bonded to a carbon atom.
There are many major industrial applications of nitration
in the strict sense; the most important by volume are for the production
of Nitroaromatic compounds such asnitrobenzene. Nitration reactions are notably used for the production
of explosives, for example the conversion of guanidine to nitroguanidine and the conversion of toluene totrinitrotoluene. However, they are of wide importance as chemical intermediates and precursors.
Millions of tons of nitroaromatics are produced annually.
Aromatic nitration
Typical nitration
syntheses apply so-called "mixed acid", a mixture of
concentrated nitric acid and sulfuric acids. This mixture produces the nitronium ion (NO2+),
which is the active species in aromatic nitration. This active
ingredient, which can be isolated in the case of nitronium tetrafluoroborate, also effects nitration without the need for the
mixed acid. In mixed-acid syntheses sulfuric acid is not consumed and hence
acts as a catalyst as
well as an absorbent for water. In the case of nitration of benzene, the reaction is conducted at 50°C.[ The
process is one example of electrophilic aromatic substitution, which involves the attack by the electron-rich benzene ring ration:
Alternative mechanisms
have also been proposed, including one involving single electron transfer
(SET).[4][5]
Process engineering: Particle size reduction
techniques and equipment
There are numerous industries that depend on
size reduction to improve performance or to meet specifications. This article
details size-reduction techniques and equipment that may assist you when
handling these materials.
The chemical,
pharmaceutical, food and mining industries all rely on size reduction. Its uses
include grinding polymers for recycling, improving extraction of a valuable
constituent from ores, facilitating separation of grain components, boosting
the biological availability of medications, and producing particles of an
appropriate size for a given use. There are many types of size-reduction
equipment, which are often developed empirically to handle specific materials
and then are applied in other situations.
Knowing the properties of the material to be processed is essential. Probably the most important characteristic governing size reduction is hardness because almost all size-reduction techniques involve somehow creating new surface area, and this requires adding energy proportional to the bonds holding the feed particles together. A common way of expressing hardness is the Mohs scale, on which talcum is a 1 and diamond is a 10. Also important is whether a material is tough or brittle, with brittle materials being easier to fracture.
Other characteristics include particle-size distribution, bulk density, abrasiveness, moisture content, toxicity, explosiveness and temperature sensitivity. Flow properties can be major factors, too, because many size-reduction processes are continuous, but often have choke points at which bridging and flow interruption can occur. For instance, most size-reduction equipment is fed by chutes, which might constrict flow. Often, the feed flows adequately, but the crushed product will compact and flow with difficulty. Intermediate storage bins might aggravate flow issues by causing compaction and bridging.
For a given feed material, it is important to determine the desired particle-size distribution of the product. In mining, for example, very fine particles can interfere with separation processes, such as froth flotation, and might result in loss of valuable components. In other operations, the objective might be to produce very fine particles. Sometimes, as in sugar grinding, very fine particles are agglomerated to increase the share of larger particles.
Many particle-size distributions can be represented by the Gaudin-Schuhmann equation:
y = 100 (x/xm)ª where y is the cumulative percentage of material that is finer than size x, xm is the theoretical maximum size, and ª is the distribution modulus, which is related to hardness and has lower values for softer materials (0.9 for quartz and 0.3 for gypsum, for instance). The equation indicates that softer materials produce more fines [1].
Nearly all size-reduction techniques result in some degree of fines. So unless producing very fine particles is the objective, it usually is more efficient to perform size reduction in stages, with removal of the desired product after each operation.
Knowing the properties of the material to be processed is essential. Probably the most important characteristic governing size reduction is hardness because almost all size-reduction techniques involve somehow creating new surface area, and this requires adding energy proportional to the bonds holding the feed particles together. A common way of expressing hardness is the Mohs scale, on which talcum is a 1 and diamond is a 10. Also important is whether a material is tough or brittle, with brittle materials being easier to fracture.
Other characteristics include particle-size distribution, bulk density, abrasiveness, moisture content, toxicity, explosiveness and temperature sensitivity. Flow properties can be major factors, too, because many size-reduction processes are continuous, but often have choke points at which bridging and flow interruption can occur. For instance, most size-reduction equipment is fed by chutes, which might constrict flow. Often, the feed flows adequately, but the crushed product will compact and flow with difficulty. Intermediate storage bins might aggravate flow issues by causing compaction and bridging.
For a given feed material, it is important to determine the desired particle-size distribution of the product. In mining, for example, very fine particles can interfere with separation processes, such as froth flotation, and might result in loss of valuable components. In other operations, the objective might be to produce very fine particles. Sometimes, as in sugar grinding, very fine particles are agglomerated to increase the share of larger particles.
Many particle-size distributions can be represented by the Gaudin-Schuhmann equation:
y = 100 (x/xm)ª where y is the cumulative percentage of material that is finer than size x, xm is the theoretical maximum size, and ª is the distribution modulus, which is related to hardness and has lower values for softer materials (0.9 for quartz and 0.3 for gypsum, for instance). The equation indicates that softer materials produce more fines [1].
Nearly all size-reduction techniques result in some degree of fines. So unless producing very fine particles is the objective, it usually is more efficient to perform size reduction in stages, with removal of the desired product after each operation.
Hardware options
Size-reducing equipment relies on compression or impact. Compression is applied via moving jaws, rolls or a gyratory cone. The maximum discharge size is set by the clearance, which is adjustable. Impact-based equipment commonly uses hammers or media. The pros and cons of several types of size-reduction equipment are shown in the table.
Rolls, in particular, can produce very fine particles. Rolls are used in flour milling, where crushing yields different-sized particles, allowing separation of purified flours. Moisture content is important so that, for example, the bran is soft and remains in large pieces, whereas the endosperm is brittle and fractures into small granules. Corn germ can be separated from starch and fiber by roller milling because the germ selectively absorbs water and is made into flakes, whereas the starch fractures.
Impact mills use revolving hammers to strike incoming particles and to break or fling them against the machine case (Figure 1). The hammers might be fixed or, more commonly, pivoted. Typically, the hammers can be reversed to provide added life before they need to be replaced.
Size-reducing equipment relies on compression or impact. Compression is applied via moving jaws, rolls or a gyratory cone. The maximum discharge size is set by the clearance, which is adjustable. Impact-based equipment commonly uses hammers or media. The pros and cons of several types of size-reduction equipment are shown in the table.
Rolls, in particular, can produce very fine particles. Rolls are used in flour milling, where crushing yields different-sized particles, allowing separation of purified flours. Moisture content is important so that, for example, the bran is soft and remains in large pieces, whereas the endosperm is brittle and fractures into small granules. Corn germ can be separated from starch and fiber by roller milling because the germ selectively absorbs water and is made into flakes, whereas the starch fractures.
Impact mills use revolving hammers to strike incoming particles and to break or fling them against the machine case (Figure 1). The hammers might be fixed or, more commonly, pivoted. Typically, the hammers can be reversed to provide added life before they need to be replaced.
In jet mills, particles strike each other as
they are transported in a stream of air or steam. For the initial reduction of
large materials, a rotating drum propels the feed into the air where the pieces
strike each other and fracture.
Ball, pebble and rod mills are rotating cylinders that are partially filled with metal or ceramic balls, flint pebbles or rods. The units are capable of producing very fine powders, such as pigments for inks and paints, but are quite energy inefficient. The crushing mechanism is a combination of impact with the grinding media and shearing between the media and the cylinder walls. A variation is a jar mill, in which relatively small ceramic containers holding some grinding media are rotated on a common machine frame. It is used for small batches of valuable chemicals and in laboratories.
Ball, pebble and rod mills are rotating cylinders that are partially filled with metal or ceramic balls, flint pebbles or rods. The units are capable of producing very fine powders, such as pigments for inks and paints, but are quite energy inefficient. The crushing mechanism is a combination of impact with the grinding media and shearing between the media and the cylinder walls. A variation is a jar mill, in which relatively small ceramic containers holding some grinding media are rotated on a common machine frame. It is used for small batches of valuable chemicals and in laboratories.
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