Transcript Zinc sulfide - IndiaStudyChannel.com

Sulphur is considered a secondary
element, along with calcium and
magnesium.
Sulphur is required in moderate
amounts by pants, but is less likely to
limit crop growth than nitrogen,
phosphorus or potassium.
Sulphur has been known from ancient
times. The ancient Indians, the Inca
Indians, Greeks and Romans used
sulphur, as a fumigant, and in certain
medicines.
The word "Sulphur" is probably
derived from the Sanskrit word
"Sulveri" meaning "enemy of copper"
as copper loses all its properties when
heated with sulphur. It has been
mentioned in the Bible on many
occasions, as "Brimstone".
In the free state, sulphur is found in areas
that have had volcanic activities.
It is mostly found in Sicily in Italy
and in Texas and Louisiana in the USA.
To a lesser extent it is found in Russia,
Canada, Japan and Poland.
In the combined state, it exists as sulphide, sulphates etc.
The following are the chief sulphur-minerals.
Galena (PbS)
Zinc blende (ZnS)
Cinnabar(HgS)
Iron pyrites (FeS2)
Gypsum (CaSO4.2H2O)
Chemistry: PbS, Lead Sulfide
Class: Sulfides
Group: Galena
Uses: Major ore of lead and
silver
Galena is a common and popular
mineral for rock hounds.
Its characteristic cubes,
distinctive cleavage and high
density
make it easy to identify and a
favorite in high school geology
labs.
The structure of Galena is
identical to that of halite, NaCl.
The two minerals have the same
crystal shapes, symmetry and
cleavage.
Some Galena may contain up to
1% silver in place of lead.
The large volume of Galena that is
processed for lead produces
enough Silver as a by product to
make Galena the leading ore of
Silver.
Zinc sulfide (or zinc sulphide) is a
chemical compound with the formula
ZnS. Zinc sulfide is a white to yellow
colored powder or crystal. It is
typically encountered in the more
stable cubic form, known also as the
mineral sphalerite. The hexagonal form
is also known both as a synthetic
material and as the mineral wurtzite.
Both sphalerite and wurtzite are
intrinsic, wide-bandgap
semiconductors. The cubic form has a
band gap of 3.54 eV at 300 K whereas
the hexagonal form has a band gap of
3.91 eV.
A transition from the sphalerite form
to the wurtzite form occurs at around
1293.15 K. Sphalerite melts at 1991 K. It
has a standard enthalpy of formation
of −204.6 kJ mol−1 at 298 K.
Cinnabar is a colorful mineral that adds
a unique color to the mineral color
palette.
Its cinnamon to scarlet red color can be
very attractive. Well shaped crystals
are uncommon and the twinned
crystals are considered classics among
collectors.
The twinning in cinnabar is distinctive
and forms a penetration twin that is
ridged
with six ridges surrounding the point of
a pryamid. It could be thought of as two
scalahedral crystals grown together
Chemistry: HgS, Mercury
with one crystal going the opposite way
Sulfide
of the
Class: Sulfides and Sulfosalts
other crystal. Cinnabar was mined by
Uses: primary ore of mercury,
the Roman Empire for its mercury
a pigment and as a minerals
content
specimen.
and it has been the main ore of mercury
throughout the centuries.
Chemistry: FeS2, Iron Sulfide
Class: Sulfides
Group: Pyrite
Uses: A very minor ore of sulfur
for sulfuric acid, used in jewelry
under the trade name
"marcasite" and as mineral
specimens.
Pyrite is the classic "Fool's Gold". There
are other shiny brassy yellow
minerals,
but pyrite is by far the most common
and the most often mistaken for gold.
Whether it is the golden look or
something else, pyrite is a favorite
among rock collectors. It can have a
beautiful luster and interesting
crystals.
It is so common in the earth's crust
that it is found in almost every
possible environment, hence it has a
vast number of forms and varieties.
Chemistry: CaSO4-2(H2O),
Hydrated Calcium Sulfate
Class: Sulfates
Uses: plaster, wall board, some
cements, fertilizer, paint filler,
ornamental stone, etc..
Gypsum is one of the more
common minerals in
sedimentary environments.
It is a major rock forming
mineral that produces
massive beds, usually from
precipitation out of highly
saline waters. Since it forms
easily from saline water,
gypsum can have many
inclusions of other minerals
and even trapped bubbles of
air and water.
Herman Frasch in 1894 devised a
brilliant technique of obtaining sulphur
from these deposits. In this process, now
called the Frasch process, a system of
three concentric pipes is sunk into the
earth to reach the sulphur beds (about
300m deep).
Outer pipe
In this pipe, superheated water (170oC) is
sent down under 10 atm pressure. This
melts the sulphur in the beds of the soil.
Inner most pipe
Hot compressed air is simultaneously
blown down this pipe, which causes the
frothing of sulphur.
Middle pipe
The simultaneous pressure of air in the
inner pipe and water of the outermost
pipe forces the sulphur froth to rise up
Impure sulphur is heated in an
iron retort. The sulphur vapors
formed at 444oC are allowed to
escape
in
a
large
brick
chamber. The vapors condense
and form 'flowers of sulphur'
on the walls of the chamber.
After sometime when the
temperature of the chamber
rises up to 120oC, these flowers
of sulphur melt and collect at
the bottom of the chamber. The
molten sulphur is poured into
cylindrical
moulds
and
allowed to solidify. Sulphur so
obtained is called roll sulphur.
Large quantities of
hydrogen sulphide are
obtained from the
purification of petroleum
gases along with other
gases such as methane and
carbondioxide. These gases
are removed and hydrogen
sulphide is oxidised to
sulphur in different stages
using heated bauxite as
catalyst.
Rhombic Sulphur (Octahedral or
Alpha Sulphur)
Rhombic sulphur is prepared by
dissolving roll sulphur in carbon
disulphide,
and
then
evaporating the solution slowly,
at room temperature. Eight
sided
crystals
of
rhombic
sulphur crystallizes out
Rhombic or octahedral sulphur
consists of rings of 8 atoms of
sulphur. It is the most stable of
all the allotropes of sulphur. It is
soluble in carbon disulphide,
benzene, chloroform, etc., but is
insoluble in water. It is nonconductor
of
heat
and
electricity. It is transparent and
pale yellow in color.
Monoclinic Sulphur (Prismatic or Beta
Sulphur)
This allotrope of sulphur is prepared by
melting roll sulphur in a dish. The molten
sulphur is allowed to cool slowly. The top
layer solidifies first and forms a crust.
Two holes are made in the crust with the
help of a heated nail. The molten sulphur
is poured out through one of the holes.
Then with the help of a knife the crust is
carefully
peeled
off.
Pale-yellow,
transparent needle shaped crystals are
seen projecting out form the inner
surface of the dish. These are the crystals
of
monoclinic
sulphur.
Monoclinic
sulphur also consists of 8 atom rings. It is
stable only above 96oC. When it cools
down below 96oC, it changes to rhombic
sulphur i.e., 96oC is the transition
temperature of this sulphur.
Milk of sulphur is prepared by the
action of dilute hydrochloric acid on
ammonium sulphide. Milk of sulphur
is also prepared by boiling roll
sulphur with an aqueous solution of
calcium hydroxide. The mixture is
then filtered and dilute hydrochloric
acid is added to the filtrate to get
milk of sulphur. Milk of sulphur is
non-crystalline and white in color. It
is soluble in carbon disulphide. When
heated it changes to the ordinary
yellow variety of sulphur. It is used
in medicine.
On heating sulphur, till almost the boiling
point and suddenly cooling it by pouring
into cold water a viscous mass is formed.
This sudden cooling does not allow
sufficient time to the molecules to
rearrange themselves to form monoclinic
or rhombic forms of sulphur. Hence the
molecules form an interwined mass,
consisting
of
both
rhombic
and
monoclinic varieties of sulphur. This is
called plastic sulphur
This type of sulphur is a dark brown or
even black, sticky substance. It is elastic.
It has no sharp melting point. It does not
dissolve
in
carbon
disulphide.
On
standing, it slowly changes to the rhombic
forms, as it gains the eight atom ring
structure.
When heated, sulphur undergoes a series
of characteristic changes.
Sulphur can melt at two different
temperatures.
a) If heated rapidly, it melts at 112.8oC.
b) But if heated slowly it will melt only at
118.6oC.
As a result the liquid is quite mobile. As
the color gradually darkens form pale
yellow to amber color At 160oC, it becomes
so viscous that it looks almost tar-like, and
it does not flow out even if the test tube is
inverted .When heating continues the
liquid once again becomes mobile .At
444oC, sulphur begins to boil giving off
light yellowish orange vapors. When the
vapor are cooled, they solidify to form a
Yellow powder of flowers of sulphur.
We find many sulphur compounds on
Earth. These include sulphur dioxide,
elemental sulphur, sulphuric acid, salts of
sulphate or organic sulphur
compounds such as dimethylsulphide and
even amino acids in our body. All these
chemical compounds do not last forever.
They are transported by physical
processes like wind or erosion by water,
by geological events like volcano
eruptions or by biological activity. They
are also transformed by chemical
reactions. But nothing is lost. Changes
often take place in cycles. Such cycles can
be chemical cycles in which a sulphur
compound A reacts to form B, B to C, C
to D and D to A again. One example is
when sulphur compounds move from the
ocean to the atmosphere, are transported
to the land, come down with the rain and
are transported by rivers to the ocean