Zinc | Properties, Compounds, & Extraction

Zinc is also the transition element with the symbol Zn and atomic number 30. It belongs to group 12 (IIB) and the 4th period in the periodic table. It is the 24th most abundant element in the world. It has a great resemblance to magnesium because both elements possess the same common oxidation state of Mg²⁺ and Zn²⁺. It is metal so, present in pure mixed ore, it is refined by the froth floatation process and ore is roasted that is extracted by electricity (electrowinning).

Brass is a very famous alloy that is formed by a mixture of copper and zinc in various proportions. Different chemists burn zinc metal in the presence of air to form a product called “philosopher’s wool” or “white snow”. Zinc is basically silver-grey in appearance with the standard atomic weight of 65.38 g/mol. The word zinc is probably named after the Greek word “Zinke” by chemist Paracelsus. Zinc metal is applicable in daily life examples. Such as electrical batteries, and galvanizing. Zinc chloride and zinc carbonate can act as dietary supplements.

In the biological classifications, zinc played an important role. Zinc is an essential mineral. The deficiency of zinc may cause retardation in growth, delayed sexual; maturation, diarrhea, and infection susceptibility. Alcohol dehydrogenase enzyme contains zinc atom as the reactive center. More consumption of zinc may cause a decreased level of copper in the body. According to the latest and current research, this element appeared to be a very important component of meeting eggs and sperms.

History & Discovery

Zinc basically appears many years ago when copper, lead, and nickel were introduced. These metals are obtained as molten metal when the oxides of these metals are heated or reacted with the charcoal as carbon form by the process of reduction in the blast furnace. The oxides of zinc cannot have the ability to reduce when the temperature is reached above the low boiling point of metal 907°C. In flues of lead blast furnace, some quantity of zinc is also produced.

Basically, it is recognized as a metal under the rule of Yasada or Jasada in the medical Lexicon. In the 13th century, in India, smelting, and extraction of impure zinc by reducing calamine with will and other organic substances. Some of the evidence proves that Greeks knew zinc’s existence as pseudargyras or “false silver”. Romans in 200 BCE produced some amount of brass. The metallurgists of Chine achieved large-scale production of zinc in the 16th century.

At the end of the 18th century, Belgium and Poland made many improvements in the furnace and also developed an electrolytic process in 1917. In 1930, an endothermic process was produced to manufacture zinc.

Occurrence of Zinc:

Every ton of Earth’s crust, zinc makes up 65 grams of it. First of all, native zinc has been reported in Australia, New Zealand, and the United States. Zinc is considered to be an essential trace element in the body, where it is found in high concentrations in Red blood cells (Erythrocytes). It is an essential component of the carbonic anhydrase enzyme. Metallic zinc is produced by roasting the sulfide ore and also either in leaching processes. Basically, zinc does not occur in its native form. Hence, it is a reactive metal. But in the combined state, zinc is widely distributed.

Zinc makes around 75 parts per million of Earth’s crust, making it the 24th most abundant element on the Earth. The native soil contains 5-770 parts per million of zinc. Seawater has just 30 parts per million and air 0.4 g/m³. Basic zinc ores are formed in the USA, Canada, Australia, and Mexico. The dominant ore of zinc is zinc blended, called Sphalerite. Some other ores are wurtzite, smithsonite, and hemimorphite.

Physical Properties

Physical properties are basically changed in the physical state of matter. So, it possesses many of the physical properties as given below:

• It is bluish-white, lustrous, and diamagnetic metal.
• It is less dense than iron and has a hexagonal crystal structure.
• At the temperature of 100°C & 150°C, it acts as a hard, and brittle metal.
• Zinc is a fair conductor of electricity because it possesses free electrons in it.
• Zinc has a low melting point of 419.5°C and also has a low boiling point of 907°C.
• It is a transition metal that has the d-orbital in it.
• Its electronic configuration is [Ar]3d104s2.
• At standard temperature and pressure of 273K and 1atm, it acts in the solid phase.
• Density is mass per unit volume which is 7.14 g/cm3.
• The heat of fusion is 7.32 KJ/mol.
• The heat of vaporization is 115 KJ/mol.
• The molar heat capacity of zinc is 25.470 KJ/mol.
• It belongs to CAS number 7440-66-6.
• It’s young’s modulus is 108 GPa & shear modulus is 43 GPa.
• Its electrical resistivity is 59.0 nΩ-m (at 20℃)

Periodicity & Periodic Properties

The periodicity of an element depends upon the atomic number or number of protons variations in the nucleus. As it is the first element of group 12, So it has a small atomic size as compared to the other transition element. It also comprises high ionization energy and also high electron affinity values. It occurs due to strong nuclear attraction. It is very difficult to remove an electron from the outermost shell due to its small atomic size.

PERIODIC PROPERTIES	ZINC ELEMENT
1. Oxidation state	-2, 0, +1, +2
2. Electronegativity	1.65 (Pauling’s scale)
3. Ionization energy	1st (906.4 KJ/mol) 2nd (1733.3 KJ/mol) 3rd (8833 KJ/mol)
4. Atomic Radius	134 pm
5. Covalent Radius	122 + 4 pm
6. Vander Waals Radius	139 pm

Reactivity of zinc

Zinc is the transition element having electronic configuration [Ar]3d¹⁰4s². It is a strong reducing agent and moderately reactive metal. The surface of pure metal tarnishes quicker so forms a protective layer of zinc carbonate Zn₅(OH)₆(CO₃)₂. Zinc reacts with air to form zinc oxide. Zinc also has the ability to react with acid, alkali, and non-metals.

At room temperature, zinc reacts with acid very slowly. It does not react with strong acid because it losses its passivating layer which releases H₂ gas when reacted with the strong acid. Basically, zinc exists mostly in the oxidation state of +2. This condition forces when the last two electrons from the 4s orbital remove forming Zn⁺². In other words, sometimes the +1 oxidation state of Zn also exists which forms when zinc is volatilized with zinc chloride above 285℃ forming ZnCl₂ which shows Zn has a +1 oxidation state.

It is also noticed that zinc compounds of the +4 oxidation state are unlikely to exist. An exception is made in which Zn (III) complexes exist in the +4 oxidation state in presence of strong electronegative trianion. the chemistry of zinc has a resemblance to copper and nickel. It is diamagnetic colorless metal. Ionic radii of copper & zinc are supported to be equal because some of their equivalent salts have some structure of the crystal.

There is some similarity between nickel, copper, and zinc. Zinc can form bonds with the greater covalency & with stable -N and -S complexes. 5-coordinate complexes of zinc are also known.

Zinc-containing compounds:

Following are the zinc-containing compounds:

Zinc oxide:

It is the most important industrial compound. It is used in rubber manufacturing, photocopier, chemical, and paints. It is also used in floor covering, as a lubricant, in rayon manufacturing, and in the ointment for external use. In nature, it is present as zincite and red zinc. It is prepared by the following reactions:

2Zn + O₂ → 2ZnO

ZnCO₃ → ZnO + CO₂

Zn(OH)₂ → ZnO + H₂O

ZnCO₃.3Zn(OH)₂ → 4ZnO + 3H₂O + CO₂

It possesses some of the properties. These are insoluble in water and sublime at 400℃. It is amphoteric oxide in nature that acts as both acids as well as the base. It is reacted with acids as well as Co(NO₃)₂. When it is reacted with acid it forms the following the products:

ZnO + H₂SO₄ → ZnSO₄ + H₂O

ZnO + HCl → ZnCl₂ + H₂O

It is also reacted by NaOH base. NaOH is the strong base that has a strong ability to donate OH ions in the water.

ZnO + 2NaOH → Na₂ZnO₂ + H₂O

Reduction of Zinc also occurs:

ZnO +H₂ → Zn + H₂O

ZnO + C → Zn + CO↑

Zinc Chloride:

Zinc chloride is basically a white deliquescent solid which is soluble in water. Anhydrous zinc chloride is used as a dehydrating agent in making dry cel, parchment papers, and adhesives. It is also used to protect the timber from different harmful microorganisms. A mixture of syrupy ZnCl2 & ZnO is used as a dental filling. Zinc Chloride is basically a compound of zinc that has zinc and chloride in it. Zinc Chloride is manufactured by many reactions. It is prepared by:

Zinc Chloride is prepared by the zinc compounds. These are prepared by the zinc by the action of HCl on it.

ZnO + 2HCl → ZnCl₂ + H₂O

ZnCO₃ + 2HCl → ZnCl₂ + CO₂ + H₂O

The solution of Zinc chloride on concentration and cooling gives a crystal of hydrated zinc chloride. Anhydrous zinc chloride can be prepared as follows:

Zn + Cl₂ → ZnCl₂

Zn + HgCl₂ → ZnCl₂ + Hg

It will give many of the other reactions and also has the effect of heat on it.

ZnCl₂.2H₂O → Zn(OH)Cl + HCl + H₂O

2ZnCl₂.2H₂O → Zn₂OCl₂ + 2HCl + 3H₂O

Zinc chloride may also react with the sodium hydroxide base.

ZnCl₂ + 2NaOH → Zn(OH)₂ + 2NaCl

Zn(OH)₂ + 2NaOH → Na₂ZnO₂ + 2H₂O

Zinc chloride is also reacted with the ammonium hydroxide or aqueous ammonia.

ZnCl₂ + 2NH₄OH → Zn(OH)₂ + 2NH₄Cl

Zn(OH)₂ + 2NH₄OH + 2NH₄Cl → [Zn(NH₃)₄]Cl₂ + 4H₂O

Zinc chloride also reacts with sodium carbonate.

4ZnCl₂ + 4Na₂CO₃ + 3H₂O → ZnCO₃.3Zn(OH)₂ + 8NaCl + 3CO₂

Zinc chloride may also react with the H₂S gas.

ZnCl₂ + H₂S → ZnS + 2HCl

Zinc Sulphate:

Zinc Sulphate is a colorless crystalline solid and is soluble in water. It is also used to prepare lithopone (a mixture of ZnO + BaSO₄) which is the well-known white pigment. It also acts as a reagent in the analytical chemistry and or bleaching of paper. In the manufacturing of rayon, it is an important component in spinning baths.

It is also used in manufacturing fungicides. Zinc sulfate is also used in skin freshness creams, glue, the textile industry, and many other applications. It also acts as a fireproof reagent, herbicide as well as fertilizer. It can be used as a supplement for humans, plants, and animals with zinc deficiency.

Zinc sulfate is also used in eye lotion. It is also prepared by the following reactions:

Zn + H₂SO₄ → ZnSO₄ + H₂

ZnO + H₂SO₄ → ZnSO₄ + H₂O

ZnCO₃ + H₂SO₄ → ZnSO₄ + H₂O + CO₂

When the solution of ZnSO₄ is reacted in the presence of a concentrated solution it is followed by crystallization giving a crystal of ZnSO₄.7H₂O. In this formula, there is seven water of crystallization.

Zinc sulfate also possesses many chemical properties. It has an adverse effect on the effect of heat.

ZnSO₄.7H₂O → ZnSO₄.6H₂O (above 40 degrees, -H₂O)

ZnSO₄.6H₂O → ZnSO₄.H₂O (above 70 degrees, -5H₂O)

ZnSO₄.H₂O → ZnSO₄ (change in heat at 280 degrees, -H₂O)

ZnSO₄ → ZnO + SO₃ or SO₂ + 1/2 O₂ (at 800 degrees)

Zinc Sulfate is also reacted with NaOH, a strong base.

ZnSO₄ + 2NaOH → Zn(OH)₂ + Na₂SO₄

It also gives a reaction with the Na₂CO₃ salt, a strong electrolyte.

4ZnSO₄ + 4Na₂CO₃ + 3H₂O → ZnCO₃ + 3Zn(OH)₂ + 4Na₂SO₄ +3CO₂

Zinc Sulfate is also reacted with sodium bicarbonate:

ZnSO₄ + 2NaHCO₃ → ZnCO₃ + Na₂SO₄ + H₂O + CO₂

Zinc acetate:

• Zinc acetate is used in the preservation of wood against termite attacks.
• It is also used as a component in adhesives, and as a reagent in testing for album, tannin, phosphate, and blood.
• It comes in the form of white granules. It has a faint vinegar odor and it decomposes at 200 degrees.
• It is soluble in water, and alcohol and crystallizes to form dilute acetic acid.

Zinc Phosphide:

• Zinc phosphide also acts as a poison in rat and field mice.
• It also acts as a stomach poison for mosquito larvae and agricultural pests. It exists in dark grey color crystals or lustrous or dry powder.
• Its melting point is greater than 420 degrees and boils at 1100 degrees. It is insoluble in alcohol and water.
• It reacts with acids more slowly with water.
• It reacts violently with acid when heated at less than 400 degrees to form phosphine gas.

Zinc Flouride:

• Zinc fluoride is used in the manufacturing of phosphorus for fluorescent light, in fluorination, in electroplating baths, in galvanizing steel, and in making steel and ceramics.
• Also used in enamels for porcelain. It exists as colorless crystal needles of white crystalline mass.
• It has a melting point of 872 degrees and a boiling point of 1502 degrees. Its specific gravity is 4.95.
• It is soluble in water, ammonium hydroxide, alkali, hydrochloric acid, and nitric acid.

Organozinc Compound

In organic chemistry, these are compounds in which the C-Zn bond exists. These are known to be the first organometallic compounds that are less reactive such as Grignard & organolithium reagents. A scientist, Edward Frankland manufactured the first organozinc compound by heating ethyl iodide in presence of zinc metal in 1848.

Organozinc possess pyrophoric nature that’s mean these compound are manufactured by air for techniques. These are isolated on the basis of the number of carbon. These are classified as:

• Diaorganozinc compounds (R₂Zn)
• Heteroleptic compounds (RZnX)
• Ionic organozinc compounds

Synthesis of organozinc compounds

Organozinc metals are manufactured by many other methods. But it can be manufactured by an organobromine precursor. These are also synthesized by dimethyl zinc, diethyl zinc, and diphenyl zinc but these are so much expensive.

1. From Zinc metal:

This method is involved in the manufacturing of diethylzinc which involves the reaction of ethyl iodide with the zinc metal. This method is basically a redox reaction. This reaction involves the reaction between ethyl iodide and the zinc metal to form diethylzinc and zinc iodide. In the below reaction, Et stands for ethyl.

2EtI + 2Zn⁰ → Et₂Zn + ZnI₂

Riecke Zinc is also produced when there is a reduction of ZnCl₂ with potassium. The catalyst used in this reaction is THF and 2KCl which only speed up the reaction. the reaction is given below:

ZnCl₂ + 2K → Zn⁰

2. Functional Group exchange:

Zinc functional group interconversion reaction occurs with halides, and boron which is catalyzed by copper iodide. Initial hydroboration reduction is followed by the diethyl zinc treatment which synthesizes boron intermediate. The organozinc reagent’s utility is shown by displaying high selectivity for the most reactive site in the molecule.

In the case of other coupling reactions of allylation, a group transfer reaction occurs. The following reaction takes place.

Negishi coupling is employed in transition metal-catalyzed cross-coupling reaction. In this process, organic halides or triflates form the C-C bond with organozinc compounds.

R-X + R₁ – ZnX’ → R – R₁

In this reaction R, X, and X’ may represent the following:

X → halogens (Cl, Br, I), leaving group, and triflates and acetoxy groups are feasible as well.

R → alkenyl, aryl, allyl, propargyl

X’ → organozinc compounds can be chloride, bromine, and iodine.

3. B-silyl organozinc compounds:

In the organozinc alkylation, two alkyl substituents are transformed. This problem is solved by using TMSM (MeSiCH2). TMSM is a non-transferable group. THF is used as a catalyst in this reaction.

4. Transmetallation:

Transmetallation is similar to the interconversion of functional groups in which zinc is exchanged with other metals such as mercury, lithium, copper, etc.

HgPh₂ + Zn → ZnPh₂ + Hg

The benefit of transmetallation is that it produces more tolerance to another functional group in the molecule due to low reactivity which increases selectivity.

Isotopes of Zinc:

Zinc is considered to be the 23rd most abundant element in the world. Therefore, it possesses five stable isotopes. Hence, isotopes have the same atomic number but different mass numbers. Following are the stable isotopes of zinc:

ISOTOPES	RELATIVE ABUNDANCE	MASS
64Zn	49.2 %	63.92
66Zn	27.7%	65.92
67Zn	4.0%	66.92
68Zn	18.5%	67.92
70Zn	0.6%	69.92

In all these isotopes, Zn-67 is often used in biological research and also uptake some quantity of zinc in the human body. But all other isotopes of zinc may use for the production of therapeutic isotopes Cu-67.

Extraction of Zinc:

Extraction of zinc may include many other processes. Just like concentration, roasting, smelting, and electrolysis process. In these processes, first of all, the zinc ore is concentrated by the froth floatation process and then heated in a blast furnace for roasting and smelting. And then in the electrolysis process, it is refined completely and extracted in pure form by electrowinning process.

1. Concentration of Zinc (Froth Floatation process)

First of all, zinc is extracted from its ore. In this process of concentration, the froth flotation process is used in this process pulverized ore is kept in a large tank that contains water and pine oil. Also, the ore in the crushed form is present in the system. When the compressed air is passed through the system the complete mixture of ore, water, and Pine oil gets agitated. The ore in the pure form mixed with oil and form froth and reach the above surface of the water but on the other hand, there must be impurities that are collected at bottom of the system due to their higher weight. So, in this way ore is concentrated.

2. Roasting

It is a unique process in which concentrated Ore is heated in excess supply of air above 900 ℃. In the blast furnace basically, zinc is present in the form of zinc sulfide ore is heated in the excess supply of oxygen to form zinc oxide along with SO₂ which is a poisonous material and harmful for apparatus.

2ZnS + 3O₂ → 2ZnO + 2SO₂

 A further small amount of zinc sulfide (ZnS) may be oxidized or added with oxygen to form ZnSO₄ above 900℃ after that reaction ZnSO₄ decomposes to form zinc oxide.

ZnS + 2O₂ → ZnSO₄

ZnSO₄ → 2ZnO + 2SO₂ + O₂ (above 900 ℃)

The roasting may be done by three types of roasters:

* Multiple-hearth Roaster:

It consists of many brick-lined cylindrical columns. As concentrated Ore is dropped into the furnace it is dried firstly and then oxidized to form calcine. The addition of fuel in the system can sustain the reaction but overall it is a slow reaction. This system requires no condition of pressure and it works at a temperature of 690℃. This method is used to remove sulfur from the system it has the ability to produce high-quality Calcine.

* Suspension roaster:

The suspension roster consists of a combustion chamber that has a resemblance to the coal chamber it also contains a refractory-lined cylindrical steel shell which has a large commercial space at the top. This roaster also shows resemblance to the multiple-hearth roaster. In this roaster, a grinder is available which is additional equipment according to the multiple heart roaster.

The role of this grinder is to ensure that heat transferred to the material is sufficient or not to desulfurize and to carry out an oxidation reaction in the blast furnace-like multiple hearth roaster. It also can not work in conditions of pressure but work at a temperature of 980 ℃.

* Fluidized bed roaster:

In this zinc sulfide, Ore is suspended and oxidized in presence of excess oxygen. In this, the rate at which desulfurization occurs is more rapid than that of a multiple-Hearth chamber. This roster work on the condition of pressure as well as temperature averaging 1,000℃. The pressure must be slightly lower than atmospheric pressure. For ignition, no additional fuel is required. It has a vast benefit to other rosters that it has a great ability to remove greater Sulphur and should possess lower maintenance.

3. Smelting:

Now in this process, ZnO formed in roasting is mixed with Coke so, it is reduced by Zn and forms CO which is a poisonous compound.

ZnO + C → Zn + CO

This reduction is carried out in vertical refort. The roasted ore is mixed with Coke in a ratio of 2:1 which forms small briquets. This should be entered from the charging door. Then briquets are heated by burning which produces gas above 1400℃. Vapors of zinc came close to the condenser where it condenses to give molten zinc called spelter zinc.

4. Electrolysis Process:

This method is also called the hydrometallurgical process in this process pure zinc is obtained it occurs in three steps:

* Leaching process:

In this process, zinc oxide obtained is reacted with Sulphur trioxide SO₃ to form ZnSO₄ zinc sulfate.

ZnO + SO₃ → ZnSO₄

ZnSO₄ is obtained by the double leaching process. First of all, to leach out zinc calcine is leached first in a neutral or slightly acidic solution some of the calcine is left over. This remaining calcine leached with the rest of the zinc out of zinc ferrite and zinc oxide. Through this process, solid and liquid products are obtained. The solid mass may be leached which contains precious metal in it as a byproduct. This leech product contains iron produced by strong acid leaching and is removed as an intermediate product.

* Purification:

To purify the Zn cementation process is utilized. It uses zinc dust and steam to remove copper, cadmium, and Nickel which interfere electrolysis process. Cementation is a type of precipitation and heterogeneous process in which ions are reduced to zero balance at solid metallurgical interference. This method is used for purification. 

After purification, the concentrates are left less than 0.05 mg/liter purification is carried out at different temperatures in between  40-85℃. It also works on the condition of the pressure of 2.4 atm. In the next process of Electrowinning, Zn is completely extracted.

* Extraction/Electrolysis/Electrowinning:

Electrowinning is the process by which complete pure zinc is extracted by zinc sulfate, zinc is extracted. This process work when the current is passed in solution in a series of cells. It contains two electrodes that are the anode and a cathode by passing an electrical current zinc deposit on the cathode which has a negative charge on it. Zinc possesses a positive charge on it and oxygen is deposited on the anode. After 1 to 2 days or 24 to 48 hours, zinc-coated cathodes are removed and rinsed. In this process, sulphuric acid in the bleaching process is used as a strong electrolyte that dissociates completely when the current is passed.

Basically, it is an electrolytic cell that works on the condition of temperature and pressure the temperature varies from 30 to 35℃. The small amount of sulphuric acid circulates by the cooling tower to cool and concentrate by evaporation of water.