A stone Like no Other
Michelangelo created astounding masterpieces with this material.His 17-feet-tall Renaissance sculpture of the biblical hero David, created between 1501 and 1504 in Florence, Italy, inspires awe even today. Emperor Shah Jahan built an edifice with the same material in Agra between 1631 and 1653 to commemorate the passing of his wife Mumtaz Mahal that the world today knows as the Taj Mahal. Odd as it may seem,this material is also used as a calcium additive in animal feed to increase the production from eggs in chickens and milk in cows. Its salt is used asanantacid to treat indigestion, as calcium supplement to strengthen bones and to treat patients with kidney diseases. In a powdered form, it is used in glues, plastics and toothpastes.Quite remarkably, this versatile material, marble, has also been found to be grounding and helps those who have trouble finding clarity and focus.
Marble is a natural stone that is extracted from the earth in tough conditions, using a variety of equipment that runs on compressed air. This glossy, durable stone has been used as a building material since the time of ancient Greece and Egypt. Today, there are countless pieces of art, sculptures and edifices in marble from ancient Europe, Africa and Asia, to modern-day buildings around the world.
From limestone to marble
The term ‘marble’ is derived from the Latin word ‘marmor’ which has its roots in the Greek word ‘marmaros,’ meaning a shining stone. Marble is capable of taking high polishbecause it contains crystalline rock predominantly made of calcite or dolomite, which in turn are metamorphosed from limestone. Limestone is produced by the sedimentation of shells of small fossilized snails, shellfish and coral over millions of years. Exposed to tremendous heat and pressure beneath the earth’s surface, limestone crystallises and becomes denser to form a metamorphic rock called marble. The impurities give marble its various colours and designs. Pure marble is snow white, with more than 99 per cent calcite with a grain structure that imparts a smooth texture, a homogeneous look and a luminous surface.
Limestone, known chemically as calcium carbonate,is found near the edges of the earth’s tectonic plates.It is subjected to high pressure and temperature during plate movement and subduction. Under these extreme conditions, calcite and other minerals within the stone begin a metamorphic transformation. Fossils and other organic debris trapped in the limestone disintegrate at the molecular level and reconstitute into small crystals of calcite. The calcite crystals expand and as they grow in size, they interlock to form a stable rock matrix that binds the marble together. Another type of marble, called ‘dolomite marble,’ forms when the mineral dolostone is exposed to extreme temperatures and pressures from molten subterranean rocks called magma, and undergo a similar metamorphosis. This takes place over eons and far from a plate boundary. This process is similar to the formation of limestoneor calcite marble, but often occurs close to hot granitic intrusions. Since these extreme conditions prevailed more during the early days of earth’s formation, marble is usually found around the oldest part of the earth’s crust. That includes places around the world namely the US, Greece, Spain, Turkey, Italy and India.
In India,economically viable deposits of marble are found in Rajasthan, Gujarat, Haryana, Telangana, Jammuand Kashmir, and Madhya Pradesh. Rajasthan has the best resources of fine quality marble; around 70 percent of the deposits in India are spread across 20 districts of the state—Udaipur, Makrana, Alwar and Jaisalmer being chief among them. Marble is found in various colours and shades.Recent estimates put total national resources of marble at around 2,300 million tonnes.
Marble mining processes
Mining of marble, or for that matter any dimension stone like granite or limestone, is different from conventional mining practices. While in conventional mining, mined minerals are obtained in small-sized debris, in marble and granite mining, large-sized intact blocks are extracted from quarries. The process begins with exploration and identification of a quarry site with sufficiently large deposits, followed by extraction of stone blocks from the quarry. Manual operations are viable in sites with small deposits,but semi-mechanisedmeansare employed in larger ones.
The various stages involved in this operation are as follows:
Removal of top soil and debris from the quarry site:Carried out using heavy earth-moving machineries. In some cases, the weathered zone is loosened using jackhammers and carted away. If the overburden is deep and hard, then explosives are charged into drilled holes and blasted under controlled conditions. The blasted debris is transported to designated waste yards using heavy earth-moving machinery.
Marking out a block for front cut:On removal of the overburden, the marble underneath is exposed. Based on the topography and further development of the quarry, a suitably located block is marked out for removal, making sure it has minimum fracture patterns or fault lines in order to ensure recovery of a large sized block.
Drilling vertical and horizontal holes:Using diesel or pneumatic drilling machines called slim drill or line drill, three holes are drilled perpendicular to each other.One vertical hole is drilled from the top down at the back of the marked block and twohorizontal holes drilled at the bottom, one from the front and the other from the side.All three holes meet at the far inner corner.Highly skilled workers manually match the holes, occasionally with laser techniques for higher precision.A specially designed wagon drilling machine or hydraulically operated crawler drills spin the carbide-tipped drill bits and impart air hammering.These machines are capable of drilling holes of upto five inches in diameter to depths of 120 feet or more.
Cutting the block: One end of adiamond-edged wire-cutting saw in the form of a long wire carrying diamond-reinforcedbeadsis threaded through the vertical hole and extracted through one of the horizontal holes using a long flexible wire retractor. The wire saw is now pooled through the driving wheel of a pneumatic wire-cutting machine stationed on the ground and the two ends of the wire saw are joined together, forming a closed loop. Once the machine is turned on, the wheel rotates and pulls the wire saw through both the vertical holes and one of the horizontal holes, travelling at speeds of 24 metres per second. This cuts the block from the back and bottomsimultaneously,and advances towards the front. A tensioning arrangement moves the machine on its rails at a speed proportional to the cutting rate that keeps the required tension on the wire saw. Next, the block is cut through the vertical and the other horizontal hole. The same procedure is repeated to cut the block at the bottom plane through the two horizontal holes, thus creating cuts on the back, side and bottom. A wire saw cuts through a block at the rate of more than a metre an hour.Debris and waste boulders are transported to dumping yards and auctioned periodically.
Separating the cut block from the rock face:A metal or rubber bag is inserted at the crevice on top between the cut block and the rock face.Fed with compressed air, the bag expands, pushes the block away from the rock faceand creates a gap. This is called an air pillow, or if water under pressure is injected into the bag, it is called a hydro bag. An excavator machine nudges the block away from the top, tilting it until it topples onto a mound of rubble positioned below. The block breaks into smaller segments on impact. These are then lifted either by a forklift or a boom lift, and placed on a haul truck that takes it to the storage area. This operation is called front cut or creating a gully.
The above process results in some wastage. The size of the blocks recovered is small and not free from defects. To overcome this, smaller blocks can be marked out,cut to the exact size required and individually lifted and removed without breakage. The separated segments or blocks are sold by weight.The blocks are slicedinto slabs of about an inch or so thick in a factory using gang saws,and then polished to a sheen. Granite quarrying involves the same steps and procedures. Pneumatically operated polishing machines with diamond abrasives impart a mirror finish to both marble and granite slabs. To make sized tiles, the slabs are trimmed and cut to size, then buffed and chamfered using different types of machines. Rajasthan produces around 1,000 million square feet of marble slabs per annum.
Compressed air is used at different stages in this operation to run a variety of equipment as a motive force to run air-operated motors in drilling machines, wire saw cutting machines and jackhammers, and in air bags to nudge outmassive blocks. With a wide range of rugged air compressors to suit every need, ELGi plays a major role in this field. Close to 1,000 ELGi trolley compressors have been powering this industry for the past 20 years. Diesel powered models like the PG 300-100, PG 300-105 and PG 400-100, and electric models like the PG22e-7 to PG45e-8 and PG75e-9.5series have been winning accolades in this tough, hugely demanding environment.
[Contributed by ELGi Equipments Limited]
