Agricultural Value of Burnt Lime

New Zealand Journal of Agriculture, Volume 84, Issue 4, 15 April 1952, Page 289

Agricultural Value of Burnt Lime

DURNT lime is made in New Zealand principally for preparing lime mortar and as an industrial chemical. Ground burnt lime sold for agricultural purposes is in many instances a second-grade product and its liming value is often no better than that of ground limestone. D. F. Waters, Senior Agricultural Chemist, Rukuhia Soil Research Station, Department of Agriculture, Hamilton, here gives the reasons for the poor quality of burnt lime and describes the differences in the various forms of lime.

THE value of lime in agriculture is widely appreciated, but not all farmers are aware : of the essential differences between the forms of lime available from merchants. For soil treatment lime may be broadly defined as a compound of the element calcium which will reduce soil acidity. Many other chemicals would neutralise acidity, but the value of lime lies in its wide occurrence, its cheapness, and most of all in the fact that calcium is necessary in the soil as a plant nutrient. In applying lime the farmer is adding calcium as well as correcting soil acidity. The common forms of lime which have these properties are: — Limestone (calcium carbonate), which occurs naturally and is ground and sold as agricultural lime; Burnt lime (calcium oxide), produced by burning limestone and sold under various names such as burnt lime, quicklime, burnt shell lime, and Roche lime; Slaked lime (calcium hydroxide), produced by the addition of water to burnt lime, and sold as hydrated or builders’ lime. Burning When limestone is to be burnt a mixture of coal and limestone rock fragments is loaded into the top of a vertical kiln. As the charge slowly sinks it burns about half-way down and the heat produced breaks up the calcium carbonate into calcium oxide and carbon dioxide, which is carried away in the flue gases. Burnt stone is removed from an opening in the bottom of the kiln at such a rate that

passage of the charge through the kiln takes from 2 to 4 days. . Unless a modern kiln is used, the burnt stone will not be of the uniformly high grade which commands a good price for. industrial purposes. Accordingly the stone is picked over by an experienced operator who selects the lighter, well-burnt stone to be packed for industry. The heavier pieces will consist of unburnt and partly burnt stone with hard-burnt stone. The last-named is stone which has been subjected to excessive heat and changed to a dense, partly fused rock which will not slake readily. These rejected pieces with smaller fragments of burnt stone are ground and bagged as agricultural burnt lime. Agricultural Use When burnt lime is slaked by the addition of a small amount of water or by exposure to damp air, each lump swells, heats considerably, and falls away to a fine powder of slaked lime or calcium hydroxide. The whole process of burning followed by slaking was for centuries the most common

method of reducing limestone to a fine condition for application to the soil. The extreme fineness of slaked lime permits thorough incorporation with the soil and as it is alkaline, it immediately neutralises soil acidity. Carbonate of lime and burnt lime have the same action in the soil; in fact burnt lime slakes and becomes carbonate rapidly. The advantage of burnt lime for agricultural purposes should be its high content of calcium oxide. The difference between burnt and unburnt lime can •be illustrated by taking two 1001 b. samples of limestone, each containing 100 per cent, of carbonate, and burning one sample perfectly. The burning drives off the carbon dioxide and reduces the weight of the sample by about 441 b., though the bulk remains about the same. The residue of burnt lime, weighing about 561 b., has the same acidity-neutralising power as the 1001 b. of unburnt lime. In theory it should therefore be possible to effect about 44 per cent, savings in freight and spreading costs by using burnt lime instead of crushed carbonate. Actually, however, most of the ground burnt lime sold for agricultural purposes originally contains considerably less than 100 per cent, of carbonate and is then so imperfectly burnt that it contains only about the same percentage of calcium oxide as does a good specimen of unburnt limestone. In practice, therefore, there is very little saving in freight. In recent years samples of burnt limes produced for industrial and agricultural purposes have been analysed. The results of - the analysis are expressed as percentages of the important constituents in each sample in the table on this page. Discussion on Table Burnt lime: A pure burnt lime would contain 100 per cent, of calcium oxide, while a commercial product made from high-grade limestone could be expected to contain about 80 to 90

per cent. Only two samples tested, the burnt lime contents of which are recorded in column 1, could be regarded as first-quality burnt lime for industrial purposes. Hydrated lime, because of the added water, would contain not more than 75 per cent, of calcium oxide, and the sample containing 63 per cent, is therefore quite satisfactory. 1 Hard-burnt lime: Such lime is useless for industrial purposes, but being slowly soluble in the soil is still useful for agricultural purposes. Only a modern kiln under close supervision can . be expected to produce a uniformly well-burnt lime without over-burning some of the rock. .. . Unburnt limestone: If there is insufficient heat and time for each piece of rock to be calcined completely, a . core of unburnt rock will remain as carbonate of lime. The calcium oxide content of unburnt stone found in the samples is given in column 3. lhe actual percentage of carbonate of lime found is given in column 4. In samples giving a high percentage of carbonate and low burnt and hard-burnt lime figures evidently the time and temperature of burning were quite inadequate.' Where carbonate and hardburnt lime are both high the cause is probably the use of high temperature for a short time. A similar product would be left by taking out well-burnt stone for industry. Importance of Calcium Oxide For general agricultural purposes the value of a liming material depends on the total calcium oxide present, that is, the sum of columns 1,2, and 3. Instead of the total calcium oxide, column 5 gives- its equivalent -as carbonate of lime, which permits a direct comparison of liming value with limestone. It is evident from column .5 that the industrial-..- burnt limes have appreciably greater; liming value than agricultural- lime. - The.- burnt limes sold for agricultural use, however, are only a little better (one is even poorer) than high-grade agricultural lime. A high-grade agricultural lime would contain about 90 per cent, of calcium carbonate, and column 5 shows several limes ranging from 86 to 105 per cent, of carbonate. The current market prices for the various lime products show that although there is not a great difference in liming value between good, well-ground, unburnt limestone and the material that is sold as ground burnt agricultural lime under various names, the difference in price in favour of the former is so great that there can be no question that the unburnt limestone is by far the most economical liming material available to the farmers of New Zealand.

1 2 3 4 5 Liming Calcium oxide value of . Type True Hardin unburnt Limestone sample as burnt lime burnt lime limestone unburnt carbonate (per cent.) Hardburnt lime (per cent.) Calcium oxide Limestone unburnt (per cent.) Liming value of sample as carbonate 'of lime in unburnt limestone (per cent.) NORTH ISLAND (per cent.) (per cent.) (per cent.) (per cent.) of lime Burnt lime . . 52.5 10.0 14.2 25.4 137 Burnt lime .. 52.5 . . 56.6 10.0 9.5 14.2 10.8 25.4 19.3 137 137 . Hydrated lime .. (= 49.2) 6.5 9.0 16.0 19 Agricultural burnt 8.2 2.9 39.5 70.5 90 Agricultural burnt .. 13.3 3.0 38.8 69.3 98 Agricultural burnt 9.4 19.0 19.6 35.0 86 Agricultural burnt . . 29.0 12.0 14.5 25.9 99 Agricultural burnt . . 30.4 14.! 14.5 25.9 105 SOUTH ISLAND Burnt lime . . 52.6 19.2 0.7 1.2 129 Burnt lime . . 69.6 2.4 4.5 8.1 137 Burnt lime .. 71.0 7.1 4.4 7.8 147 Burnt lime . . 79.0 10.5 1.0 1.8 161 Burnt lime .. 85.2 1.3 0.9 1.6 156 Hydrated lime .. (= 63.1) 2.7 2.7 1.8 122 Agricultural burnt .. 49.4 7.0 13.7 24.4 125 Agricultural burnt .. 21.4 4.1 32.4 57.9 103 ' Agricultural burnt . . 30.0 5.6 25.7 45.9 109 Agricultural burnt .. 57.0 7.7 5.8 10.4 126

CONSTITUENTS OF BURNT LIME SAMPLES