Charcoal

CHARCOAL. Charcoal is the residue obtained when car bonaceous materials, of either animal or vegetable origin, are partially burned or heated so that the tarry and volatile matter is removed : in most cases the residues may be roughly described as impure carbon (q.v.). The same process of "carbonization" or "destructive distillation" is applied to coal in the manufacture of ordinary illuminating gas, but the residue obtained in this case is termed "coke." It is customary to speak of "animal charcoal," "sugar char coal" and "wood charcoal," the origin of the material being indi cated in this way. The first of these products is obtained by strongly heating blood or bones, and accordingly may be either "blood charcoal" or "bone charcoal." Sipce bones consist very largely of phosphate of lime and other inorganic constituents which are not driven off on heating, bone charcoal contains rela tively little carbon—only about Io–I2% deposited in a finely divided state over the surface of the inorganic material.

Of the various charcoals, that obtained from sugar most nearly approaches pure carbon, and on this account is of considerable scientific interest, although of minor significance from the indus trial standpoint. When carefully prepared from pure sugar, it is practically free from inorganic constituents, and, accordingly on burning, leaves behind a negligible proportion of ash.

Wood Charcoal.

Wood charcoal is a material the production of which has been practised for centuries in regions where timber is abundant. The old procedure consisted in building the cut wood into heaps or stacks with a vent in the middle, and then setting it alight. The heat developed by the combustion of part of the wood served to carbonize the remainder, the extent of the com bustion being limited by controlling the access of air and by cover ing the stack with moist turf or soil. In modern methods, on the other hand, the wood is heated in retorts, and this procedure makes possible the recovery of certain valuable by-products, such as wood tar, acetone, wood spirit and acetic acid.

The proportion of charcoal obtainable from wood is roughly about one-quarter of the weight of the wood, varying naturally with the method of carbonization employed. The relative reduc tion in volume, however, is not so great, for wood charcoal is an extremely porous material, and its bulk is approximately two thirds of the bulk of the wood from which it has been prepared.

The composition and properties of the product depend on the temperature of carbonization. The higher the temperature and the longer the duration of carbonization, the greater is the propor tion of carbon in the charcoal obtained, and by protracted treat ment at a red heat the proportion of hydrogen may be reduced to a comparatively small figure. A charcoal, for example, obtained by heating birch wood for six hours at 75o° C was found to con tain only 0.5% of hydrogen. The amount of ash left on the com plete combustion of wood charcoal varies with the wood employed and with the conditions of its carbonization, but is generally not more than 2-3%.

Charcoal was originally produced mainly for use as fuel. Since during carbonization the bulk of the tarry and volatile mat ter is driven off, charcoal is a clean fuel, and the heat given out in the combustion of a fixed weight—the so-called calorific value —is twice as great for charcoal as for the original wood. While charcoal prepared at a high temperature is difficult to ignite, the product obtained by carbonization at, say, 300° C, retaining a considerable proportion of volatile inflammable matter, it ignites readily.

In earlier days large quantities of wood charcoal were prepared for the reduction of iron from its ores, an operation in which coke is now mainly employed. So great was the demand for char coal where iron ores were found that serious inroads were made on the timber in these districts. This was the case, for example, in the county of Sussex, where at one time the smelting of iron was extensively practised.

Another purpose for which wood charcoal has been required in large quantities is the manufacture of gunpowder. The constitu ents of this material are nitre, sulphur and charcoal, and the last mentioned ingredient is obtained by the carbonization, at a fairly low temperature, of certain light woods, such as alder and willow.

Wood charcoal or charred wood differs from ordinary wood in exceptional resistance to decay—a characteristic which was known to the ancients. Some time ago a quantity of oak stakes were found in the bed of the river Thames at the spot where, according to Tacitus, the Britons fixed a vast number of such stakes in order to oppose the passage of a Roman army. These stakes, which were found to be charred superficially, had retained their form, and were sound at the heart. In this connection it is further note worthy that the wooden piles on which many Venetian houses stand have all been charred in order to secure their preservation.

An extremely interesting property of charcoal is its power of adsorbing gases and of removing colouring matters from solution. It has long been known that charcoal will take up many times its own volume of a gas—in the case of ammonia 100-200, and at a low temperature its adsorptive capacity is very strikingly in creased. Dewar found, for example, that the amount of nitrogen taken up by a certain charcoal was ten times as great at —185° C as at o° C. This fact has been utilized in the production of high vacua. Freshly prepared charcoal, when exposed to air, gradually takes up water vapour and increases notably in weight—a further illustration of the adsorptive properties of this material.

Equally remarkable is the ability of charcoal to remove colour ing matters from solutions, a property discovered more than years ago and applied first in the treatment of beet-sugar juice. Coloured solutions, such as red wine or indigo, when treated with animal charcoal and then filtered, yield colourless liquids.

Activated Charcoal.

In recent years there have been notable developments in the applications of wood charcoal, dating back mainly to the World War of 1914-18. On the introduction of gas warfare in 1915 it became an urgent necessity to provide every soldier with some form of protection, and before long charcoal was generally adopted as an adsorbent for this purpose. Animal charcoal was first employed, since this material, according to a commonly accepted view, was more efficient than wood charcoal for absorbing gases. On grounds of economy, however, it was desired to use the latter material if possible, and investigations soon showed that when ordinary wood charcoal is kept for some time at a red heat with limited access of air, its power of absorb ing gases is increased in a quite remarkable degree. It is said to be "activated." It is worth while giving a few details in order to emphasize this marked effect of the "heat treatment" of wood charcoal. The efficiency of a particular birch-wood charcoal was tested by exposing a sample to a "dose" of poison gas. The material proved to be comparatively inefficient, retaining only 8o% of the poison gas. The charcoal was then heated for 8 hours at 925° C with limited access of air, and a specimen of the product was tested in exactly the same way, showing a very different result, for the charcoal now adsorbed completely four consecutive doses, and retained even the half of a fifth dose.

The respirators supplied to the troops of all countries in the later stages of the war contained activated charcoal. Similar or even greater improvement in absorptive power can be achieved by heating wood charcoal in carbon dioxide or superheated steam, while impregnation with certain chemicals, such as zinc chloride, sulphuric acid or phosphoric acid, followed by exposure for some time to a high temperature, also yields an "active" product. This increase in absorptive capacity has been the subject of much investigation, and it appears that heat treatment such as that described augments the available surface of the charcoal. Activa tion is accompanied by a decrease of the bulk density, that is, a given weight of charcoal occupies more space after activation than it did before : its porosity is greater. It is probable also that the carbon molecules themselves undergo some modification on heating.

Activated charcoals are also of great importance from the industrial standpoint, more especially in connection with the adsorption of hydrocarbons such as gasoline, and the decolorizing of sugar juices. The well-known commercial materials, "Eponit" and "Norit," may be quoted as examples of these products, and evidence of the industrial significance of active charcoals is furnished by the numbers of fresh patents taken out for their manufacture from all sorts of vegetable substances. (See AD SORPTION.) (J. C. P.)