Method of producing burned crystalline carbonate minerals
United States Patent Dahl Apr. 22, 1975 METHOD OF PRODUCING BURNED [56] References Cited CRYSTALLINE CARBONATE MINERALS UNITED STATES PATENTS [75] Inventor: Erik Qvale Dahl, Vagsbygd, Norway 3.233.017 2/1966 Weaver et al 264/63 q 3.368.010 2/1968 Cummin s 264/63 [7J1 Asslgneei Elkem&#39;splgel&#39;verkel 3.666.851 5/1972 Chaklad r 264/66 Norway 3.712.599 1/1973 Davies et al. 264/66 [22] Filed, Oct 4 1972 3.729.329 4/1973 Morris ct al. 264/63 [21] Appl. No: 294,873 Primary E.\-aminer-Donald J. Arnold Alturney. Agent, or Firm-Eyre, Mann &amp; Lucas 30 F A l l O t :rleg l p llllcatlon Priority Data 699m ABSTRACT C (mm) A process for the production of burned crystalline car- [52] US Cl M63, 764/59. bonate materials is disclosed. The crystalline mineral 6 is pulverized to a size smaller than the size of its single [51] lm Cl C04) 35/64 co4bh33l3o crystals. admixed with water and a binding agent and [S8] Fie&#39;ld 764/56 5 66 then pelletized and burned at temperatures above 65 900C. to form a mecanically strong agglomerate.  
 3 Claims, No Drawings METHOD OF PRODUCING BURNED CRYSTALLINE CARBONATE MINERALS The present invention relates to a new and useful method of producing mechanically strong agglomerates from crystalline carbonate minerals.  
  Carbonate minerals are used extensively in the smelting industry as raw materials for the production of calcium carbide and metal products such as magnesium metal and alloys comprising magnesium and/or calcium as alloying components. The carbonate minerals are used as slagforming and fluxing materials, as refining materials in the steel industry and in refractory materi als. The carbonate minerals are usually calcined before they are used in the smelting industry. During the calcination process, which usually takes place at temperatures of l000-l500C., the carbonates are decomposed and carbon dioxide is liberated.  
  Carbonate minerals occur in nature in both the amorphous and crystalline form. The amorphous types of carbonate material will generally maintain their form and lump size during the calcining process but the crystalline types will tend to decrepitate, i.e., be broken up into single crystals. Since these crystals are very small they are normally referred to as dust or fines and, be cause of the known problems encountered with dust or fines, they have been considered unusable in smelting processes. This problem is especially evident with crystalline limestone and crystalline dolomite.  
  There has now been discovered a process by which mechanically strong sintered agglomerates can be formed from crystalline decrepitating carbonates. The invention comprises pulverizing the crystalline carbonate to a size smaller than the size of its single crystals, combining the thus pulverized material with water and a binding agent, pelletize, and then burning the admixture at temperatures above 900C. whereby a sintered agglomerate is formed which can be used in the smelting industry in calcination processes without the usual problems of dust or fines.  
  The absolutely critical step in the process is that the crystalline carbonate be pulverized to a size smaller than its single crystal size. While it would be expected that this would cause additional dusting and fines problems, it has been discovered that quite the contrary is true in the process of the present invention and that an agglomerate material is formed which has very little dust and fines, usually even less than the strong type of amorphous carbonates. It has been found that pulveribe added to form agglomerates is within the skill of the art and is generally from about 5 to about 15 parts by weight of carbonate material. Similarly the amount of binding agent to be added to form the agglomerates is within the skill of the art and should generally be from about 0.5 to about 5 parts by weight of carbonate material.  
  After the carbonate material is agglomerated, it is burned at temperatures above 900C. and preferably at temperatures of l000-l500C. Normally the agglomerated material is pelletized before burning and the agglomerates are dried before the burning process. Burning is suitably done in a rotary kiln or a vertical shaft furnace and the drying can also take place in a lower temperature zone of the rotary kiln or shaft furnace or the drying can be done in a separate drying furnace if desired.  
  The binding agent of the present invention may be any binding agent which will hold the pulverized carbonate mineral in agglomerated condition during the burning process. Suitable binding agents include bentonite, sulphite lye, molasses, silica-dust, alumina, or burned carbonate material such as burned dolomite, burned lime or burned magnesia. It is especially of advantage to use the dust which is formed during the burning process as the binder for subsequent formation of agglomerates. Where the process employed is a continuous one, part of the dust formed during the burning process can be returned to the process for use as the binder material. This has obvious economic advantage and it also has technical advantage since there is no introduction of a foreign binding agent.  
  When forming the agglomerated material, it is sometimes desirable to add other materials such as reducing agents, fluxes, or alloying components to be used in the subsequent smelting process so that the agglomerated material obtained is better suited for the subsequent smelting process.  
  Quite surprisingly the burned agglomerates of the present invention have very high strength compared to amorphous carbonates found in nature. Pellets formed according to the present invention from pulverized crystalline dolomite with 3% returned dust and 1% bentonite and calcined in a rotary kiln at temperatures up to 1100C. were compared to natural crystalline dolomite, amorphous dolomite of the usual type and amorphous dolomite of the strong type and the following sieve analysis was obtained.  
 zation of crystalline dolomite or crystalline limestone to a specific surface of more than 5000 cm per cm is suitable for the present invention. The pulverization is done in known manner and may be by either the wet or dry method.  
  After the crystalline carbonate is pulverized it is combined with water and a binding agent to form agglomerates of the pulverized material. The amount of water to It has also been quite surprisingly found that the calcined agglomerates of the present invention have far better handling strength than natural carbonate materials. Handling strength of the materials was tested in a rotary drum equipped with internal ribs in order to effect abrasion. Pellets produced from crystalline dolomite in accordance with the present invention gave 0.53% dust after 200 revolutions in the rotary drum and ll% after 1000 revolutions. Lump amorphous lime burned in a similar manner gave l0-40% dust after 200 revolutions and burned amorphous dolomite gave 20-90% fines after 200 revolutions.  
  The following examples are illustrative of the present invention and in each of the examples the compression strength of the burned agglomerate is given as well as dust formation measured as percentage of thru l mm material after 1000 revolutions in the rotary drum equipped with internal ribs.  
 EXAMPLE 1 Crystalline dolomite was pulverized below its single crystal size and was admixed with by weight water, 2% by weight bentonite and 2% by weight silica-dust, the latter two being the binding agents. The composition was formed into pellets and the raw pellets had a compression strength of 1.6 kilos. The pellets were dried in a drying furnace and in the dried state the compression strength was 7 kilos. The pellets were then transferred to a rotary kiln wherein they were burned at temperatures up to 1200C. Samples taken out at ll00, ll50 and 1200C. had compression strengths of l2, l6 and 18 kilos respectively. The dust formation (percent material thru 1 mm) for the material burned at ll50C. was 3.5% after the 1000 revolutions of the ribbed rotary drum.  
 EXAMPLE 2 Crystalline dolomite was pulverized to below its single crystal size and combined with 8% alumina, 5% returned dust and sufficient water to form agglomerates. The raw pellets obtained had a compression strength of 2.8 kilos and after drying the pellets had a compression strength of 20 kilos. After burning in a rotary kiln at 1200C., the burned pellets had a compression strength of 26 kilos. The dust formation was 3%. (Drum test) EXAMPLE 3 Pellets were produced from pulverized crystalline dolomite which had been reduced below its single crystal size combined with 8% alumina, 3% bentonite and sufficient water to form the agglomerates. The pellets were dried in a low temperature zone of a rotary kiln and were then burned in the rotary kiln at temperatures up to 1100C. The raw and dried pellets had compression strengths of 1.5 and l l kilos respectively whereas the compression strength of the burned pellets was 55 kilos. Sieve analysis showed that 99% of the burned material had a grain size above 4 mm and dust formation after treatment in the ribbed rotary drum was 4.5%.  
 EXAMPLE 4 Pellets produced in accordance with Example 3 were Burning temperature C. 1250 I350 Compression strength, kilos 33 60 Dust formation (Drum test) 5.0% 4.0%  
 EXAMPLE 5 Pellets were produced from crystalline limestone pulverized below its single crystal size with 3% binder and sufficient water to form the agglomerate. The pellets were dried and then burned in a shaft furnace at 1325C. The compression strength of the burned pellets was 46 kilos and dust formation was 1%.  
  It will be understood that the claims areintended to cover all changes and modifications of the preferred embodiments of the invention, herein chosen for the purpose of illustration, which do not constitute departures from the spirit and scope of the invention.  
 What is claimed is:  
  1. The method of producing burned crystalline carbonate materials to form agglomerates suitable for use in smelting processes comprising:  
 a. pulverizing a carbonate material selected from the group consisting of crystalline dolomite and crystalline limestone to a size less than the single crystal size of the carbonate material.  
 b. admixing the pulverized carbonate material with binding agent in an amount from about 0.5 to about 5 parts by weight of carbonate material and sufficient water to form a raw agglomerate, at least a part of the binding agent being dust formed from a prior burning of the same carbonate material,  
 0. drying the raw agglomerate,  
 d. burning the agglomerate at a temperature from about l000 to about l500C., and  
 e. using the burned agglomerate material obtained in a smelting process.  
  2. The method of claim 1 wherein the carbonate material is pulverized to a specific surface of I at least 10,000 cm per cm.  
  3. The method of claim 1 wherein the binding agent includes a member selected from the group consisting of bentonite, sulphite lye, molasses, silica-dust, and alumina.