CA1073214A - Process and apparatus for production of slag material of reduced density - Google Patents

Abstract

Abstract of the Disclosure A process is disclosed for the production of light-weight aggregate by pelletization of slag materials, particularly blast furnace slags of base/acid ratio below 1.1, which otherwise have proven difficult if not impossible to pelletize to the commercially required density specification for the pelletized material. The molten slag is poured to form a stream and from about 50 to about 400 pounds per ton of finely-divided inert mineral particulate is added thereto, together with or followed by sufficient water to obtain foaming and a desired pyroplastic state. The exact amount of added material will depend among other factors such as the desired density and particle size of the pelletized material. The pyroplastic slag mixture is then impinged upon a rotating projector, which throws it into the air for a distance sufficient to ensure that it will cool and harden to the extent that the particles retain their individual identities upon reaching the ground. The added mineral material acts to control the cooling rate of the molten slag; due to the similar values of their specific gravities they blend with the molten slag much more readily than the water. Suitable materials for addition are, for example, crushed or screened fine slag and sands, for example waste foundry sand.

Description

- ~V'~3~

PROCESS AND APPARATUS ~OR PRODUCTION OF SLAG MAT~RIAL
~ OF R~DUCED DENSITY
Field of the Invention The present inven-tion is concerned with processes and apparatus for the production of slag material of reduced density by the pelletization thereof, and especially to such processes and apparatus capable of producing generally-spherical, closed-surface "lightweight" pellets from such materials.
Description of the Prior Art The handling and disposal of the large quantities of metallurgical slag such as blast furnace slag that results from iron and steel production is a continuing problem, and much research has been carried out into methods of converting the slags into commercially usable forms. In one process that has been extensively used hither-to the slag is quenched rapidly by dumping it into large quantities of water in an open pit. The cooled material that results is of random size, from powder to large lumps, and generally is of porous, brittle nature. The cooled slag removed from the pit is subjected to a series of sieving and grinding operations, whereby it is sorted into different, uniform particle sizes. The resulting products may be used for different purposes, depending largely upon the particle size, for example, as an aggregate for concrete or as ~ a constituent of concrete blocks.
'l 25 There has been disclosed in our U.S. Patent Serial ' ' No: 3,594,142, a process and apparatus for the spherulization of such slag materials. In such a process the material is fed to the apparatus at a controlled rate, is mixed with a controlled quantity of water, and is allowed to mix with the water for a period, causlng it to achieve a desired foamed .

.,, ~ .

, . - .: - ,, . . - . . .:, , -, . . . , . , . : . .. " . . . . ..

~Ot~3Z~

pyroplastic state, before it is impinged upon a rotating water-cooled projector. The projector projects the foaming pyroplastic slag into th~ air in the form of separate particles at a velocity and inclination, and with a motion, such that while in the air the individual particles are spherulized and are also cooled sufficiently to retain thelr individual identities upon reaching the ground, .i~e. there is negligible tendency to agglomerate or coalesce with other partlcles.
This process and apparatus have proven very successful and, besides providing a commercially-desirable "lightweight", closed-surface aggregate, has three very significant advantages:
a) The material can be produced with use of only about 2-15% of the quantity of water employed in prior art -granulation processes, b) The residual moisture within the pellets is low, generally 3-10% by weight, as compared to the values of 25-50~ for conventional quanulation processes, c) The pellets retain in their interiors large quantities of the potentially gaseous sulphur compound-q produced by reaction of the slag with water, drastically reducing the quantities that are released into the air and the consequent atmospheric pollution.
~ he North American steel industry has a special ~ystem for classifying slags and makes use o a base/acid (B/A) ratio measured by the following formula:
: ;. c ~
S102 + A12 The slag is classed as "basic" if the ratio is more than one, and "acidic" if the ratio is less than one~ It will be u~der-~ si~i~
stood that ~a~ ~ or acidity a~ expxessed by this ratio is not to be confused with alkalinity ~ acidity a~ expressed . .

- 2 ' . , '~

.
... ... .. ~ , . :
.:

~ 3~

by the pH value. It has now been ~ound -that the process of our U.S. Patent No: 3,59~,142 is highly effective with slags of s/A ratio 1.1 and higher, but becomes less effective as the ratio decreases below 1.1, to the extent that pelletiza-tion to the required low density proved extremely difficult if the ratio had the value 1.0 or less.
Summary of the ~Invention It is an object of the present invention to provide a new process and apparatus for the pelletization of metallurgical and like slags.
It is a more specific object to provide a new process and apparatus for the pelletization of blast furnace slags of B/A ratio less than 1.1 in order to achieve required low pellet densities.
In accordance wi-th the present invention there is provided ; a process for production of slag material of reduced density by the pelletization thereof, comprising the steps of:
a) forming a stream of molten liquid slag of controlled rate of flow;
b) delivering into the said stream of slag a controlled ;~ quantity of finely divided inert mineral material;
c) mixing the said stream of slag and added material with a controlled quantity of water such as to plasticize and expand the slag to a pyroplastic state and viscosity for , 25 sperulization thereof and permitting interaction between the slag and the water to proceed until the mixed material is in , the said pyroplastic state and viscosity; and ` d) thereafte;r impinging the mixed material in the said state on a rotating mechanical projecting device and thereby projecting the mixed material through the air at a speed and ' '' ..

~t~3~

for a distance sufficient for the ~enerally spherical pellets formed thereby to be sufficiently cooled to be self-sustaining and to retain their individual identities upon reaching the ground.
It is found that the process of the invention permits successful pelletization to the required density specifieation over a wider range of slag ehemistry than has been possible hitherto. The proeess is especially applieable to slags of lower base/aeid ratios, whieh hitherto have proven diffieult to pelletize, but is not limited to use with such low ratio slags, as it also shows signifieant improvements in density reduetion and pellet produrtion for slags of higher ratios.

. - 4 -V

Descri~t.ion of the Drawi~
Processes and apparatus which are particular preferred embodiments of the invention will now be de cribed, by way of example, with reference to the accompanying diagrammatic drawing wherein:-FIGURE 1 is a general perspective view from thefront and to one side, .s.
FIGURE 2 is a side e].evation to a smaller scale and showing the pouring of slag into the apparatus from a ladle, and FIGURE 3 is a schematic figure to how the relation between the various parts of a piece of the apparatus.

Description of the Preferred Embodiments The apparatus illustrated taXes the form of a structure indicated generally by the refere~ce 10, thi~
structure being mounted in a special recess l2 at the front edge of a pit 14. Means for mounting this structure i~ the : pit consist of a base 16 ha~ing vertical standards 18 upstanding therefrom. The standards support cross-members 20 each of which in tuxn supports a respective set of helica~
coil compression sprLngs 22 mounte~ with their compression axes vertical and upon which tha structure 10 is mounted.
Referring to F:igure 2, in the illustrated embodiment, a uniform stream 24 of ~lag is delivered to the structure 10 .

':

~.

.
' ' .

~ ~ ' . ' ' . ' . .. : ' ~t~3Z~

by controlled pouring from a laclle 26 mounted upon a vehicle 28. In other embodiments this stream 24 may instead be delivered directly from the furnace via a runner.
The downwardly-falling stream of slag impinges upon dow~wardly-inclined front surface 30 and rear surface 32, which are ~ormed of a suitable hleat-resistant material (such as graphite). The structure 10 is mounted by the springs 22 for free vibration relative to the base portion, and controlled vibration is provided by a suitable vibrator (not illustrated) connected to the struc~ure. This ~ibration i8 highly effective in facilitating the mixing and preventing sticking of the ~lag to the surfaces 30 and 32. At the same time water at a controlled rate of flow, related to the controlled rate of flow of the slag, is added to the 31ag lS stream from nozzles 3~, 38 and 40 fed by supply plpes 41.
The water streams mix and interact with the ~lag an~, at the very high temperature of the Ylag (usually ~bout 2500F) begin to vaporize and commence its expansion and/or disinteg-ration and cooling.
~ controlled quantity of finely divided inert mineral material coolant is deliver~d~into the slag stream and in this embodiment the means for-effecting this addition thereto include a hopper 42 into which the bùlk material is delivered through a grating 44 which removes large lumps therefrom.
The hopper di charge drops the material onto a vibrating feeder 46 operated by a motor 48, the feeder moving the fine material at a controlled rate onto the lower end of a belt conveyor 49. The upper end of he conveyor diæcharges into a chu~e S0 which deliver the material into the open mouth ; 3G o~ the structure 10, ~o ~hat it falls in~o the ~lag ~ream .
- 6 - ~ ~

~ ' , ' ' " ' ' ~

, ~3 and is entrained thereby.
The downwardly-moving mixed stream of slag, water and finely divided material is impinged radially on to the surface of a motor-driven rotary drum 52, which ls mounted by bearings 54 with the axis of rotation of its axle horizontal.
The motor for driving the drum is not illustrated. The periphery of the drum is provided with radiall~-extehding flinging or projecting vanes 56, which engage he downwardly-moving stream and flins the contents thereof through the air over and into the pit 14~ The interior of ~he drum 52 is supplied with water through plpe 57~ which exits therefrom via slots (not shown) in its periphery.
In the operation of a process in accordance with this invention the ladle 26 that has been illed with moltsn slag from the furnace i8 moved as quickly a~ possible to the pelletizing apparatus, and is operated to pour as uniform a stream of slag as possible through the apparatus, while the rate of feed of the water and of the finely-divided material - are maintained reasonably constant, thereby producing operat-ing conditions that are as constant as possible. Because of the high temperature of the slag thare is an immediate reaction as the three components mlx, with the result that the slag is expanded and is in a desired pyroplastic state with the finely-divided materia:L entrained therein by the time that it contacts the periphery oiE the rotating drum. The rate of flow of the ; slag stream, the quantity of mineral material and water fed into the streaml the peripheral speed of the drum, the shape -of the vanes 56J and the angle at which the mixture is projeated from 1:hP drum are all controlled so as to cause the slag to remain iLn the air for a time and distance sufficient to . .

. .

~ 32.~

cool it, and to form it into generally-~pherical, expanded self-sustaining pellets that will rekain their individual identitie~ upon reaching th~ ~round. An examination o the.
interior~ of the pellet~ produced by the apparatus ~how~ that they have one or more o~ the particles e~b2dded therein, showing that the entrained particles have acted as nucleatlng centres around wh~ch molten pyroplaBti~ 8`1ag has coll~ted and coalesced.
In ~he e~3mples which ~ollow, lllustra~ing the inven~io~, the ladle 26 ha~ a capacity o~ about 25 tsn~ and thQ glag iB poured at a rate of about;3/4 to a~out 1-1~2 ton per minute. Water i9 ~ed to the nozzles 36, 38 and 40 and to the drum 52 to gi~e a total ~low af about 100 to 200 gallons per minute, while the drum 52 ~8 ro~a ed to give a peripheral ~pe~d at ~he ~ip~ of the vanes o~ about 40 ~o about 55 feet pex second. ~t has been found that by employ-ment of a proces~ ln accordance with the inve~tion ~lag~ ofbase/acid ratio of less than 1.1 can readily be pelletized without agglomeratlon in the pi~, at lea~t approximately 70 ~0 of the total weight of the ~lag th~t 13 poured through the apparatu~ being oonverted ~o ~he fo~n of generally-spherica~
expanded pellet~ having a range in~dl~meter of from about 0.~ ~
lnch to about 0.06 inch, with the majority having a diam~ter of about 0.25 inch . MoreoYer ~ ~ t ~ 3 found that the pellets produ~ed by the process tend to have a clo~ed or smooth rela ~voly non-porou3 surface wi~h a mor~ porou~ ~nterio$ coreO
. llhe process of the invention is particularly recom-- mended for'use with near-neutral or acid slags of base/acid ratio less than one, which are sometimes referred to in the art as "silicious" slags. With these slags difficulty is usually '' '''_ \

~ 3Z~l~

experienced in obtainin~ sufficient expansion and in pre venting agglomeratlon of the peilets which remain plastic after they strike the floor of the pit. It can also be employed with basic slags, for example, to incr~ase the yield and reduce the density of the pellets. It is at pre~ent believed that the principal reason for di~ficulty in obtaining completely satisfactory pelletization with acid ~la~s, using a prior process as disclosed in our U.S. Patent Serial No.

3,594,142, is that the melting point of th~ slag decreases markedly with its acidity. It appears that the water added has difficulty in penetrating into the liquid slag, and there is therefore much less possibility of satisfactory foaming i and cooling steps. Th~ resulting pellets are highly "vitrified" or "glassy" in physical structure (i.e. not of the desired porous core structure described above).
It is not ~ound necessary or even desirable for the added material to modify the chemistry of, or to react chemically with, th~ slag so that it is believed to be pre-dominantly a physical e~fect that is taking place, the added ~0 material providing a "pre-cooled" nucleus around which a spherical pellet can more easily grow. It is also b lieved that ~ome of the beneficial effect is due to the higher specific gravityl 50 that the particles thereof can penetrate more easily than water droplets into the body of the liquid slag. All commercially-available particulate material, unless pre-dried, has ~ome moisture content, usually 5-10%
by weight, and the particulate material is able to convey this small but perhaps significant amount of moisture into the interior of the cooling particle, a~ described abo~e.
A further significant advantage is that the positive 9 _ ~

.

~3Z~

introduction of cold particles into the molten Rlag more quickly reduces the temperature of the slag to the poin-t at which it will solidify ~ufficiently during its projection through the air.
Examples 1 - 7 Untreated Treated Perceht Means Density Density of No. lbs/cb.ft lbs/cb.ft Reductio~ ndin~
1 61.7 52.6 14.7 Alr injection 2 69.0 59 14.5 "
3 56.0 49.7 11.2 Conveyor

4 57.5 52.6 8.5 62.8 55.~ 12.4 "
: 6 54.8 50.8 7.8 n 7 53.4 44.8 16.1 Average 59.31 51.64 12.9 "
In all of examples 1 to 7 the slag employed was only very slightly basic (base/acid ratio 1.05).. The particulate material used was damp crushed pelletized slag of particle size less than 3/16 inch, the material being fed to the slag stream at a rate o~ 200~pounds per 2000 pounds of slag. In ; the slag ~ndu~try the principal requirement for coars~ expanded lightweight slag is a density les than 55 pounds per cubic foot. It will be seen that only examples 6 and ~ of the '. 25 untreated slags are below this value, while only examplè 2 of the treated slags (which was very dense`to begin with) is above. With examples 1 and 2 the particulate material was air injected to assist p~netration of the slag, but was found to be unneces~ary and the simple conveyor addition pro~edure employed i~ comp:Letely satisfactory. . ~ ~

.
.: :
~, - 10 - ' ' ~ 3 Examples 8 - 12 Base/ Untreated Treated Percent Acid density density Reduct-No. ratio lbs/cb.ft lbs/cb.ft ion Material 8 1.00 61.7 56.2 8.5 ~ Black Waste ) Foundry 9 1~16 56.1 51.2 805 ) Sand 1.20 52.8 49.0 9.0 ) Yellow 11 1.22 49.0 41.6 15 ) Fine 12 1.17 47.9 39.4 18 ) 5and In example 10 using yellow fine sand the sand was saturated with water before addition, but this did not appear to assist the process and the improved results of examples 11 and 12 with naturally occuring moist~re contents may be noted~
The various parameters to be considered will now be discussed ; 15 individually.
Basicit~ of the Slag ' ~ s shown by the examples slagq of base/acid ratio of greater than 1.1 have been successfully pelletized, and the ; excellent density reduction of examples 11 and 12 will be noted to show ~hat the process does have application to slags of value greater than 1,.1. The process may also be found of value with basic slags in which pelletization to the desired density is inhibited by some other actor e.g. low slag temperatur~. In commPrcial practic~ tha resultant basicity of the slag from the iron- and steel-making process is not of prime concern to the iron and steel maker, and the slag processor must operate with the ~lag as delivered from the furnace, since i.t i~ not practical to transport heated slag ` any substantial distance or to reheat cooied slag. It i~
normal ~o blend different slag "castsl' to achieve an averag~
~.

~lre value and the fact that a few of these casts i~-slightly i~ r above the desired density is thexefore not significant.
Nature of Added Particulate Material As stated above, it ir, not desired for the material to have any significant chemical reaction with the slag, and it may therefore be characterizPd as chemlcally inert.
Crushed slags and various sands have been employed successfully, and the materials to be employed may be defined as mineral particulates. It is generally desirable that the density of the added material be as low as possible, since the use of high density material opposes the objective of lower density for the expanded slag. The density of slag i5 usually of the order of 2.0 to 2.5. The ùsual valuP for sands is in the range 2.5 to 2.8. Silicious materials appear to be the most desirable commercially as being economical or being available in the form of waste products of the right particle size, ~uch as the above-mentioned foundry sand. Othex possible materials - are crushed and screened stone or gravel.
Particle Size of Added Material As described above the process is found to work satisfactorily with typical crushed slag particles that will pass through a 3/16 inch screen, and with foundry and natural fine sands. It is ound that the process shows improvement as the particle size is reduced, but he economics of the process do not justify the cost of grinding any other than material a:Lready available "on-site", such as slag.
Particles much smaller than will pass through a 200 mesh screen (75 microns) are believed to be too small to be effective for the purpose of density reduction.
~ ~d ~atc~
Commercially $t is only practicabls to Qpecify the ..

-~, . ~, . .. , .. .. ... ~ .: .

~LOr~32 1~

weight of added material per ton of slag processed, although it is the number of particles oE suitable size present that determines its effectiveness, and a smaller weight of fine material will contain as many particles as a larger weight of coarse material. Similarly the preferred less dense mat-erials will weigh lighter for a given nu~ber ~f particles than the more dense materials~ To achieve specification densities in slags processed according to this invention using less dense fine particle materials will require not less than 50 pounds per ton of slag, although a vaLue of 150-200 pounds per ton is preferred. The addition of more than 400 pounds per ton appears to merely involve wastage of part of the added material.
Point of Feed .
Clearly it is desirable to provide as much time as possible for the particulate material to be mixed with the slag and to take part in the foaming reaction, so as to ensure even mixing and distribution. The simple operation illustrated of pouring from above into the descending slag stream has been found~to be ade~uate with the slags investigated. "

~ :.

~ .
' ' , . :

.', ' .

. .

:
.
- . ~ . .

Claims (6)

1. l. A process for the production of slag material of reduced density by the pelletization thereof comprising the steps of:
   a) forming a stream of molten liquid slag of controlled rate of flow;
   b) delivering into the said stream of slag a controlled quantity of finely divided inert mineral material;
   c) mixing the said stream of slag and added material with a controlled quantity of water such as to plasticize and expand the slag to a pyroplastic state and viscosity for spherulization thereof and permitting interaction between the slag and the water to proceed until the mixed material is in the said pyroplastic state and viscosity; and d) thereafter impinging the mixed material in the said state on a rotating mechanical projecting device and thereby projecting the mixed material through the air at a speed and for a distance sufficient for the generally spherical pellets formed thereby to be sufficiently cooled to be self-sustaining and to retain their individual identities upon reaching the ground.
2. 2. A process as claimed in claim l, wherein the finely divided inert material is added in the amount of from 50 to 400 pounds per ton of slag processed.
3. 3. A process as claimed in claim l or 2, wherein the finely divided material is slag, stone or gravel crushed or screened so as to pass through a sieve of 3/16 inch opening.
4. 4. A process as claimed in claim 1 or 2, wherein the finely divided material is slag, stone or gravel crushed or screened so as to pass through a sieve of 3/16 inch opening, and wherein the amount of added crushed slag is about 150 to 200 pounds per ton of slag processed.
5. 5. A process as claimed in claim 1 or 2, wherein the added material is a fine natural sand.
6. 6. A process as claimed in claim 1 or 2, wherein the added material is waste foundry sand.