FIELD OF INVENTION
The present invention generally relates to tube or tumbling mills for crushing and grinding of coal, and In particular to an improved ceramic bail adaptable to tube mill for crushing and grinding of coal.
BACKGROUND OF THE INVENTION
Majority of Thermal Power Plants use pulverized coal for fossil fuel fired boilers. The pulverization of coal is done in a number of equipment like Bowl Mill, Tumbling or Tube Mills, Bali and Ring, Tyre Type Mills, etc. The coal pulverizing equipment is chosen based on the advantages it offers in terms of reliability, power consumption, minimum maintenance, a wide range of capacity, low wear of replaceable grinding media, etc. Tube mill is one such mill which has the advantage of producing good fineness of pulverized fuel, without requirement of any coal reject handling equipment. Such mills do not generate any noise during operation and maintains clean environment. However, it has the disadvantage of high specific power consumption and high running cost compared to some of the Other coal pulverizing mills like bowl mill.
In a tube mill, primarily crushed coal having average size range of 25 mm major dimension are introduced through a trunnbn of a screw conveyor into a rotating drum/tube lined with replaceable wear resistant liners and filled with unattached toughened, impact resistant, wear resistant balls. The bails have the function of crushing and grinding the coal by their kinematics motion within the milt.
Hot primary air is introduced into the mill throu^^ a control tube. It completes the coal ck-yhg and carries the pulverized coal out of thw mill via the trimnicNi annulus around a central tube counter to a raw coal feeding device of the mill.
The coal crushing takes place by the impact of the balls on coal bed as they tumble down with the rotation of the drum. The grinding takes place as the coat particles are dragged between balls as they move with the rotating drum. Both the actions cause we» of the ^hiding media. The wear is more if the coal contains a hi^ percentage of alM-asive ash or if a very fme grirKl is necessary.
The total rx>wer c(MisumpticHi cm be maMy contrKnited to ttie erMirgy rec^ired to move the ball charge m the mill snd the frk:ticm of tlw balls. The other components of power con^jmptkm btrng tht bearNigs &rd ti% transmission gears which atthou^^ are ccmiparatively less.
As a ^lecific example one can mentkm that tiw avera^ power consitfnption of a 4.7 meter diameter tube mill rotating at 120 rpm with 78/90 ton of steel ball and 85 tons of coal fbw per hour is 1.3 MW.
The steel balls used are prone to ri^kl wear with aixds'wt coal as mentioned e^rljet^-AttoQipt has been made by Aineryy Cy«i»Tikl Ccanpany (US Patent No.751,'^458)^ overcome the pr(^lem of high we»^ and also to prevent
contaminatkHi by steel balte whkh is ctetrimental to tl% ^ound |M'ockK:t. The US
reference suggested a grvidHig medkvn fcMrJumnbiing mills com^ising a core of
lead and an outer wearing surfacr romprisinq a hard ceramics, average specific
gravity beuig 7. The g-wKlwig media can be rods for rod mills or balls fcM- ball
mills. However, this type of balls pose maruifactifflng diffkiulties or breakage
pi' probtems. Moreover, having specific gravity ntds 7 does reduce the power
consumption.
OUECTS OF THE IMVENTION
It is therefore dm object of the invention to ixopose an improved ceramic bail ad^t^le to tube mills for crushkig and sH'inding of coal which elmninates the disadvantages of the prior art.
Another ob^ct of the inventk>n is to propose e mills for crushw^i and grnxtv^g of coat which has high wear nresistant properly.
A stili ^flhSf dbjie! of ti% InventkHi is to propose an improved ceramic ball adaptable to tube mitts for crushing and grinding of coal which substantially reduces the power consumption of the tube milt.
still another (^ject of th« invtntion is to |xqK>se an improved ceramic bail adaptable to tube mills for crushing and grinding of coal which entails less manufactiffing cost liHJt provittes a k^ngfit service life.
SUMNARY OF THE INVENTION
Accordingly there is provided an Nfnproved ceramic ball adaptable to tube mill for crushing and grindrig coal, ccxnpreing a solid ball configured using a of hard ceramic havirH2 diameter between 30 to 50 mm, wherein the specific p^avity of the ball being matoitaried in a range between 3 to 5.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - Shows a ball tube with Air and coal circuit.
Figure 2 - Shows a ti^ mill bcxiy Kcommodatv^ the ceramic balls.
Figure 3 - Shows a ceramic ball KCordN^ to the invention.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 shows the schematic diagram of Tube Mill showing the fbw of coal and primary air inside the mill with ball ch«^ge. The raw coal is tarfcen from a hof^r by a feeder (1) whose ^peed is Mifcomaticaily ccxitrolled. At the feeder discharge tiie coal falls into a mixnig box (2) where it is {xre (ihried by the hot by pass air. The coal then enters the mill by the trunnion (3), whose screw conveyor (4)
pushes the coai inside tiie mill by its rotational movement. The coai is then pulverized by the movement of the b»alts (5) inside the rotating body (6) lined with wea* res»t«it tiles or Ikiers (7).
The hot primary air is introckiced rito the mill tivcHjgh the central tube of the trunnion. It completes the coal ikymg axvi civries the pulverized coal out of the mill via the trunnion annuhis around the central tubt, counter to the raw coal feeding the mill.
Tl% pulverized coal «kI f^'knary anr mixed with by pass air from the mixing box enter the cl^sifier (8) vistalied above the mills, often at the feeder level. F^e 2 shows movement and tumblir^ of bails inskk the dri^ section of the ti^ mill as it rotates. Because of kiteractk)n of centrifugal fcNrce generated by rotation of the mill body, frictkm fortt between the balls «kI ti% coal bed «kI tfw Ihiers p|jK:ed inside the mill body and the reacticNi fm^ce from the iNM^r, the bails are lifted up along with the mill body m the dvectk>n of rotation of the mill. After lifting to a hei^^t specified by the inters^tion of the above mentioned forces, the balls tumble down onto the coal bed as shown in Pi^re 2. The pulverization takes place initially by the imp^t of the falling balks atvi then by the finding action of the moving balls. Figure 3 shows a spherical ceramk ball (5).
The prkN* art finding medkffn in the f«^m of a ball compritesjijcore having lead, and an outer wearing surfacejc^niic^iDa a hard ceramk. The specific gravity of gravity of such fx'm art balls remahis afH^oxmnately 7'. According, the i^esent inventton, the im^Mroved cerwnk: t»ll constitutes a solkl ball thereby eliminating die complkated process of m«%ifactiM'in@: a connposite structiN-e of a ceramic
outer layer and a lead core. Secondly the specific gravity of the improved ceramic ball has been maintained according to the invention in the range of 3 to 5 the solid ceramic ball of reduced density brings down the mass of the bail charge by 30 to 60% over the composite ball mentioned in the patent aboye or the presently steel balte in ti^e mills for ccmiI pulverization.
As the total mass of ceramic bail charge reduces with specific gravity change between 3 to 5 from 8 for the same filling ratkwl as that used with steel ball charge, the power consumption will come down drastically as the bulk of the energy required to rvn the pulverizer is consimied in liftrig the mediimi to a height which ensures tNs maxNniNn potential energy of the medium prior to free fall.
Two factors have been consictered for the pulverization of coal, impactrig velocity and mass. The b^eic issue involved here is tiw relationship of impact velocity and impacting mass with the efficiency of coal breakage. It Is established that iiKreisNig the medtiKTi vokMtie is (Jtetrmfientel to brearica^ e^iciency while impact vebcity plays a beneficial role. It is known that given a critkal impact energy rec^ired to riitiate crack nucleatkMi, the breaka^ at a const«it impact energy increases with impact velocity. As a matter of fact due to the strong relatbn whkh exists between impact vebcity and stress wave propagatbn in coal it has been consklered that the role of impact vebcity in coal breakage s^Mircedes that of impi^t energy whk^ only sets the initial bvet of stress to initiate fracture. If a critical level of impact energy is mainteined then it is the vebcity and not the mass which controls bres^age and reduce power consumptbn. Thus, taking full advantage of the 'additbnal stress wave'related
effect of impact vebcity, the bail mass can be reduced by reducing medium density. Based on this Iftidr^, in a pulverizer mill, where the impact velocity is high, it is possible to reduce the dtns'itf ^f^the medium without i^eatly sacriftcing the breaka^ efficiency and sNDultaneousiy gauiir^ s^ificantly ni reckling power consumption.
It is thus possible to ckicrease ^wer consiffnption in a mill wHhout sacrifKing grirKling efTicierKy and it is possttile by ushg a hi^ly wen- resistant medkim. The additional cost of (he latter is signi^witiy offiset by rechiction ni power consiwnption cost by reducit^ the der»ity of tiie medkffn.
The prior art entailed vicreasNig ball size, wettsig the medium, changing mill parameters «id fnally usir^ vtem resistait medkNU. All of these Hicrease costs without decreasing power cor^imption. The overall cost therefore irKreased with each rmovation. The present fftven^m teads to a decrease in cost. The Hicrease in cost per material wei^t of ceramk media will be compensated by the decrease m total wei^t of the ball charge «kI the hicreased life of cermnic balls will rechjce the consiffnption ccMst. In ackiitioi, there will be atleast 15% saving in power consumption cost.
The communication process consists of a large chimk bre^age arKi small particle ^induig driven by friction. The latter which constitutes a small compcwient of power consimiption is influenced by the wei^t of the medkim. Oecre^mg the density of the medkmi may bad to some sacrifice of ^ overall breakage effickency.
WE CLAIM
1. An improved ceramic ball adaptable to tube mill for crushing and grinding coal comprising a solid ball configured using a hard ceramic having diameter between 30 to 50 mm, characterized in that the specific gravity of the ball being maintained in a reduced range between 3 to 5, to achieve pulverization of coal with reduced power consumption of the pulverizing mill due to reduced density of the medium and enhanced impact velocity of the balls.
2. The improved ceramic ball as claimed in claim 1 wherein the hard ceramic comprises at least 90% alumina.
3. The improved ceramic ball as claimed in claims 1 & 2 wherein the power consumption saving of the tube mill is at least 15%, on reduced density of the balls bringing down the mass of the ball charge by 30 to 60% over the composite ball used in the existing practice.

This invention relates to an improved ceramic ball adaptable to tube mill for crushing and grinding coal, comprising a solid ball (5) configured using a hard ceramic having diameter between 30 to 50 mm, wherein the specific gravity of the ball being maintained in a range between 3 to 5.