Method for operating a fluidized bed boiler

Abstract

The invention relates to a method for operating a fluidized bed boiler (6), comprising the steps of: a) providing fresh ilmenite particles having a shape factor of 0.8 or lower as bed material to the fluidized bed boiler (6); b) carrying out a fluidized bed combustion process; c) removing at least one ash stream comprising ilmenite particles from the fluidized bed boiler; d) separating ilmenite particles from the at least one ash stream, wherein the separation includes a step of using a magnetic separator (12) comprising a field strength of 2,000 Gauss or more; e) recirculating separated ilmenite particles into the bed of the fluidized bed boiler; wherein the average residence time of ilmenite particles in the fluidized bed is 100 h or more.

Claims

1. A method for operating a fluidized bed boiler (6), comprising the steps of: a) providing fresh rock ilmenite particles having a shape factor of 0.75 or lower as bed material to the fluidized bed boiler (6); b) carrying out a fluidized bed combustion process; c) removing at least one ash stream comprising ilmenite particles from the fluidized bed boiler; d) separating ilmenite particles from the at least one ash stream, wherein the separation includes a step of using a magnetic separator (12) comprising a rare earth roll or rare earth drum magnet and a field strength of 2,000 Gauss or more, wherein the separation efficiency of step d) is at least 0.5 by mass for ilmenite, and wherein the separation is at least one of a one-stage magnetic separation including an axial or radial magnetic field, and a two-stage magnetic separation with a first step using an axial magnetic field and a second step using a radial magnetic field; e) recirculating separated ilmenite particles into the bed of the fluidized bed boiler; the method further comprises a pre-classification step, in which the particles in the at least one ash stream are pre-classified before magnetic separation of the ilmenite particles from the ash stream, wherein the pre-classification comprises mechanical particle classification comprising sieving with a mesh size from 200 to 1,000 μm, wherein the average residence time of ilmenite particles in the fluidized bed is 100 h or more.

2. The method of claim 1, characterized in that the fresh ilmenite particles comprise a particle size distribution with a maximum at 100 to 400 μm.

3. The method of claim 1, characterized in that the at least one ash stream is selected from the group consisting of bottom ash stream and fly ash stream.

4. The method of claim 1, characterized in that the pre-classification further comprises fluid driven particle classification.

5. The method of claim 4, characterized in that the mechanical particle classification comprises sieving with a mesh size from 300 to 800 μm.

6. The method of claim 1, characterized in that the separation includes a step of using a magnetic separator (12) comprising a field strength of 4,500 Gauss or more.

7. The method of claim 1, characterized in that the average residence time of the ilmenite particles in the fluidized bed boiler (6) is at least 120 h.

8. The method of claim 1, characterized in that the average residence time of the ilmenite particles in the fluidized bed boiler (6) is less than 600 h.

9. The method of claim 1, characterized in that the boiler (6) is a circulating fluidized bed boiler (CFB).

10. The method of claim 1, characterized in that the separation efficiency of step d) is at least 0.7 by mass for ilmenite.

11. The method of claim 1, characterized in that the fraction of ilmenite in the bed material is 25 wt. % or more.

12. The method of claim 2, characterized in that the fresh ilmenite particles comprise a particle size distribution with a maximum at 150 to 300 μm.

13. The method of claim 4, characterized in that the pre-classification further comprises gas driven particle classification.

14. The method of claim 5, characterized in that the mechanical particle classification comprises sieving with a mesh size from 400 to 600 μm.

15. The method of claim 5, characterized in that the mechanical particle classification uses a rotary sieve.

16. The method of claim 7, characterized in that the average residence time of the ilmenite particles in the fluidized bed boiler (6) is at least 200 h.

17. The method of claim 16, characterized in that the average residence time of the ilmenite particles in the fluidized bed boiler (6) is at least 300 h.

18. The method of claim 8, characterized in that the average residence time of the ilmenite particles in the fluidized bed boiler (6) is less than 500 h.

19. The method of claim 18, characterized in that the average residence time of the ilmenite particles in the fluidized bed boiler (6) is less than 400 h.

20. The method of claim 19, characterized in that the average residence time of the ilmenite particles in the fluidized bed boiler (6) is less than 350 h.

21. The method of claim 11, characterized in that the fraction of ilmenite in the bed material is 30 wt. % or more.

Description

(1) Embodiments of the invention are now shown by way of example with reference to the figures.

(2) It is shown in:

(3) FIG. 1: a schematic illustration of a system for practicing the invention;

(4) FIG. 2: a schematic illustration of magnetic drum separator;

(5) FIG. 3: a schematic illustration to show the mass streams in an embodiment of the process according to the invention;

(6) FIG. 4: SEM micrographs of ilmenite particles used as bed material during the experiments: a) sand ilmenite; b) rock ilmenite;

(7) FIG. 5: SEM micrographs of cross-section of ilmenite particles extracted after 2 and 15 days of exposure where a) and b) are sand ilmenite and c) and d) are rock ilmenite;

(8) FIG. 6: sieving curves obtained through sieving of sand and rock ilmenite:

(9) FIG. 7: accumulated attrition measured on sand and rock ilmenite;

(10) FIG. 8: accumulated attrition plotted against time for rock-ilmenite and sand-ilmenite.

EXAMPLE 1

(11) In this example the composition and particle size distribution of bottom ash is analyzed. The bottom ash was taken from a 75 MW municipal solid waste fired boiler operating with the bed material comprising silica sand and 16 wt. % rock ilmenite.

(12) The bottom ash was sieved through a 500 μm mesh which removed the particle fraction coarser than 500 μm (about 50 wt. % of the original sample).

(13) The bottom ash sample, excluding particulates coarser than 500 μm, of 8.3 kg was analyzed for ranges of material content of bed materials (ilmenite, silica oxide, calcium oxide, aluminum oxide) and particle size distribution.

(14) Material Composition (Ranges, Wt. %):

(15) TABLE-US-00001 Ilmenite: 10-20% Silica oxide: 40-60% Calcium oxide:  5-10% Aluminum oxide:  5-10%
Particle Size Distribution (Wt. %):

(16) TABLE-US-00002 355-500 μm:  ~7% 250-355 μm ~17% 125-250 μm: ~69% <125 μm:  ~7%

(17) This analysis shows typical percentages of ilmenite in the bottom ash which can be retrieved according to the invention and also shows that the particle size distribution of the bottom ash does allow an initial mechanical classification to remove coarse particles with e.g. a mesh size of 500 μm.

EXAMPLE 2

(18) In this example the effectiveness of magnetic separation processes is tested. The following test equipment was used:

(19) Eriez® 305 mm dia.×305 mm wide model FA (Ferrite Axial) magnetic drum. Field strength ca. 2000 Gauss (drum #1).

(20) Eriez® 305 mm dia.×305 mm wide model RA (Rare Earth Axial) magnetic drum. Field strength ca. 4500 Gauss (drum #2).

(21) Eriez® 305 mm dia.×305 mm wide model RR (Rare Earth Radial) magnetic drum. Field strength ca. 4000 Gauss (drum #3).

(22) FIG. 2 shows an arrangement of two magnetic separation drums or rolls in sequential order.

(23) Material is fed through a feed 3 on a magnetic drum 1 rotating into the direction indicated by the arrow (counterclockwise). Magnetic particles tend to adhere to the drum longer than nonmagnetic particles which is indicated by the arrows nonmagnetics 1 and magnetics 1 in the drawing. A mechanical separator blade 4 helps to separate the magnetic and nonmagnetic particle fractions.

(24) When using a two-stage process, the nonmagnetic particle fraction from the first drum 1 can be fed to a second drum 2 for a second magnetic separation step.

(25) Three tests were carried out, the first test using a two-step separation process and the second and third test using single step separation processes. The tests were carried out with bottom ash as analyzed in example 1.

(26) Test 1

(27) A 2.5 kg bottom ash sample was passed over a ferrite magnetic drum (drum #1) with an axial magnet arrangement. This causes the strongly magnetic material to tumble as it passes from north to south poles, releasing any entrapped nonmagnetic or paramagnetic materials, thus providing a cleaner magnetic fraction.

(28) The nonmagnetic fraction from this first separation step was then passed over a second drum (drum #2), with a stronger Rare Earth axial magnetic field.

(29) Test 2

(30) A 1.25 kg bottom ash sample was passed over a drum (drum #2), with a strong Rare Earth axial magnetic field.

(31) Test 3

(32) A 1.25 kg bottom ash sample was passed over a drum (drum #3), with a strong Rare Earth radial magnetic field.

(33) Both tests 2 and 3 utilized single step magnetic separation.

(34) The test results are shown in the following table. The table also indicates the splitter position in terms of the distances A and B of the leading edge of the mechanical splitter from the rotational axis of the drum (see FIG. 2) and the drum speed in terms of min.sup.−1 and surface speed in m/min. Table 1 also indicates the results of the magnetic separation.

(35) TABLE-US-00003 Test Drum Feed Rate Splitter Position Drum Speed Sample Weight % of Feed No. Type (t/hr) A B RPM M/Min. Description No. (g) Weight 1 FA 1.5 125 mm  140 mm ~63 60 Feed 100 2498 Magnetics 1 101 716 28.7 Non Magnetics 1 102 1782 71.3 RA 1.5 70 mm 160 mm ~63 60 Magnetics 2 103 236 16.8 Non Magnetics 2 104 764 54.5 2 RA 1.5 70 mm 160 mm ~63 60 Feed 1248 Magnetics 1 201 593 47.5 Non Magnetics 1 202 655 52.5 3 RR 1.5 115 mm  170 mm ~63 60 Feed 1247 Magnetics 1 301 736 59.0 Non Magnetics 1 302 511 41.0

EXAMPLE 3

(36) FIG. 1 shows schematically an embodiment of a system for practicing the invention.

(37) A boiler 6 is fed with fuel (waste) at 7 and rock ilmenite bed material at 8.

(38) Bottom ash is retrieved via 9 and fed to a rotary sieve 10 having a mesh size of 500 μm. The coarse fraction comprising mostly ash and some lost ilmenite material is discarded at 11.

(39) The fine particle size fraction is fed to a magnetic separator 12 comprising a rare earth roll magnet (as shown above). The nonmagnetic fraction from the magnetic separator 12 is discarded at 13. The magnetic fraction is recirculated as bed material (ilmenite) to the boiler at 14.

EXAMPLE 4

(40) This example serves to illustrate material stream calculations in a further embodiment of the invention shown in FIG. 3.

(41) The system of FIG. 3 corresponds to that of FIG. 1 but additionally comprises a classifier 15 wherein the finer particles from the bottom ash are entrained by an airflow and carried back to the boiler.

(42) A bottom ash mass balance, taking into account coarse ash, fine ash, and ilmenite was constructed for the system shown in FIG. 3.

(43) Coarse ash components (A) include large particles that are easily separated by the existing recirculation system and are not accumulated, fine ash components (As) include inert sand and small agglomerates of ash that can be accumulated by the existing recirculation system, the ilmenite (I) can also, of course, be accumulated by the existing recirculation system.

(44) For the purposes of this example, the boiler is a 75 MW municipal solid waste fired boiler with a classifier that operates at 95% separation efficiency for ilmenite and fine ash. The material streams of interest are denoted in FIG. 3. Another material stream, not included in the model, consists of the very fine particles that are carried out of the furnace by the flue gas and separated as fly ash in the flue gas treatment plant, e.g. a bag-house filter or an electrostatic precipitator. This material stream consists of very fine particles from the fuel, very fine particles of fresh bed material and very fine bed material particles formed by attrition in the furnace.

(45) C denotes the classifier 15, B the boiler 6, R the rotary sieve 10, and M the magnetic separator 12. The indexes e and r denotes exiting and returning respectively. The separation efficiencies of the classifier and rotary sieve are assumed to be equal for ilmenite and fine ash while the magnetic separator is described using two different efficiencies for ilmenite and fine ash (optimally 0% for ash). The separation efficiency is varying in relation to the inflow for all separators of the system: classifier, mechanical and magnet. The coarse ash is assumed to pass both the classifier and the mechanical sieve without any fraction of it being separated (η.sub.C,A=0 and η.sub.R,A=0).

(46) The mass balances for ilmenite and fine ash are similar and therefore only that of ilmenite is described as follows:

(47) $\begin{array}{cc}\frac{d{m}_i}{dt}=I_i+I_{C,r}+I_{M,r}-I_{B,e}& \left(1\right)\\ I_{B,e}=\left(A_i+I_i+A{s}_i+I_{C,r}+I_{M,r}+A{s}_{C,r}+A{s}_{M,r}\right)*\frac{m_i}{m_{tot}}& \left(2\right)\\ I_{C,r}=I_{B,e}*{\eta }_C& \left(3\right)\\ I_{C,e}=I_{B,e}-I_{C,r}& \left(4\right)\\ I_{R,r}=I_{C,e}*{\eta }_R& \left(5\right)\\ I_{R,e}=I_{C,e}-I_{R,r}& \left(6\right)\\ I_{M,r}=I_{R,r}*{\eta }_{M,I}& \left(7\right)\\ I_{M,e}=I_{R,r}-I_{M,r}& \left(8\right)\end{array}$
where m.sub.i denotes the mass of ilmenite inside the boiler and m.sub.tot is the total mass of the bed inventory, including the coarse ash (m.sub.A) and the fine ash m.sub.As). At steady state the transient term dmi/dt is equal to zero.

(48) Upon deriving a matching set of equations for the fine ash (As), the system is calculated to yield the fraction of ilmenite in the boiler, eqn. (9), and the average time that the ilmenite spends inside the system (identical to the average residence time of the ilmenite particles in the boiler (T.sub.Res,ilmenite) as defined above), eqn. (10).

(49) $\begin{array}{cc}{F}_i=\frac{m_i}{m_{tot}}& \left(9\right)\\ {\tau }_i=\frac{m_t}{I_i}& \left(10\right)\end{array}$

(50) Four cases are defined: 1) The base case, with only the classifier as separator. 2) Also mechanical sieve and magnetic separator. Same addition rate of fresh ilmenite. 3) Also mechanical sieve and magnetic separator. Reduced flow of added fresh ilmenite, so that it gives the same fraction of ilmenite in the bed as in the base case. 4) Also mechanical sieve and magnetic separator. Increased efficiency of the mechanical and magnetic separator.

(51) Cases 1) to 3) are comparative examples, case 4) is according to the invention. The mass flow data are typical values measured over long time in the particular boiler, Table 2.

(52) In case 4, it is utilized the superior attrition resistance (less accumulated attrition rate, see below) of rock ilmenite compared with sand ilmenite by applying a recovery system with a higher efficiency (η), as seen from the data in Table 2. This case is applicable at ilmenite residence time exceeding around 7 days (168 h).

(53) TABLE-US-00004 TABLE 2 Input data for the four cases. Case 1 2 3 4 Comment The same ilmenite Decreased Invention Base fraction ilmenite using rock case in the bed addition ilmenite Mass flows (kg/s) I.sub.i 225 225 81 56 A.sub.Si 1000 1000 1000 1000 A.sub.i 4000 4000 4000 4000 Bed inventory, 25000 25000 25000 25000 m.sub.tot (kg) Separation efficiencies (—) η.sub.c 0.95 0.95 0.95 0.95 η.sub.R 0 0.8 0.8 0.96 η.sub.Mi 0 0.8 0.8 0.96 η.sub.MAs 0 0 0 0

(54) The calculated data, Table 3, describe the fraction of ilmenite in the boiler, the average residence time of ilmenite within the system (including the effects of recirculation), and the possible reduction in the amount of introduced ilmenite that maintains the ilmenite fraction of the base case.

(55) TABLE-US-00005 TABLE 3 Derived data for the base case and for operation with the proposed system. Case 1 2 3 4 Fraction of ilmenite 15.8 34.2 15.8 37.4 in the bed [%] Average residence 17.5 38.0 48.7 166 time of ilmenite in the system [h] Possible reduction in 144 (64) 169 (75) ilmenite feed [kg/h] (% of case 1)

EXAMPLE 5

(56) This example compares the composition of sand ilmenite (not according to the invention) and rock ilmenite.

(57) Sand ilmenite, which originated from Australia, was provided by Sibelco, while rock ilmenite originated from Norway and was provided by Titania A/S. The elemental composition of the fresh materials are presented in Table 4, with the main crystal phase identified being FeTiO.sub.3.

(58) TABLE-US-00006 TABLE 4 Elemental specification of sand and rock ilmenite as-received from supplier Sand Ilmenite Rock Ilmenite Element wt. % wt. % Fe 34.20 33.29 Ti 27.93 23.85 Mg 0.44 1.83 Si 0.15 0.94 Al 0.19 0.34 Mn 0.48 0.13 Ca 0.06 0.26 K 0.07 0.07 Na 0.04 0.08 P >0.01 >0.01

EXAMPLE 6

(59) This example examines the attrition properties of sand ilmenite (shape factor 0.91) and rock ilmenite (shape factor 0.7). Sand ilmenite is a comparative example, both sand and rock ilmenite are those from example 5.

(60) The tests were carried out in a 12 MW.sub.th CFB-boiler situated in the Chalmers university campus predominantly used for district heating of campus facilities from November to April. The furnace has a cross section of 2.25 m.sup.2 and a height of 13.6 m. A detailed description of the system is provided in Thunman, H. Lind, F. Breitholtz, C. Berguerand, N. Seemann, M. 2013; Using an oxygen-carrier as bed material for combustion of biomass in a 12-MW.sub.th circulating fluidized-bed boiler; Fuel 113, 300-309.

(61) The system is equipped with a number of extraction ports where bed material and bottom ashes can be extracted at the dense state of the bed using a water-cooled suction probe. Bed material samples were extracted from the dense bed, the first one shortly after start-up and then on a daily basis for 15 days. In the present paper, only the results from the second and 15.sup.th days are presented. During the experimental period, controlled amount of new bed material was added when required in order to keep constant operational conditions.

(62) Two experimental runs have been performed, one with each of sand and rock ilmenite. For both of the experimental sessions 100% of the respective ilmenite was used in the boiler as bed material. During the experiments, the boiler was fired with wood-chips that had a moisture content in the range of 38.5-45.3 wt. % based on the as-received fuel and the bed temperature was held around 850° C. Furthermore, to withhold stable operational conditions, the bed height was held constant through continuous supply of additional fresh material. The total bed inventory in the boiler was held around 3000 kg throughout the experiments.

(63) A selection of the extracted bed material samples were immobilized in epoxy resin and polished to obtain a cross-sectional surface of the particles, which was evaluated with Scanning Electron Microscopy (SEM) analysis. Quanta 200FEG equipped with an Oxford EDS system was used for SEM imaging and elemental composition analysis. 50-60 g of the sampled bed material was sieved during 20 min to obtain the size distribution. Sieving plates of the following mesh size was used; 355 μm, 250 μm, 180 μm, 125 μm, 90 μm and a bottom plate for fractions below 90 μm. Particles in the range of 125-180 μm were collected during the sieving, from which a sample of 5 g was tested for mechanical stability in a customized jet cup, described in detail in Rydén, M. Moldenhauer, P. Lindqvist, S. Mattisson, M. Lyngfelt, A. 2014; Measuring attrition resistance of oxygen carrier particles for chemical looping combustion with a customized jet cup; Powder Technology 256, 75-86. The apparatus is constructed to simulate the mechanical stress that particles undergo in a FBC. A filter collecting the fine particles that leave the device at the top, was continuously measured, providing the rate of attrition of the bed material particles.

(64) Cross-sectional SEM micrographs of fresh sand and rock ilmenite particles are shown in FIGS. 4a) and b), respectively. The materials differ in particle morphology where the sand ilmenite particles have rounded edges and the rock ilmenite particles have sharp edges. The shape factors are 0.91 and 0.7 respectively.

(65) The difference in particle shape is influenced by the origin of the materials. The sand ilmenite, which has been used in the as-received form, has prior to collection been exposed to natural weathering, erosion and attrition, whereof the particles have obtained a rounded shape. This is not the case for rock ilmenite particles which have been mined and ground and are thus sharp-edged. Analysis with SEM-EDX show that both materials have a homogeneous distribution of Fe and Ti over the cross-section with no local enrichment of either of the elements.

(66) The change in morphology of the particles have been followed on samples of both sand and rock ilmenite extracted from the boiler after 2 and 15 days. The cross-sectional micrographs of these are presented in FIG. 5 a)-d). After 2 days of exposure, (FIG. 5 a)), small voids are formed at the outer parts of the sand ilmenite particle. This phenomenon is further developed over time and is more prominent after 15 days (FIG. 5 b)) where the voids have evolved to larger cavities that are widespread at the inside of the particles. The rock ilmenite particles have formed distinct cracks that were extended along the inside of the particles, after 2 days (FIG. 5 c)). During further exposure, the cracks in the rock ilmenite expanded further, which led to break-up of the majority of the particles (FIG. 5 d).

(67) The different morphologies developed during exposure points on the importance that the initial structural morphology of the particles have on their mechanical performance during exposure. Small cavities are expected to form within the bed material particles, as a result of inter-diffusion of elements during high temperature exposure. Further, formation of cracks has also been reported previously as a result of the thermal and mechanical stress that the particles undergo within the reactor (Knutsson, P. Linderholm, C. 2015; Characterization of ilmenite used as oxygen carrier in a 100 kW chemical-looping combustor for solid fuels; Applied Energy 157, 368-373). During the mining and the grinding process that the rock ilmenite has undergone prior to exposure, the material has accumulated mechanical stress. The further thermal and chemical stress during exposure to the conditions in the combustion chamber, adds to this accumulated stress and leads, most probably, to cracks opening as a form of stress release. The initial material preparation, could therewith be used as an explanation for the mechanism observed for the rock ilmenite.

(68) Particle size distributions have been obtained through sieving of the materials prior to exposure as well as sieving the collected samples that have been used in the boiler. In FIG. 6, the results from the sieving of fresh sand and rock ilmenite, as well as the materials collected after 2 and 15 days, are presented. R stands for rock, S for sand, F for fresh material, 2 and 15 are the amount of days that the material has been used as bed material before extraction. The sieving curves of the fresh materials reveal that the sand ilmenite contains considerably higher amount of finer fractions than the rock ilmenite, which is in line with the supplier's specifications. The lower amount of fines in the case of rock ilmenite, can be explained with the more narrowed size distribution that is obtained through grinding.

(69) After 2 days of exposure the rock ilmenite shows a noticeable increase in the amount of coarse particles (particles above 250 μm), which further expands with time. This trend is accompanied by an initial decrease in the finer particle fractions (below 125 μm), followed by a moderate increase after 15 days. For the sand ilmenite, a drastic decrease of finer fractions can be observed after two days, as well as a significant increase of particles over 180 μm. These trends are consistent and sustained after 15 days.

(70) The enlargement of particle size for both sand and rock ilmenite, with increased time in the combustion chamber, can be explained by the ash layer growth around the particles. Increase in the porosity of the ilmenite particles (both sand and rock) has also been observed and previously reported, which would also lead to increase in the size of the bed material particles. The drastic decrease of finer fractions could mainly be explained by particle loss due to their entrainment with the flue gases and with the fly ash. Some of the particles are also expected to increase in size due to the factors described previously and thereby be accounted for in higher size fractions within the sieving curve.

(71) FIG. 7 shows the results of the attrition tests performed on both sand, S (solid lines) and rock, R (dashed lines) ilmenite in “as-received” conditions and after 2 and 15 days of exposure. Diamond markers represent fresh material, F, square and circular markers represent material that has been extracted after 2 and 15 days of operation in the combustor, respectively.

(72) FIG. 8 plots the rightmost data points from FIG. 7 against residence time in the boiler for sand and rock ilmenite.

(73) The fresh materials are worn equally in the beginning, followed by a slight increase in the measured attrition for the case of fresh rock ilmenite. The increase in the latter case was expected due to the observed sharp edged particles morphology which is thus more easily worn off than the rounded structure of the fresh sand ilmenite particles. Accordingly, used rock ilmenite particles obtain a more rounded shape with exposure time in the combustion chamber, which is also confirmed by the results in FIG. 5. For both materials, the measured attrition is increased after exposure in the boiler. The materials show higher attrition after 2 days than after 15 days of exposure. The highest accumulated attrition is found for the rock ilmenite after 2 days, which with further exposure decreases below the attrition for the exposed sand ilmenite samples.

(74) The attrition of both materials is highest after 2 days, which is reasonably due to that the inherent stress in the particles is released early in their exposure to boiler conditions. This is confirmed by the observation that the attrition is higher for the rock ilmenite which in its as-received form is also expected to contain a higher degree of inherent stress. With further exposure, the attrition of both materials is decreased. The reason for this could be coupled to that the particles are stabilized by the formation of ash layers. However, the rock ilmenite becomes considerably more resistant to mechanical stress with time in comparison to sand ilmenite. The reason being that cavities found in sand ilmenite are built up over time while the cracks in the rock ilmenite are formed earlier on.

(75) The obtained results point to that the sand and rock ilmenite differ in their structural development, which has impact on their corresponding mechanical stability. It is found that rock ilmenite is initially less resistant to mechanical stress, but with increased exposure becomes more resistant to it in comparison to sand ilmenite.