System for preparing weighting agent for well drilling from industrial carnallite

Abstract

The utility model provides a system for preparing a weighting agent for well drilling from industrial carnallite, and relates to the technical field of industrial waste salt recycling, the system comprises a feeding unit, a storage unit, a multi-stage crushing unit, a vacuum drying unit and a vacuum conveying unit, the storage unit comprises a raw material buffer storage bin and a compound medicament bin; the feeding unit is used for feeding materials to the raw material temporary storage bin, and the compound agent bin is used for containing an anti-caking complexing agent; the multi-stage crushing unit at least comprises a first screw conveyor and a crusher which are arranged in sequence, one end of the first screw conveyor is arranged in the raw material temporary storage bin, and the compound medicament bin is located above the first screw conveyor; the vacuum drying unit comprises a drying machine, and the drying machine is arranged between the first spiral conveyor and the crusher; by means of the device, industrial carnallite generated by offshore drilling can be treated, and the purpose that the industrial carnallite is prepared into the weighting agent for drilling is achieved.

Description

Technical Field

[0001] This utility model relates to the field of industrial waste salt resource utilization technology, and in particular to a system for preparing drilling weighting agents from industrial mixed salts. Background Technology

[0002] Drilling fluid weighting agents play a crucial role in oil and gas drilling, increasing the density of the drilling fluid to control downhole pressure and prevent hazards such as blowouts and well collapses. However, traditional weighting agent production methods typically require high-purity raw materials such as barium sulfate, barite, sodium chloride, and potassium chloride. These raw materials are expensive and scarce, leading to high production costs. Furthermore, traditional methods generate significant amounts of waste during processing, causing environmental pollution.

[0003] Offshore drilling processes generate large amounts of solid waste such as drilling mud and rock cuttings. These wastes have a high salt content, and the resulting crystalline salts typically appear as end-of-pipe solid waste. Industrial salts, as waste, are generally considered useless and difficult to manage. However, industrial salts are inexpensive, abundant, and have similar compositions, making them potentially valuable in the preparation of drilling fluid weighting agents. However, their high water content, high agglomeration rate, and low chelation value limit their application in weighting agent preparation.

[0004] Currently, there is limited research on the resource utilization of industrial salts generated during drilling. Existing technologies are insufficient to effectively treat and utilize them to meet the requirements of drilling fluid weighting agents. These salts are generally disposed of as industrial solid waste through outsourcing.

[0005] Therefore, there is an urgent need for an industrial mixed salt preparation system for drilling weighting agents, which can convert the mixed salt into a product that meets the requirements of drilling fluid weighting agents, thereby realizing resource utilization, reducing production costs, and reducing environmental pollution. Utility Model Content

[0006] The purpose of this invention is to provide a system for preparing drilling weighting agents from industrial mixed salts, addressing the existing limitations in research on the resource utilization of industrial mixed salts generated during drilling, the difficulty in effectively processing and utilizing them to meet the requirements of drilling fluid weighting agents, and the technical problem that such mixed salts are generally outsourced for disposal as industrial solid waste. The various technical effects of the preferred solutions among the many technical solutions provided by this invention are detailed below.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] This utility model provides a system for preparing drilling weighting agents from industrial mixed salts, comprising: a feeding unit, a storage unit, a multi-stage crushing unit, a vacuum drying unit, and a vacuum conveying unit, wherein:

[0009] The storage unit includes a raw material buffer silo and a compound medicine silo;

[0010] The feeding unit is used to feed the raw material buffer silo, and the compound agent silo is used to contain the anti-caking complexing agent.

[0011] The multi-stage crushing unit includes at least a first screw conveyor and a crusher arranged in sequence, with one end of the first screw conveyor located in the raw material buffer silo and the compound agent silo located above the first screw conveyor;

[0012] The vacuum drying unit includes a dryer, which is disposed between the first screw conveyor and the crusher;

[0013] A discharge hopper is provided between the vacuum conveying unit and the crusher.

[0014] Preferably, the feeding unit includes:

[0015] Support arm;

[0016] The suspension arm is movably connected to the support arm;

[0017] An electric chain hoist is mounted on the suspension arm.

[0018] Preferably, the compound drug compartment is equipped with a drug dosing spiral structure.

[0019] Preferably, the multi-stage crushing unit further includes a second screw conveyor, wherein:

[0020] The second screw conveyor is located between the vacuum drying unit and the crusher.

[0021] Preferably, the first screw conveyor is an inclined screw conveyor, and the second screw conveyor is a U-shaped screw conveyor.

[0022] Preferably, the vacuum drying unit further includes a heating device, a gas-liquid separation device, and an induced draft fan, wherein:

[0023] The heating device is located at the bottom of the dryer, and the gas-liquid separation device is connected to the dryer through a pipeline;

[0024] The induced draft fan is used to connect the dryer and the gas processing device.

[0025] Preferably, the vacuum drying unit further includes a heat exchanger, a vacuum pump, a circulating pump, and a cooling tower, wherein:

[0026] The heat exchanger is connected to the vacuum pump, the circulating pump, and the gas-liquid separation device, respectively.

[0027] The circulating pump is connected to the cooling tower.

[0028] Preferably, the inner layer of the dryer is made of 2205 stainless steel.

[0029] Preferably, the vacuum conveying unit includes a pulse vacuum pump, the pipeline between the crusher and the discharge hopper is connected to an arch breaker, and a filter bag is installed at the inlet of the discharge hopper.

[0030] Preferably, the first screw conveyor is used to crush materials to a particle size of no more than 20 mm;

[0031] The crusher is a auger crusher used to crush materials to a particle size between 0.4mm and 1mm.

[0032] This invention provides a system for preparing drilling weighting agents from industrial mixed salts. Through a combination of multi-stage crushing, vacuum drying, compounding property conversion, and pulsed vacuum conveying processes, the system treats industrial mixed salts generated during offshore drilling, reducing water content and agglomeration rate, controlling particle size, and improving fluidity, calcium chelation value, and ability to stabilize ferric ions. This allows for the preparation of drilling weighting agents from industrial mixed salts. The product's performance is significantly improved after resource utilization. Its saturated solution density, calcium chelation value, and ability to stabilize ferric ions all meet the requirements of the standard "Performance Indicators of Completed Fluids - Solid-Free Water-Based Completed Fluids" (Q / HS2014.1-2004). The product after resource utilization by the system can be directly used as a weighting agent for offshore drilling. This system is suitable for the resource utilization of drilling weighting agents prepared from such industrial mixed salts and has good application prospects. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of this utility model or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a schematic diagram of an embodiment of the industrial mixed salt preparation system for drilling weighting agents according to this utility model;

[0035] Figure 2 This is a process flow diagram of an industrial salt preparation system for drilling weighting agents.

[0036] In the diagram: 1. Feeding unit; 11. Support arm; 12. Suspension arm; 13. Electric hoist;

[0037] 2. Storage unit; 21. Raw material buffer silo; 22. Compound drug silo;

[0038] 3. Multi-stage crushing unit; 31. First screw conveyor; 32. Crusher; 33. Second screw conveyor;

[0039] 4. Vacuum drying unit; 41. Dryer; 42. Heating device; 43. Gas-liquid separation device; 44. Exhaust fan; 45. Heat exchanger; 46. Vacuum pump; 47. Circulating pump; 48. Cooling tower; 49. Gas processing device;

[0040] 5. Vacuum conveying unit; 6. Discharge hopper; 7. Condensate; 8. Weighting agent. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this utility model clearer, the technical solutions of this utility model will be described in detail below. Obviously, the described embodiments are only a part of the embodiments of this utility model, and not all of them. Based on the embodiments of this utility model, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this utility model.

[0042] In the description of this utility model, it should be understood that the terms "center", "side", "length", "width", "height", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "side", etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this utility model and simplifying the description, and are not intended to indicate or imply that the device or component referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this utility model.

[0043] In the description of this utility model, it should also be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this utility model according to the specific circumstances.

[0044] In this embodiment, the industrial mixed salt used for resource utilization is mud and rock cuttings generated during offshore drilling. After pretreatment, the mixture is evaporated and concentrated by mechanical evaporation equipment, and then centrifuged by centrifuge to produce crystalline mixed salt. The specific composition is shown in Table 1.

[0045] Table 1 Components of Industrial Miscellaneous Salts

[0046] Serial Number project Components unit 1 chloride ions 1.36×104 mg / L 2 phosphate 0.122 mg / L 3 sulfate 9.02×103 mg / L 4 Total Chromium 0.06 mg / L 5 nickel 25.1 mg / kg 6 cadmium 0.1L mg / kg 7 Potassium 1.17×105 mg / kg 8 sodium 3.1×105 mg / kg 9 Hexavalent chromium 0.04L mg / L 10 magnesium 70.8 mg / kg 11 arsenic 0.0175 mg / L 12 mercury 0.0004 mg / L 13 calcium 9.05×103 mg / kg 14 manganese 7.0 mg / kg 15 copper 0.4L mg / kg 16 tin 0.8L mg / L 17 antimony 0.5L mg / kg 18 lead 1.4L mg / kg

[0047] Note: 1. L indicates that the detection result is less than the detection limit, and its value is the detection limit for this item.

[0048] This embodiment provides a system for preparing drilling weighting agents using industrial mixed salts. Figure 1 This is a structural schematic diagram of this embodiment, as shown below. Figure 1 As shown, the industrial mixed salt preparation system for drilling weighting agents includes a feeding unit 1, a storage unit 2, a multi-stage crushing unit 3, a vacuum drying unit 4, and a vacuum conveying unit 5.

[0049] The storage unit 2 includes a raw material buffer silo 21 and a compound agent silo 22; the feeding unit 1 is used to feed the raw material buffer silo 21, and the compound agent silo 22 is used to contain the anti-caking complexing agent.

[0050] The feeding unit 1 in this embodiment includes a support arm 11, a suspension arm 12, and a chain electric hoist 13. The support arm 11 supports the suspension arm 12 and the chain electric hoist 13. The suspension arm 12 is movably connected to the support arm 11, and the chain electric hoist 13 is disposed on the suspension arm 12.

[0051] In use, the operator adjusts the angle and position of the suspension arm 12 and the support arm 11 to lower the electric chain hoist 13 and lift the raw materials (in ton bags) to the top of the raw material buffer silo 21 via the suspension arm 12. The weight range of the suspension arm 12 is 0.5t-2t. The operator unloads the raw materials into the raw material buffer silo 21 using the unloading knife.

[0052] In this embodiment, the raw material buffer silo 21 is made of 2205 stainless steel, and its volume is 3m³. 3 It is used to store materials.

[0053] The compound reagent compartment 22 is used to contain the anti-caking complexing agent. The compartment 22 has a volume of 100L and is made of 2205 stainless steel. It features a dosing spiral structure. The main components of the anti-caking complexing agent in the compartment 22 are: tetrasodium EDTA, crystallization inhibitor HPMC, water loss reducing agent acrylamide, and corrosion inhibitor sodium gluconate. This anti-caking complexing agent helps prevent clumping during the drying process, while also improving the product's chelation value and reducing the clumping rate.

[0054] In this embodiment, the multi-stage crushing unit 3 includes at least a first screw conveyor 31 and a crusher 32 arranged in sequence. One end of the first screw conveyor 31 is located in the raw material buffer silo 21, and the compounding agent silo 22 is located above the first screw conveyor 31, which can realize material compounding during the material conveying and drying process.

[0055] Among them, the first screw conveyor 31 is an inclined screw conveyor, which is a primary crusher used to crush materials to a particle size of no more than 20mm. The crusher 32 is an auger crusher, which is a secondary crusher used to crush materials to a particle size between 0.4mm and 1mm.

[0056] Because the mixed salts have a high water content and low hardness, a vacuum drying unit 4 is provided, including a dryer 41. In this embodiment, the dryer 41 is a drum dryer. The inner layer of the dryer 41, which comes into contact with the material, is made of 2205 stainless steel. The volume of the dryer is 4.5m³. 3 The dryer 41 is positioned between the first screw conveyor 31 and the crusher 32.

[0057] Specifically, in use, after primary crushing using an inclined screw conveyor, the particle size of the crushed material is no higher than 20mm. After drying, it enters the secondary crushing - a chisel crusher, and the particle size after crushing is between 0.4mm and 1mm.

[0058] Optionally, the multi-stage crushing unit 3 further includes a second screw conveyor 33, wherein the second screw conveyor 33 is disposed between the vacuum drying unit 4 and the crusher 32. The second screw conveyor 33 is a U-shaped screw conveyor.

[0059] The vacuum drying unit 4 in this embodiment also includes a heating device 42, a gas-liquid separation device 43, and an induced draft fan 44.

[0060] The heating device 42 is an electric heating oil furnace, which is located at the bottom of the dryer 41. The gas-liquid separation device 43 is connected to the dryer 41 through a pipeline. The induced draft fan 44 is used to connect the dryer 41 and the gas processing device 49.

[0061] Optionally, the vacuum drying unit 4 further includes a heat exchanger 45, a vacuum pump 46, a circulating pump 47, and a cooling tower 48, wherein the heat exchanger 45 is connected to the vacuum pump 46, the circulating pump 47, and the gas-liquid separation device 43 respectively; and the circulating pump 47 is connected to the cooling tower 48.

[0062] In this embodiment, a discharge hopper 6 is provided between the vacuum conveying unit 5 and the crusher 32. The vacuum conveying unit 5 includes a pulse vacuum pump, and the pipeline between the crusher 32 and the discharge hopper 6 is connected to an arch breaker. A filter bag is installed at the inlet of the discharge hopper 6. After being crushed, the raw material is conveyed to the discharge hopper 6 through the vacuum conveying unit 5. At the same time, the arch breaker is activated once every 60 seconds to prevent pipeline blockage. The volume of the discharge hopper 6 is 1.5 m3, and during operation, it is maintained between -0.06 MPa and -0.1 MPa.

[0063] like Figure 2 As shown, the working process of this industrial mixed salt system for preparing drilling weighting agents includes the following steps:

[0064] Step 1: The operator operates the feeding unit 1 with a handheld remote control, adjusts the angle and position of the suspension arm 12 and the support arm 11, lowers the electric hoist 13, and hoists the raw materials through the suspension arm 12 to the raw material buffer hopper 21 of the storage unit 2. The operator unloads the raw materials into the raw material buffer hopper 21 using the unloading knife. 3 bags are unloaded, totaling 3 tons.

[0065] Step 2: The operator starts the first screw conveyor 31 to transport 3t of material to the dryer 41 and performs preliminary crushing. The particle size of the crushed material is no higher than 20mm. During the conveying process, the material is compounded. At the same time, the dosing screw structure is started to add anti-caking complexing agent through the compounding agent bin 22. The dosage is 3%-5%.

[0066] Step 3: The operator starts the dryer 41 and pre-starts the heating device 42. The oil furnace operating temperature reaches 140℃, and feeding begins. After the heated raw material enters the drum dryer 41, the vacuum pump 46 is turned on to evacuate the equipment to -300pa. The drive motor of the drum dryer 41 starts working, and the drying process begins. During the drying process, the operating temperature is between 140℃ and 200℃, and the vacuum degree is maintained between -300pa and -1000pa. Drying takes 3-6 hours, and the material moisture content is not higher than 6% (the moisture content can be determined by the liquid level change of the gas-liquid separator 43 or by judging the material flowability through the sight glass). Then, the material is discharged. The air compressor, feed pneumatic valve, cooling tower fan, and circulating pump are turned on in advance. The drawn air is cooled by the plate heat exchanger 45 and then enters the gas-liquid separator 43 to achieve gas-liquid separation. The liquid is collected and processed. The temperature of the gas after heat exchange is not higher than 60℃. The gas is then connected to the VOCs treatment system via the induced draft fan 44 on the gas-liquid separator 43.

[0067] Step 3: The operator starts the pulse vacuum pump, crusher 32, and second screw conveyor 33 of vacuum conveying unit 5. The pulse vacuum pump draws the vacuum degree of discharge hopper 6 to below -0.06Mpa, and crusher 32 crushes the material. After drying, the discharge valve is activated to unload the material. The material is conveyed to the reamer crusher via 6U screw conveyor for crushing. The crushed particle size is between 0.4mm and 1mm. The crushed material is pumped into discharge hopper 6 by pulse vacuum pump, filtered by filter bags, and falls into discharge hopper 6 under gravity. After unloading, the pulse vacuum pump is turned off, the discharge pneumatic valve is opened, and the product weighting agent is unloaded into ton bags, completing the preparation of drilling weighting agent from industrial miscellaneous salt.

[0068] This embodiment utilizes industrial mixed salt produced from offshore drilling as a raw material, reducing reliance on high-purity raw materials and significantly lowering production costs. The direct drying, crushing, and compounding process simplifies the process, reducing complex processing steps and expensive equipment investment.

[0069] After adding an anti-caking complexing agent to the compound, the main component, tetrasodium EDTA (Na₄EDTA), improves the density and stability of the weighting agent. EDTA molecules have six coordination sites, enabling them to chelate divalent and trivalent metal ions in drilling fluids, such as divalent calcium ions (Ca). 2+ or ferric ions Fe 3+ This process forms stable coordination compounds with metal ions, preventing these ions from causing drilling fluid instability under high temperature and pressure conditions. This chelation reaction improves the chemical stability of the drilling fluid by forming stable chelates (such as CaEDTA and FeEDTA), preventing wellbore collapse and mud contamination. The chemical reaction equation is as follows:

[0070] For divalent metal ions (such as Ca) 2+ ):

[0071] Na4EDTA+Ca 2+ →[CaEDTA] 2- +4Na +

[0072] For trivalent metal ions (such as Fe) 3+ ):

[0073] Na4EDTA+Fe 3+ →[FeEDTA] - +4Na +

[0074] After adding an anti-caking complexing agent, the main components HPMC and acrylamide improve the rheological properties of the drilling fluid, enhancing its suspension capacity and fluid loss reduction performance. Acrylamide reacts with water to form polyacrylamide (PAM), a highly effective fluid loss reducer. PAM can form a polymer chain structure in the drilling fluid, effectively sealing formation fractures and reducing fluid loss. The reaction is as follows:

[0075] Polymerization of acrylamide:

[0076] Initiator generates free radicals:

[0077] K₂S₂O₈→₂K⁺₂SO₄ -

[0078] Polymerization of acrylamide:

[0079] nC3H5NO→(C3H5NO)n

[0080] The long-chain structure of polyacrylamide (PAM) is formed as: (-CH2-CH(CONH2)-)n

[0081] HPMC (hydroxypropyl methylcellulose) helps maintain the homogeneity and stability of drilling fluids by increasing their viscosity and rheological properties. Furthermore, HPMC effectively prevents the crystallization and precipitation of solid particles, improving the suspension capacity of the drilling fluid. The reaction is as follows:

[0082] The thickening effect of HPMC:

[0083] HPMC + H₂O → HPMC·H₂O

[0084] Note: This is a physical process; hydrogen bonds are formed, increasing viscosity.

[0085] After adding an anti-caking complexing agent to the compound, the main component, sodium gluconate, enhances corrosion inhibition. As an effective corrosion inhibitor, sodium gluconate prevents metal corrosion by forming stable gluconate complexes with metal ions (such as iron ions), thereby significantly reducing the corrosion of drilling fluid on drilling tools and equipment. The chemical reaction equation is as follows:

[0086] For ferrous ions (Fe) 2+ ):

[0087] Fe 2+ +C6H 11 NaO7→[Fe(C6H 11 O7) 2 ] 2- +Na +

[0088] For ferric ions (Fe 3+ ):

[0089] Fe 3+ +C6H 11 NaO7→[Fe(C6H 11 O7) 3 ] 3- +Na +

[0090] Samples were taken from different batches of the weighting agent, and the product moisture content, agglomeration rate, solution density, calcium chelation value, and ability to stabilize ferric ions were tested sequentially, as shown in Figure 2.

[0091] Table 2 Test Data Recording Table

[0092]

[0093] As can be seen from the table above, the weighting agent prepared in this embodiment has a water content of 2.84%, a significant drying effect, a saturated solution density of 1.248 g / cm3 at 20℃, an average agglomeration rate of 3.58%, and average calcium chelation capacity and ferric ion stabilization capacity of 19370 mg / kg and 55893 mg / kg, respectively. All parameters meet the requirements of the standard "Performance Index of Completed Fluid - Solid-Free Water-Based Completed Fluid" (Q / HS2014.1-2004). This process and equipment are suitable for the resource utilization of drilling weighting agents prepared from such industrial mixed salts.

[0094] The above description is merely a specific embodiment of this utility model, but the protection scope of this utility model is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this utility model should be included within the protection scope of this utility model. Therefore, the protection scope of this utility model should be determined by the protection scope of the claims.

Claims

1. A system for preparing drilling weighting agents from industrial mixed salts, characterized in that, include: The unit includes a feeding unit, a storage unit, a multi-stage crushing unit, a vacuum drying unit, and a vacuum conveying unit, among which: The storage unit includes a raw material buffer silo and a compound medicine silo; The feeding unit is used to feed the raw material buffer silo, and the compound agent silo is used to contain the anti-caking complexing agent. The multi-stage crushing unit includes at least a first screw conveyor and a crusher arranged in sequence, with one end of the first screw conveyor located in the raw material buffer silo and the compound agent silo located above the first screw conveyor; The vacuum drying unit includes a dryer, which is disposed between the first screw conveyor and the crusher; A discharge hopper is provided between the vacuum conveying unit and the crusher.

2. The industrial mixed salt preparation system for drilling weighting agents according to claim 1, characterized in that, The feeding unit includes: Support arm; The suspension arm is movably connected to the support arm; An electric chain hoist is mounted on the suspension arm.

3. The industrial mixed salt preparation system for drilling weighting agents according to claim 1, characterized in that, The compound drug compartment is equipped with a drug dosing spiral structure.

4. The system for preparing drilling weighting agents from industrial mixed salts according to any one of claims 1-3, characterized in that, The multi-stage crushing unit also includes a second screw conveyor, wherein: The second screw conveyor is located between the vacuum drying unit and the crusher.

5. The industrial mixed salt preparation system for drilling weighting agents according to claim 4, characterized in that: The first screw conveyor is an inclined screw conveyor, and the second screw conveyor is a U-shaped screw conveyor.

6. The system for preparing drilling weighting agents from industrial mixed salts according to any one of claims 1-3, characterized in that: The vacuum drying unit also includes a heating device, a gas-liquid separation device, and an induced draft fan, wherein: The heating device is located at the bottom of the dryer, and the gas-liquid separation device is connected to the dryer through a pipeline; The induced draft fan is used to connect the dryer and the gas processing device.

7. The industrial mixed salt preparation system for drilling weighting agents according to claim 6, characterized in that, The vacuum drying unit also includes a heat exchanger, a vacuum pump, a circulating pump, and a cooling tower, wherein: The heat exchanger is connected to the vacuum pump, the circulating pump, and the gas-liquid separation device, respectively. The circulating pump is connected to the cooling tower.

8. The industrial mixed salt preparation system for drilling weighting agents according to claim 7, characterized in that, The inner layer of the dryer is made of 2205 stainless steel.

9. The system for preparing drilling weighting agents from industrial mixed salts according to any one of claims 1-3, characterized in that, The vacuum conveying unit includes a pulse vacuum pump, the pipeline between the crusher and the discharge hopper is connected to an arch breaker, and a filter bag is installed at the inlet of the discharge hopper.

10. The system for preparing drilling weighting agents from industrial mixed salts according to any one of claims 1-3, characterized in that: The first screw conveyor is used to crush materials to a particle size of no more than 20 mm; The crusher is a auger crusher used to crush materials to a particle size between 0.4mm and 1mm.

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