He sprinkled - Rather, he cast. See Leviticus 1:5.

And Moses took half of the blood, and put it in basins,.... Half of the blood of the above sacrifices, this he put into basins, and set by, in order to sprinkle on the people:

and half of the blood he sprinkled on the altar; the Targum of Onkelos adds, to atone for the people. But the altar here seems to represent the Lord, who was one of the parties covenanting, and therefore is sprinkled with blood as a ratification of the covenant on his part, and the promises of it.

The blood was divided into two parts. One half was swung by Moses upon the altar (זרק to swing, shake, or pour out of the vessel, in distinction from הזּה to sprinkle) the other half he put into basins, and after he had read the book of the covenant to the people, and they had promised to do and follow all the words of Jehovah, he sprinkled it upon the people with these words: "Behold the blood of the covenant, which Jehovah has made with you over all these words." As several animals were slaughtered, and all of them young oxen, there must have been a considerable quantity of blood obtained, so that the one half would fill several basins, and many persons might be sprinkled with it as it was being swung about. The division of the blood had reference to the two parties to the covenant, who were to be brought by the covenant into a living unity; but it had no connection whatever with the heathen customs adduced by Bhr and Knobel, in which the parties to a treaty mixed their own blood together. For this was not a mixture of different kinds of blood, but it was a division of one blood, and that sacrificial blood, in which animal life was offered instead of human life, making expiation as a pure life for sinful man, and by virtue of this expiation restoring the fellowship between God and man which had been destroyed by sin. But the sacrificial blood itself only acquired this signification through the sprinkling or swinging upon the altar, by virtue of which the human soul was received, in the soul of the animal sacrificed for man, into the fellowship of the divine grace manifested upon the altar, in order that, through the power of this sin-forgiving and sin-destroying grace, it might be sanctified to a new and holy life. In this way the sacrificial blood acquired the signification of a vital principle endued with the power of divine grace; and this was communicated to the people by means of the sprinkling of the blood. As the only reason for dividing the sacrificial blood into two parts was, that the blood sprinkled upon the altar could not be taken off again and sprinkled upon the people; the two halves of the blood are to be regarded as one blood, which was first of all sprinkled upon the altar, and then upon the people. In the blood sprinkled upon the altar, the natural life of the people was given up to God, as a life that had passed through death, to be pervaded by His grace; and then through the sprinkling upon the people it was restored to them again, as a life renewed by the grace of God. In this way the blood not only became a bond of union between Jehovah and His people, but as the blood of the covenant, it became a vital power, holy and divine, uniting Israel and its God; and the sprinkling of the people with this blood was an actual renewal of life, a transposition of Israel into the kingdom of God, in which it was filled with the powers of God's spirit of grace, and sanctified into a kingdom of priests, a holy nation of Jehovah (Exodus 19:6). And this covenant was made "upon all the words" which Jehovah had spoken, and the people had promised to observe. Consequently it had for its foundation the divine law and right, as the rule of life for Israel.

And Moses took half of the blood, and put it in basins; and half of the blood he sprinkled on the altar.

24:6
1. The blood of the sacrifice which the people offered was (part of it) sprinkled upon the altar, which signified the people's dedicating themselves to God, and his honour. In the blood of the sacrifices, all the Israelites were presented unto God as living sacrifices, Rom 12:1.
2. The blood of the sacrifice which God had owned and accepted was (the remainder of it) sprinkled, either upon the people themselves, or upon the pillars that represented them, which signified God's conferring his favour upon them, and all the fruits of that favour, and his giving them all the gifts they could desire from a God reconciled to them, and in covenant with them. This part of the ceremony was thus explained, Behold the blood of the covenant; see here how God sealed to you to be a God, and you seal to be to him a people; his promises to you, and yours to him, are yea and amen. Thus our Lord Jesus, the Mediator of the new covenant (of whom Moses was a type) having offered up himself a sacrifice upon the cross, that his blood might be indeed the blood of the covenant, sprinkled it upon the altar in his intercession (Heb 9:12,) and sprinkles it upon his church by his word and ordinances, and the influences and operations of the Spirit of promise by whom we are sealed.

6. Moses took half of the blood . sprinkled-Preliminary to this was the public reading of the law and the renewed acceptance of the terms by the people; then the sprinkling of the blood was the sign of solemn ratification-half on each party in the transaction.

24:1-8 A solemn covenant was made between God and Israel. Very solemn it was, typifying the covenant of grace between God and believers, through Christ. As soon as God separated to himself a peculiar people, he governed them by a written word, as he has done ever since. God's covenants and commands are so just in themselves, and so much for our good, that the more we think of them, and the more plainly and fully they are set before us, the more reason we may see to comply with them. The blood of the sacrifice was sprinkled on the altar, on the book, and on the people. Neither their persons, their moral obedience, nor religious services, would meet with acceptance from a holy God, except through the shedding and sprinkling' of blood. Also the blessings granted unto them were all of mercy; and the Lord would deal with them in kindness. Thus the sinner, by faith in the blood of Christ, renders willing and acceptable obedience.